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  • 1
    Publication Date: 2024-04-26
    Description: The sinking of particulate matter from the upper ocean dominates the export and sequestration of organic carbon by the biological pump, a critical component of the Earth's carbon cycle. Controls on carbon export are thought to be driven by ecological processes that produce and repackage sinking biogenic particles. Here, we present observations during the demise of the Northeast Atlantic Ocean spring bloom illustrating the importance of storm-induced turbulence on the dynamics of sinking particles. A sequence of four large storms caused upper layer mean turbulence levels to vary by more than three orders of magnitude. Large particle (>0.1 to 10 mm) abundance and size changed accordingly: increasing via shear coagulation when turbulence was moderate and decreasing rapidly when turbulence was intense due to shear disaggregation. Particle export was also tied to storm forcing as large particles were mixed to depth during mixed layer deepening. After the mixed layer shoaled, these particles, now isolated from intense surface mixing, grew larger and subsequently sank. This sequence of events matched the timing of sinking particle flux observations. Particle export was influenced by increases in aggregate abundance and porosity, which appeared to be enhanced by the repeated creation and destruction of aggregates. Last, particle transit efficiency through the mesopelagic zone was reduced by presumably biotic processes that created small particles (〈0.5 mm) from larger ones. Our results demonstrate that ocean turbulence significantly impacts the nature and dynamics of sinking particles, strongly influencing particle export and the efficiency of the biological pump.
    Type: Article , NonPeerReviewed
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  • 2
    Publication Date: 2024-05-13
    Description: Warmer temperatures and higher sea level than today characterized the Last Interglacial interval [Pleistocene, 128 to 116 thousand years ago (ka)]. This period is a remarkable deep-time analog for temperature and sea-level conditions as projected for 2100 AD, yet there has been no evidence of fossil assemblages in the equatorial Atlantic. Here, we report foraminifer, metazoan (mollusks, bony fish, bryozoans, decapods, and sharks among others), and plant communities of coastal tropical marine and mangrove affinities, dating precisely from a ca. 130 to 115 ka time interval near the Equator, at Kourou, in French Guiana. These communities include ca. 230 recent species, some being endangered today and/or first recorded as fossils. The hyperdiverse Kourou mollusk assemblage suggests stronger affinities between Guianese and Caribbean coastal waters by the Last Interglacial than today, questioning the structuring role of the Amazon Plume on tropical Western Atlantic communities at the time. Grassland-dominated pollen, phytoliths, and charcoals from younger deposits in the same sections attest to a marine retreat and dryer conditions during the onset of the last glacial (ca. 110 to 50 ka), with a savanna-dominated landscape and episodes of fire. Charcoals from the last millennia suggest human presence in a mosaic of modern-like continental habitats. Our results provide key information about the ecology and biogeography of pristine Pleistocene tropical coastal ecosystems, especially relevant regarding the—widely anthropogenic—ongoing global warming.
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  • 3
    Publication Date: 2024-06-05
    Description: Significance Particulate organic carbon (POC) formed by photosynthesis in the sunlit surface ocean fuels the ecosystems in the dark ocean below. We show that mesoscale fronts and eddies, which are ubiquitous physical features in subtropical oceans, generate three-dimensional intrusions connecting the surface to deep ocean. Intrusions are enriched in total POC due to enhancement of small, nonsinking photosynthetic plankton and free-living bacteria that resemble surface microbial communities. Flow-driven export of POC, estimated using an approximation of eddy physics, is the same order of magnitude as export by sinking POC, which was previously thought to dominate export. These observations reveal coupling of surface and deep ocean productivity and biodiversity and give insight into mechanisms by which the ocean transports carbon to depth. Abstract Subtropical oceans contribute significantly to global primary production, but the fate of the picophytoplankton that dominate in these low-nutrient regions is poorly understood. Working in the subtropical Mediterranean, we demonstrate that subduction of water at ocean fronts generates 3D intrusions with uncharacteristically high carbon, chlorophyll, and oxygen that extend below the sunlit photic zone into the dark ocean. These contain fresh picophytoplankton assemblages that resemble the photic-zone regions where the water originated. Intrusions propagate depth-dependent seasonal variations in microbial assemblages into the ocean interior. Strikingly, the intrusions included dominant biomass contributions from nonphotosynthetic bacteria and enrichment of enigmatic heterotrophic bacterial lineages. Thus, the intrusions not only deliver material that differs in composition and nutritional character from sinking detrital particles, but also drive shifts in bacterial community composition, organic matter processing, and interactions between surface and deep communities. Modeling efforts paired with global observations demonstrate that subduction can flux similar magnitudes of particulate organic carbon as sinking export, but is not accounted for in current export estimates and carbon cycle models. Intrusions formed by subduction are a particularly important mechanism for enhancing connectivity between surface and upper mesopelagic ecosystems in stratified subtropical ocean environments that are expanding due to the warming climate.
    Type: Article , PeerReviewed
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  • 4
    Publication Date: 2023-10-26
    Description: Constraining secular variation of the Earth's magnetic field strength in the past is fundamental to understanding short-term processes of the geodynamo. Such records also constitute a powerful and independent dating tool for archaeological sites and geological formations. In this study, we present 11 robust archaeointensity results from Pre-Pottery to Pottery Neolithic Jordan that are based on both clay and flint (chert) artifacts. Two of these results constitute the oldest archaeointensity data for the entire Levant, ancient Egypt, Turkey, and Mesopotamia, extending the archaeomagnetic reference curve for the Holocene. Virtual Axial Dipole Moments (VADMs) show that the Earth's magnetic field in the Southern Levant was weak (about two-thirds the present field) at around 7600 BCE, recovering its strength to greater than the present field around 7000 BCE, and gradually weakening again around 5200 BCE. In addition, successful results obtained from burnt flint demonstrate the potential of this very common, and yet rarely used, material in archaeomagnetic research, in particular for prehistoric periods from the first use of fire to the invention of pottery.
    Description: Published
    Description: e2100995118
    Description: 1A. Geomagnetismo e Paleomagnetismo
    Description: JCR Journal
    Keywords: Jordan ; Neolithic ; Pre-Pottery Neolithic ; archaeointensity
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 5
    Publication Date: 2023-03-08
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Saunders, J. K., McIlvin, M. R., Dupont, C. L., Kaul, D., Moran, D. M., Horner, T., Laperriere, S. M., Webb, E. A., Bosak, T., Santoro, A. E., & Saito, M. A. Microbial functional diversity across biogeochemical provinces in the central Pacific Ocean. Proceedings of the National Academy of Sciences of the United States of America, 119(37),(2022): e2200014119, https://doi.org/10.1073/pnas.2200014119.
    Description: Enzymes catalyze key reactions within Earth’s life-sustaining biogeochemical cycles. Here, we use metaproteomics to examine the enzymatic capabilities of the microbial community (0.2 to 3 µm) along a 5,000-km-long, 1-km-deep transect in the central Pacific Ocean. Eighty-five percent of total protein abundance was of bacterial origin, with Archaea contributing 1.6%. Over 2,000 functional KEGG Ontology (KO) groups were identified, yet only 25 KO groups contributed over half of the protein abundance, simultaneously indicating abundant key functions and a long tail of diverse functions. Vertical attenuation of individual proteins displayed stratification of nutrient transport, carbon utilization, and environmental stress. The microbial community also varied along horizontal scales, shaped by environmental features specific to the oligotrophic North Pacific Subtropical Gyre, the oxygen-depleted Eastern Tropical North Pacific, and nutrient-rich equatorial upwelling. Some of the most abundant proteins were associated with nitrification and C1 metabolisms, with observed interactions between these pathways. The oxidoreductases nitrite oxidoreductase (NxrAB), nitrite reductase (NirK), ammonia monooxygenase (AmoABC), manganese oxidase (MnxG), formate dehydrogenase (FdoGH and FDH), and carbon monoxide dehydrogenase (CoxLM) displayed distributions indicative of biogeochemical status such as oxidative or nutritional stress, with the potential to be more sensitive than chemical sensors. Enzymes that mediate transformations of atmospheric gases like CO, CO2, NO, methanethiol, and methylamines were most abundant in the upwelling region. We identified hot spots of biochemical transformation in the central Pacific Ocean, highlighted previously understudied metabolic pathways in the environment, and provided rich empirical data for biogeochemical models critical for forecasting ecosystem response to climate change.
    Description: Funding for this research was provided by the Gordon and Betty Moore Foundation (grants 3782 and 8453), the US NSF (NSF grants OCE-1924554, 2123055, 2125063, 2048774, and 2026933), the Center for Chemical Currencies on a Microbial Planet (NSF grant OCE-2019589), and the US NIH General Medicine (grant GM135709-01A1). J.K.S. was supported by a NASA Postdoctoral Program Fellowship with the NASA Astrobiology Program, administered by Universities Space Research Association under contract with NASA. A.E.S. was supported by the Sloan Foundation, the Simons Foundation, and NSF grant OCE-1437310. A portion of this research used resources at the US Department of Energy JGI sponsored by the Office of Biological and Environmental Research and operated under contract DE-AC02-05CH11231 (JGI). C.L.D. and D.K. were supported by NSF grants OCE-1558453 and OCE-2049299. T.H. was supported by NSF grant OCE-2023456.
    Keywords: Marine microbial ecology ; Metaproteomics ; Mesopelagic ; Nitrification ; Methylotrophy
    Repository Name: Woods Hole Open Access Server
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  • 6
    Publication Date: 2024-01-08
    Description: The South Shetland Trough, Antarctica, is an underexplored region for microbiological and biotechnological exploitation. Herein, we describe the isolation and characterization of the novel bacterium Lacinutrix shetlandiensis sp. nov. WUR7 from a deep-sea environment. We explored its chemical diversity via a metabologenomics approach, wherein the OSMAC strategy was strategically employed to upregulate cryptic genes for secondary metabolite production. Based on hybrid de novo whole genome sequencing and digital DNA–DNA hybridization, isolate WUR7 was identified as a novel species from the Gram-negative genus Lacinutrix. Its genome was mined for the presence of biosynthetic gene clusters with limited results. However, extensive investigation of its metabolism uncovered an unusual tryptophan decarboxylase with high sequence homology and conserved structure of the active site as compared to ZP_02040762, a highly specific tryptophan decarboxylase from Ruminococcus gnavus. Therefore, WUR7's metabolism was directed toward indole-based alkaloid biosynthesis by feeding it with L-tryptophan. As expected, its metabolome profile changed dramatically, by triggering the extracellular accumulation of a massive array of metabolites unexpressed in the absence of tryptophan. Untargeted LC-MS/MS coupled with molecular networking, followed along with chemoinformatic dereplication, allowed for the annotation of 10 indole alkaloids, belonging to β-carboline, bisindole, and monoindole classes, alongside several unknown alkaloids. These findings guided us to the isolation of a new natural bisindole alkaloid 8,9-dihydrocoscinamide B (1), as the first alkaloid from the genus Lacinutrix, whose structure was elucidated on the basis of extensive 1D and 2D NMR and HR-ESIMS experiments. This comprehensive strategy allowed us to unlock the previously unexploited metabolome of L. shetlandiensis sp. nov. WUR7.
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  • 7
    Publication Date: 2024-02-07
    Description: Significance Assessing change in Southern Ocean ecosystems is challenging due to its remoteness. Large-scale datasets that allow comparison between present-day conditions and those prior to large-scale ecosystem disturbances caused by humans (e.g., fishing/whaling) are rare. We infer the contemporary offshore foraging distribution of a marine predator, southern right whales (n = 1,002), using a customized stable isotope-based assignment approach based on biogeochemical models of the Southern Ocean. We then compare the contemporary distributions during the late austral summer and autumn to whaling catch data representing historical distributions during the same seasons. We show remarkable consistency of mid-latitude distribution across four centuries but shifts in foraging grounds in the past 30 y, particularly in the high latitudes that are likely driven by climate-associated alterations in prey availability. Abstract Assessing environmental changes in Southern Ocean ecosystems is difficult due to its remoteness and data sparsity. Monitoring marine predators that respond rapidly to environmental variation may enable us to track anthropogenic effects on ecosystems. Yet, many long-term datasets of marine predators are incomplete because they are spatially constrained and/or track ecosystems already modified by industrial fishing and whaling in the latter half of the 20th century. Here, we assess the contemporary offshore distribution of a wide-ranging marine predator, the southern right whale (SRW, Eubalaena australis), that forages on copepods and krill from ~30°S to the Antarctic ice edge (〉60°S). We analyzed carbon and nitrogen isotope values of 1,002 skin samples from six genetically distinct SRW populations using a customized assignment approach that accounts for temporal and spatial variation in the Southern Ocean phytoplankton isoscape. Over the past three decades, SRWs increased their use of mid-latitude foraging grounds in the south Atlantic and southwest (SW) Indian oceans in the late austral summer and autumn and slightly increased their use of high-latitude (〉60°S) foraging grounds in the SW Pacific, coincident with observed changes in prey distribution and abundance on a circumpolar scale. Comparing foraging assignments with whaling records since the 18th century showed remarkable stability in use of mid-latitude foraging areas. We attribute this consistency across four centuries to the physical stability of ocean fronts and resulting productivity in mid-latitude ecosystems of the Southern Ocean compared with polar regions that may be more influenced by recent climate change.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 8
    Publication Date: 2024-02-07
    Description: Prochlorococcus is a key member of open-ocean primary producer communities. Despite its importance, little is known about the predators that consume this cyanobacterium and make its biomass available to higher trophic levels. We identify potential predators along a gradient wherein Prochlorococcus abundance increased from near detection limits (coastal California) to 〉200,000 cells mL-1 (subtropical North Pacific Gyre). A replicated RNA-Stable Isotope Probing experiment involving the in situ community, and labeled Prochlorococcus as prey, revealed choanoflagellates as the most active predators of Prochlorococcus, alongside a radiolarian, chrysophytes, dictyochophytes, and specific MAST lineages. These predators were not appropriately highlighted in multiyear conventional 18S rRNA gene amplicon surveys where dinoflagellates and other taxa had highest relative amplicon abundances across the gradient. In identifying direct consumers of Prochlorococcus, we reveal food-web linkages of individual protistan taxa and resolve routes of carbon transfer from the base of marine food webs.
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  • 9
    Publication Date: 2024-02-23
    Description: Significance Oceans represent 70% of our planet’s surface, housing a large spectrum of microorganisms that interact with the above atmosphere. Ocean microorganisms were proposed in the late 80’s to be at the center of a climate feedback loop involving dimethyl sulfide (DMS) that would form aerosols and modify cloud properties (CLAW hypothesis). In the present paper, we report observational evidence from semicontrolled experiments in the South Pacific that nitrate ions, yet hitherto not considered, is a key species involved in aerosol nucleation in the pristine marine atmosphere and which precursors are coemitted with DMS. Our results further indicate that nitrate ion formation would be related to short-term microbial processes, sensitive to environmental stressors, therefore potentially “closing the loop”. Abstract Our understanding of ocean–cloud interactions and their effect on climate lacks insight into a key pathway: do biogenic marine emissions form new particles in the open ocean atmosphere? Using measurements collected in ship-borne air–sea interface tanks deployed in the Southwestern Pacific Ocean, we identified new particle formation (NPF) during nighttime that was related to plankton community composition. We show that nitrate ions are the only species for which abundance could support NPF rates in our semicontrolled experiments. Nitrate ions also prevailed in the natural pristine marine atmosphere and were elevated under higher sub-10 nm particle concentrations. We hypothesize that these nucleation events were fueled by complex, short-term biogeochemical cycling involving the microbial loop. These findings suggest a new perspective with a previously unidentified role of nitrate of marine biogeochemical origin in aerosol nucleation.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 10
    Publication Date: 2023-03-08
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Schorn, S., Ahmerkamp, S., Bullock, E., Weber, M., Lott, C., Liebeke, M., Lavik, G., Kuypers, M. M. M., Graf, J. S., & Milucka, J. Diverse methylotrophic methanogenic archaea cause high methane emissions from seagrass meadows. Proceedings of the National Academy of Sciences of the United States of America, 119(9), (2022): e2106628119, https://doi.org/10.1073/pnas.2106628119.
    Description: Marine coastlines colonized by seagrasses are a net source of methane to the atmosphere. However, methane emissions from these environments are still poorly constrained, and the underlying processes and responsible microorganisms remain largely unknown. Here, we investigated methane turnover in seagrass meadows of Posidonia oceanica in the Mediterranean Sea. The underlying sediments exhibited median net fluxes of methane into the water column of ca. 106 µmol CH4 ⋅ m−2 ⋅ d−1. Our data show that this methane production was sustained by methylated compounds produced by the plant, rather than by fermentation of buried organic carbon. Interestingly, methane production was maintained long after the living plant died off, likely due to the persistence of methylated compounds, such as choline, betaines, and dimethylsulfoniopropionate, in detached plant leaves and rhizomes. We recovered multiple mcrA gene sequences, encoding for methyl-coenzyme M reductase (Mcr), the key methanogenic enzyme, from the seagrass sediments. Most retrieved mcrA gene sequences were affiliated with a clade of divergent Mcr and belonged to the uncultured Candidatus Helarchaeota of the Asgard superphylum, suggesting a possible involvement of these divergent Mcr in methane metabolism. Taken together, our findings identify the mechanisms controlling methane emissions from these important blue carbon ecosystems.
    Description: This project was funded by theMax Planck Society.
    Repository Name: Woods Hole Open Access Server
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  • 11
    Publication Date: 2024-02-07
    Description: Significance A substantial component of the global nitrogen cycle is the production of biologically inaccessible dinitrogen attributed to anaerobic denitrification by prokaryotes. Recent evidence identified a eukaryote, foraminifera, as new key players in this “loss” of bioavailable nitrogen. The evolution of denitrification in eukaryotes is a rare event, and the genetic mechanisms of the denitrification pathway in foraminifera are just starting to be elucidated. We present large-scale sequencing analyses of 10 denitrifying foraminiferal species, which reveals the high conservation of the foraminiferal denitrification pathway. We further find evidence for a complementation of denitrification by the foraminiferal microbiome. Together, these findings provide insights into the early evolution of a previously overlooked component in the marine nitrogen cycle. Abstract: Benthic foraminifera are unicellular eukaryotes that inhabit sediments of aquatic environments. Several foraminifera of the order Rotaliida are known to store and use nitrate for denitrification, a unique energy metabolism among eukaryotes. The rotaliid Globobulimina spp. has been shown to encode an incomplete denitrification pathway of bacterial origin. However, the prevalence of denitrification genes in foraminifera remains unknown, and the missing denitrification pathway components are elusive. Analyzing transcriptomes and metagenomes of 10 foraminiferal species from the Peruvian oxygen minimum zone, we show that denitrification genes are highly conserved in foraminifera. We infer the last common ancestor of denitrifying foraminifera, which enables us to predict the ability to denitrify for additional foraminiferal species. Additionally, an examination of the foraminiferal microbiota reveals evidence for a stable interaction with Desulfobacteraceae, which harbor genes that complement the foraminiferal denitrification pathway. Our results provide evidence that foraminiferal denitrification is complemented by the foraminifera-associated microbiome. The interaction of foraminifera with their resident bacteria is at the basis of foraminiferal adaptation to anaerobic environments that manifested in ecological success in oxygen depleted habitats.
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  • 12
    Publication Date: 2024-02-07
    Description: Significance: Adaptive radiation, the evolutionary process whereby a lineage diversifies over a short period of time, often occurs in geographically isolated or newly formed habitats where colonizing species encounter unoccupied niches and reduced selective pressures. Rapid radiations may also occur in diverse and complex environments, but these cases are less well documented. Here, we show that the hamlets, a group of Caribbean reef fishes, radiated within the last 10,000 generations in a burst of diversification that ranks among the fastest in fishes. Genomic analysis suggests that color pattern diversity is generated by different combinations of alleles at a few genes with large effect. Such a modular genomic architecture of diversification is emerging as a common denominator to a variety of radiations. Abstract: Rapid diversification is often observed when founding species invade isolated or newly formed habitats that provide ecological opportunity for adaptive radiation. However, most of the Earth’s diversity arose in diverse environments where ecological opportunities appear to be more constrained. Here, we present a striking example of a rapid radiation in a highly diverse marine habitat. The hamlets, a group of reef fishes from the wider Caribbean, have radiated into a stunning diversity of color patterns but show low divergence across other ecological axes. Although the hamlet lineage is ∼26 My old, the radiation appears to have occurred within the last 10,000 generations in a burst of diversification that ranks among the fastest in fishes. As such, the hamlets provide a compelling backdrop to uncover the genomic elements associated with phenotypic diversification and an excellent opportunity to build a broader comparative framework for understanding the drivers of adaptive radiation. The analysis of 170 genomes suggests that color pattern diversity is generated by different combinations of alleles at a few large-effect loci. Such a modular genomic architecture of diversification has been documented before in Heliconius butterflies, capuchino finches, and munia finches, three other tropical radiations that took place in highly diverse and complex environments. The hamlet radiation also occurred in a context of high effective population size, which is typical of marine populations. This allows for the accumulation of new variants through mutation and the retention of ancestral genetic variation, both of which appear to be important in this radiation.
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  • 13
    Publication Date: 2024-02-07
    Description: Significance Resilience to global change will require adaptation to multiple concurrent environmental changes. However, it is unclear if adaptations to multiple stressors can be predicted from the sum of single-stressor adaptation. To answer this question, we experimentally evolved a marine copepod to warming, acidification, and their combination, finding that copepods were able to adapt to all conditions over 25 generations. Warming was a much stronger selective pressure than acidification alone and under multiple-stressor conditions. Nevertheless, the multiple-stressor response to selection was synergistic and unique from either single stressor. Thus, adaptation to single stressors may not reveal adaptive potential or mechanisms of adaptation under multiple stressors, demonstrating the complexity of predicting adaptive responses under multifaceted environmental change. Abstract Metazoan adaptation to global change relies on selection of standing genetic variation. Determining the extent to which this variation exists in natural populations, particularly for responses to simultaneous stressors, is essential to make accurate predictions for persistence in future conditions. Here, we identified the genetic variation enabling the copepod Acartia tonsa to adapt to experimental ocean warming, acidification, and combined ocean warming and acidification (OWA) over 25 generations of continual selection. Replicate populations showed a consistent polygenic response to each condition, targeting an array of adaptive mechanisms including cellular homeostasis, development, and stress response. We used a genome-wide covariance approach to partition the allelic changes into three categories: selection, drift and replicate-specific selection, and laboratory adaptation responses. The majority of allele frequency change in warming (57%) and OWA (63%) was driven by shared selection pressures across replicates, but this effect was weaker under acidification alone (20%). OWA and warming shared 37% of their response to selection but OWA and acidification shared just 1%, indicating that warming is the dominant driver of selection in OWA. Despite the dominance of warming, the interaction with acidification was still critical as the OWA selection response was highly synergistic with 47% of the allelic selection response unique from either individual treatment. These results disentangle how genomic targets of selection differ between single and multiple stressors and demonstrate the complexity that nonadditive multiple stressors will contribute to predictions of adaptation to complex environmental shifts caused by global change.
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  • 14
    Publication Date: 2024-05-22
    Description: Orbital cyclicity is a fundamental pacemaker of Earth’s climate system. The Newark–Hartford Basin (NHB) lake sediment record of eastern North America contains compelling geologic expressions of this cyclicity, reflecting variations of climatic conditions in tropical Pangea during the Late Triassic and earliest Jurassic (~233 to 199 Ma). Climate modeling enables a deeper mechanistic understanding of Earth system modulation during this unique greenhouse and supercontinent period. We link major features of the NHB record to the combined climatic effects of orbital forcing, paleogeographic changes, and atmospheric p CO 2 variations. An ensemble of transient, orbitally driven climate simulations is assessed for nine time slices, three atmospheric p CO 2 values, and two paleogeographic reconstructions. Climatic transitions from tropical humid to more seasonal and ultimately semiarid are associated with tectonic drift of the NHB from ~ 5 ° N to 20 ° N . The modeled orbital modulation of the precipitation–evaporation balance is most pronounced during the 220 to 200 Ma interval, whereas it is limited by weak seasonality and increasing aridity before and after this interval. Lower p CO 2 at around 205 Ma contributes to drier climates and could have led to the observed damping of sediment cyclicity. Eccentricity-modulated precession dominates the orbitally driven climate response in the NHB region. High obliquity further amplifies summer precipitation through the seasonal shifts in the tropical rainfall belt. Regions with other proxy records are also assessed, providing guidance toward an integrated picture of global astronomical climate forcing in the Late Triassic and ultimately of other periods in Earth history.
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  • 15
    Publication Date: 2024-02-07
    Description: During the last glacial interval, marine sediments recorded reduced current ventilation within the ocean interior below water depths of approximately 〉1,500 m [B. A. Hoogakker et al., Nat. Geosci. 8, 40–43 (2015)]. The degree of the associated oxygen depletion in the different ocean basins, however, is still poorly constrained. Here, we present sedimentary records of redox-sensitive metals from the southwest African margin. These records show evidence of continuous bottom water anoxia in the eastern South Atlantic during the last glaciation that led to enhanced carbon burial over a prolonged period of time. Our geochemical data indicate that upwelling-related productivity and the associated oxygen minimum zone in the eastern South Atlantic shifted far seaward during the last glacial period and only slowly retreated during deglaciation times. While increased productivity during the last ice age may have contributed to oxygen depletion in bottom waters, especially on the upper slope, slow-down of the Late Quaternary deep water circulation pattern [Rutberg et al., Nature 405, 935–938 (2000)] appears to be the ultimate driver of anoxic conditions in deep waters.
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  • 16
    Publication Date: 2024-02-07
    Description: Animal gastrointestinal tracts harbor a microbiome that is integral to host function, yet species from diverse phyla have evolved a reduced digestive system or lost it completely. Whether such changes are associated with alterations in the diversity and/or abundance of the microbiome remains an untested hypothesis in evolutionary symbiosis. Here, using the life history transition from planktotrophy (feeding) to lecithotrophy (nonfeeding) in the sea urchin Heliocidaris, we demonstrate that the lack of a functional gut corresponds with a reduction in microbial community diversity and abundance as well as the association with a diet-specific microbiome. We also determine that the lecithotroph vertically transmits a Rickettsiales that may complement host nutrition through amino acid biosynthesis and influence host reproduction. Our results indicate that the evolutionary loss of a functional gut correlates with a reduction in the microbiome and the association with an endosymbiont. Symbiotic transitions can therefore accompany life history transitions in the evolution of developmental strategies.
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  • 17
    Publication Date: 2024-02-07
    Description: Significance: A central goal in invasion genomics is to identify and determine the mechanisms that underlie the successful colonization, establishment, and subsequent range expansion of invasive populations of nonindigenous species. Using a whole-genome approach, we evaluate the importance of genetic diversity for the successful establishment of nonindigenous species. Our study shows that genetic diversity per se is not the major factor driving invasions, since we observed all possible scenarios with invasive populations showing reduced, similar but also increased, genetic diversity relative to the native population. Using coalescent methods, we reconstruct the demographic history of the invasion and infer the source population of each invasion event, which shows that propagule pressure and multiple introductions play an important role in determining invasion success. Abstract: Invasion rates have increased in the past 100 y irrespective of international conventions. What characterizes a successful invasion event? And how does genetic diversity translate into invasion success? Employing a whole-genome perspective using one of the most successful marine invasive species world-wide as a model, we resolve temporal invasion dynamics during independent invasion events in Eurasia. We reveal complex regionally independent invasion histories including cases of recurrent translocations, time-limited translocations, and stepping-stone range expansions with severe bottlenecks within the same species. Irrespective of these different invasion dynamics, which lead to contrasting patterns of genetic diversity, all nonindigenous populations are similarly successful. This illustrates that genetic diversity, per se, is not necessarily the driving force behind invasion success. Other factors such as propagule pressure and repeated introductions are an important contribution to facilitate successful invasions. This calls into question the dominant paradigm of the genetic paradox of invasions, i.e., the successful establishment of nonindigenous populations with low levels of genetic diversity.
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  • 18
    Publication Date: 2024-02-07
    Description: With over 18,000 species, the Acanthomorpha, or spiny-rayed fishes, form the largest and arguably most diverse radiation of vertebrates. One of the key novelties that contributed to their evolutionary success are the spiny rays in their fins that serve as a defense mechanism. We investigated the patterning mechanisms underlying the differentiation of median fin Anlagen into discrete spiny and soft rayed domains during the ontogeny of the direct-developing cichlid fish Astatotilapia burtoni. Distinct transcription factor signatures characterize these two fin domains, whereby mutually exclusive expression of hoxa13a/b with alx4a/b and tbx2b marks the spine to soft-ray boundary. The soft-ray domain is established by BMP inhibition via gremlin1b, which synergizes in the posterior fin with shh secreted from a zone of polarizing activity. Modulation of BMP signaling by chemical inhibition or gremlin1b CRISPR/Cas9 knockout induces homeotic transformations of spines into soft rays and vice versa. The expression of spine and soft-ray genes in nonacanthomorph fins indicates that a combination of exaptation and posterior expansion of an ancestral developmental program for the anterior fin margin allowed the evolution of robustly individuated spiny and soft-rayed domains. We propose that a repeated exaptation of such pattern might underly the convergent evolution of anterior spiny fin elements across fishes.
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  • 19
    Publication Date: 2021-10-26
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  • 20
    Publication Date: 2021-10-29
    Description: Mentalizing, the ability to infer the mental states of others, is a cornerstone of adaptive social intelligence. While functional brain mapping of human mentalizing has progressed considerably, its evolutionary signature in nonhuman primates remains debated. The discovery that the middle part of the macaque superior temporal sulcus (mid-STS) region has a connectional fingerprint most similar to the human temporoparietal junction (TPJ)—a crucial node in the mentalizing network—raises the possibility that these cortical areas may also share basic functional properties associated with mentalizing. Here, we show that this is the case in aspects of a preference for live social interactions and in a theoretical framework of predictive coding. Macaque monkeys were trained to perform a turn-taking choice task with another real monkey partner sitting directly face-to-face or a filmed partner appearing in prerecorded videos. We found that about three-fourths of task-related mid-STS neurons exhibited agent-dependent activity, most responding selectively or preferentially to the partner’s action. At the population level, activities of these partner-type neurons were significantly greater under live-partner compared to video-recorded–partner task conditions. Furthermore, a subset of the partner-type neurons responded proactively when predictions about the partner’s action were violated. This prediction error coding was specific to the action domain; almost none of the neurons signaled error in the prediction of reward. The present findings highlight unique roles of the macaque mid-STS at the single-neuron level and further delineate its functional parallels with the human TPJ in social cognitive processes associated with mentalizing.
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  • 21
    Publication Date: 2021-10-29
    Description: The human ERG (hERG) K+ channel has a crucial function in cardiac repolarization, and mutations or channel block can give rise to long QT syndrome and catastrophic ventricular arrhythmias. The cytosolic assembly formed by the Per-Arnt-Sim (PAS) and cyclic nucleotide binding homology (CNBh) domains is the defining structural feature of hERG and related KCNH channels. However, the molecular role of these two domains in channel gating remains unclear. We have previously shown that single-chain variable fragment (scFv) antibodies can modulate hERG function by binding to the PAS domain. Here, we mapped the scFv2.12 epitope to a site overlapping with the PAS/CNBh domain interface using NMR spectroscopy and mutagenesis and show that scFv binding in vitro and in the cell is incompatible with the PAS interaction with CNBh. By generating a fluorescently labeled scFv2.12, we demonstrate that association with the full-length hERG channel is state dependent. We detect Förster resonance energy transfer (FRET) with scFv2.12 when the channel gate is open but not when it is closed. In addition, state dependence of scFv2.12 FRET signal disappears when the R56Q mutation, known to destabilize the PAS–CNBh interaction, is introduced in the channel. Altogether, these data are consistent with an extensive structural alteration of the PAS/CNBh assembly when the cytosolic gate opens, likely favoring PAS domain dissociation from the CNBh domain.
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  • 22
    Publication Date: 2021-10-29
    Description: Fevers are considered an adaptive response by the host to infection. For gregarious animals, however, fever and the associated sickness behaviors may signal a temporary loss of capacity, offering other group members competitive opportunities. We implanted wild vervet monkeys (Chlorocebus pygerythrus) with miniature data loggers to obtain continuous measurements of core body temperature. We detected 128 fevers in 43 monkeys, totaling 776 fever-days over a 6-year period. Fevers were characterized by a persistent elevation in mean and minimum 24-h body temperature of at least 0.5 °C. Corresponding behavioral data indicated that febrile monkeys spent more time resting and less time feeding, consistent with the known sickness behaviors of lethargy and anorexia, respectively. We found no evidence that fevers influenced the time individuals spent socializing with conspecifics, suggesting social transmission of infection within a group is likely. Notably, febrile monkeys were targeted with twice as much aggression from their conspecifics and were six times more likely to become injured compared to afebrile monkeys. Our results suggest that sickness behavior, together with its agonistic consequences, can carry meaningful costs for highly gregarious mammals. The degree to which social factors modulate the welfare of infected animals is an important aspect to consider when attempting to understand the ecological implications of disease.
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  • 23
    Publication Date: 2021-10-29
    Description: The rapid development of nanotechnology has greatly benefited modern science and engineering and also led to an increased environmental exposure to nanoparticles (NPs). While recent research has established a correlation between the exposure of NPs and cardiovascular diseases, the intrinsic mechanisms of such a connection remain unclear. Inhaled NPs can penetrate the air–blood barrier from the lung to systemic circulation, thereby intruding the cardiovascular system and generating cardiotoxic effects. In this study, on-site cardiovascular damage was observed in mice upon respiratory exposure of silica nanoparticles (SiNPs), and the corresponding mechanism was investigated by focusing on the interaction of SiNPs and their encountered biomacromolecules en route. SiNPs were found to collect a significant amount of apolipoprotein A-I (Apo A-I) from the blood, in particular when the SiNPs were preadsorbed with pulmonary surfactants. While the adsorbed Apo A-I ameliorated the cytotoxic and proinflammatory effects of SiNPs, the protein was eliminated from the blood upon clearance of the NPs. However, supplementation of Apo A-I mimic peptide mitigated the atherosclerotic lesion induced by SiNPs. In addition, we found a further declined plasma Apo A-I level in clinical silicosis patients than coronary heart disease patients, suggesting clearance of SiNPs sequestered Apo A-I to compromise the coronal protein’s regular biological functions. Together, this study has provided evidence that the protein corona of SiNPs acquired in the blood depletes Apo A-I, a biomarker for prediction of cardiovascular diseases, which gives rise to unexpected toxic effects of the nanoparticles.
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  • 24
    Publication Date: 2021-10-29
    Description: Gram-negative bacteria are surrounded by a protective outer membrane (OM) with phospholipids in its inner leaflet and lipopolysaccharides (LPS) in its outer leaflet. The OM is also populated with many β-barrel outer-membrane proteins (OMPs), some of which have been shown to cluster into supramolecular assemblies. However, it remains unknown how abundant OMPs are organized across the entire bacterial surface and how this relates to the lipids in the membrane. Here, we reveal how the OM is organized from molecular to cellular length scales, using atomic force microscopy to visualize the OM of live bacteria, including engineered Escherichia coli strains and complemented by specific labeling of abundant OMPs. We find that a predominant OMP in the E. coli OM, the porin OmpF, forms a near-static network across the surface, which is interspersed with barren patches of LPS that grow and merge with other patches during cell elongation. Embedded within the porin network is OmpA, which forms noncovalent interactions to the underlying cell wall. When the OM is destabilized by mislocalization of phospholipids to the outer leaflet, a new phase appears, correlating with bacterial sensitivity to harsh environments. We conclude that the OM is a mosaic of phase-separated LPS-rich and OMP-rich regions, the maintenance of which is essential to the integrity of the membrane and hence to the lifestyle of a gram-negative bacterium.
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  • 25
    Publication Date: 2021-10-29
    Description: Electrolyte-gated transistors (EGTs) hold great promise for next-generation printed logic circuitry, biocompatible integrated sensors, and neuromorphic devices. However, EGT-based complementary circuits with high voltage gain and ultralow driving voltage (110) under a supply voltage of only 0.7 V. Furthermore, NAND and NOR logic circuits on both rigid and flexible substrates are realized, enabling not only excellent logic response with driving voltages as low as 0.2 V but also impressive mechanical flexibility down to 1-mm bending radii. Finally, the HCIN was applied in electrooculographic (EOG) signal monitoring for recording eye movement, which is critical for the development of wearable medical sensors and also interfaces for human–computer interaction; the high voltage amplification of the present HCIN enables EOG signal amplification and monitoring in which a small ∼1.5 mV signal is amplified to ∼30 mV.
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  • 26
    Publication Date: 2021-10-29
    Description: The anterior end of the mammalian face is characteristically composed of a semimotile nose, not the upper jaw as in other tetrapods. Thus, the therian nose is covered ventrolaterally by the “premaxilla,” and the osteocranium possesses only a single nasal aperture because of the absence of medial bony elements. This stands in contrast to those in other tetrapods in whom the premaxilla covers the rostral terminus of the snout, providing a key to understanding the evolution of the mammalian face. Here, we show that the premaxilla in therian mammals (placentals and marsupials) is not entirely homologous to those in other amniotes; the therian premaxilla is a composite of the septomaxilla and the palatine remnant of the premaxilla of nontherian amniotes (including monotremes). By comparing topographical relationships of craniofacial primordia and nerve supplies in various tetrapod embryos, we found that the therian premaxilla is predominantly of the maxillary prominence origin and associated with mandibular arch. The rostral-most part of the upper jaw in nonmammalian tetrapods corresponds to the motile nose in therian mammals. During development, experimental inhibition of primordial growth demonstrated that the entire mammalian upper jaw mostly originates from the maxillary prominence, unlike other amniotes. Consistently, cell lineage tracing in transgenic mice revealed a mammalian-specific rostral growth of the maxillary prominence. We conclude that the mammalian-specific face, the muzzle, is an evolutionary novelty obtained by overriding ancestral developmental constraints to establish a novel topographical framework in craniofacial mesenchyme.
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  • 27
    Publication Date: 2021-10-14
    Description: Deserts exert strong selection pressures on plants, but the underlying genomic drivers of ecological adaptation and subsequent speciation remain largely unknown. Here, we generated de novo genome assemblies and conducted population genomic analyses of the psammophytic genus Pugionium (Brassicaceae). Our results indicated that this bispecific genus had undergone an allopolyploid event, and the two parental genomes were derived from two ancestral lineages with different chromosome numbers and structures. The postpolyploid expansion of gene families related to abiotic stress responses and lignin biosynthesis facilitated environmental adaptations of the genus to desert habitats. Population genomic analyses of both species further revealed their recent divergence with continuous gene flow, and the most divergent regions were found to be centered on three highly structurally reshuffled chromosomes. Genes under selection in these regions, which were mainly located in one of the two subgenomes, contributed greatly to the interspecific divergence in microhabitat adaptation.
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  • 28
    Publication Date: 2021-10-29
    Description: Cells cooperate as groups to achieve structure and function at the tissue level, during which specific material characteristics emerge. Analogous to phase transitions in classical physics, transformations in the material characteristics of multicellular assemblies are essential for a variety of vital processes including morphogenesis, wound healing, and cancer. In this work, we develop configurational fingerprints of particulate and multicellular assemblies and extract volumetric and shear order parameters based on this fingerprint to quantify the system disorder. Theoretically, these two parameters form a complete and unique pair of signatures for the structural disorder of a multicellular system. The evolution of these two order parameters offers a robust and experimentally accessible way to map the phase transitions in expanding cell monolayers and during embryogenesis and invasion of epithelial spheroids.
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  • 29
    Publication Date: 2021-10-29
    Description: Bacterial cell wall peptidoglycan is essential, maintaining both cellular integrity and morphology, in the face of internal turgor pressure. Peptidoglycan synthesis is important, as it is targeted by cell wall antibiotics, including methicillin and vancomycin. Here, we have used the major human pathogen Staphylococcus aureus to elucidate both the cell wall dynamic processes essential for growth (life) and the bactericidal effects of cell wall antibiotics (death) based on the principle of coordinated peptidoglycan synthesis and hydrolysis. The death of S. aureus due to depletion of the essential, two-component and positive regulatory system for peptidoglycan hydrolase activity (WalKR) is prevented by addition of otherwise bactericidal cell wall antibiotics, resulting in stasis. In contrast, cell wall antibiotics kill via the activity of peptidoglycan hydrolases in the absence of concomitant synthesis. Both methicillin and vancomycin treatment lead to the appearance of perforating holes throughout the cell wall due to peptidoglycan hydrolases. Methicillin alone also results in plasmolysis and misshapen septa with the involvement of the major peptidoglycan hydrolase Atl, a process that is inhibited by vancomycin. The bactericidal effect of vancomycin involves the peptidoglycan hydrolase SagB. In the presence of cell wall antibiotics, the inhibition of peptidoglycan hydrolase activity using the inhibitor complestatin results in reduced killing, while, conversely, the deregulation of hydrolase activity via loss of wall teichoic acids increases the death rate. For S. aureus, the independent regulation of cell wall synthesis and hydrolysis can lead to cell growth, death, or stasis, with implications for the development of new control regimes for this important pathogen.
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  • 30
    Publication Date: 2021-10-29
    Description: Many intrinsically disordered proteins (IDPs) may undergo liquid–liquid phase separation (LLPS) and participate in the formation of membraneless organelles in the cell, thereby contributing to the regulation and compartmentalization of intracellular biochemical reactions. The phase behavior of IDPs is sequence dependent, and its investigation through molecular simulations requires protein models that combine computational efficiency with an accurate description of intramolecular and intermolecular interactions. We developed a general coarse-grained model of IDPs, with residue-level detail, based on an extensive set of experimental data on single-chain properties. Ensemble-averaged experimental observables are predicted from molecular simulations, and a data-driven parameter-learning procedure is used to identify the residue-specific model parameters that minimize the discrepancy between predictions and experiments. The model accurately reproduces the experimentally observed conformational propensities of a set of IDPs. Through two-body as well as large-scale molecular simulations, we show that the optimization of the intramolecular interactions results in improved predictions of protein self-association and LLPS.
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  • 31
    Publication Date: 2021-10-06
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  • 32
    Publication Date: 2021-10-18
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  • 33
    Publication Date: 2021-10-14
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  • 34
    Publication Date: 2021-10-29
    Description: In this article, we advance divide-and-conquer strategies for solving the community detection problem in networks. We propose two algorithms that perform clustering on several small subgraphs and finally patch the results into a single clustering. The main advantage of these algorithms is that they significantly bring down the computational cost of traditional algorithms, including spectral clustering, semidefinite programs, modularity-based methods, likelihood-based methods, etc., without losing accuracy, and even improving accuracy at times. These algorithms are also, by nature, parallelizable. Since most traditional algorithms are accurate, and the corresponding optimization problems are much simpler in small problems, our divide-and-conquer methods provide an omnibus recipe for scaling traditional algorithms up to large networks. We prove the consistency of these algorithms under various subgraph selection procedures and perform extensive simulations and real-data analysis to understand the advantages of the divide-and-conquer approach in various settings.
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  • 35
    Publication Date: 2021-10-29
    Description: Plants and animals use cell surface receptors to sense and interpret environmental signals. In legume symbiosis with nitrogen-fixing bacteria, the specific recognition of bacterial lipochitooligosaccharide (LCO) signals by single-pass transmembrane receptor kinases determines compatibility. Here, we determine the structural basis for LCO perception from the crystal structures of two lysin motif receptor ectodomains and identify a hydrophobic patch in the binding site essential for LCO recognition and symbiotic function. We show that the receptor monitors the composition of the amphiphilic LCO molecules and uses kinetic proofreading to control receptor activation and signaling specificity. We demonstrate engineering of the LCO binding site to fine-tune ligand selectivity and correct binding kinetics required for activation of symbiotic signaling in plants. Finally, the hydrophobic patch is found to be a conserved structural signature in this class of LCO receptors across legumes that can be used for in silico predictions. Our results provide insights into the mechanism of cell-surface receptor activation by kinetic proofreading of ligands and highlight the potential in receptor engineering to capture benefits in plant–microbe interactions.
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  • 36
    Publication Date: 2021-10-29
    Description: Mammalian sperm migration within the complex and dynamic environment of the female reproductive tract toward the fertilization site requires navigational mechanisms, through which sperm respond to the tract environment and maintain the appropriate swimming behavior. In the oviduct (fallopian tube), sperm undergo a process called “hyperactivation,” which involves switching from a nearly symmetrical, low-amplitude, and flagellar beating pattern to an asymmetrical, high-amplitude beating pattern that is required for fertilization in vivo. Here, exploring bovine sperm motion in high–aspect ratio microfluidic reservoirs as well as theoretical and computational modeling, we demonstrate that sperm hyperactivation, in response to pharmacological agonists, modulates sperm–sidewall interactions and thus navigation via physical boundaries. Prior to hyperactivation, sperm remained swimming along the sidewalls of the reservoirs; however, once hyperactivation caused the intrinsic curvature of sperm to exceed a critical value, swimming along the sidewalls was reduced. We further studied the effect of noise in the intrinsic curvature near the critical value and found that these nonthermal fluctuations yielded an interesting “Run–Stop” motion on the sidewall. Finally, we observed that hyperactivation produced a “pseudo-chemotaxis” behavior, in that sperm stayed longer within microfluidic chambers containing higher concentrations of hyperactivation agonists.
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  • 37
    Publication Date: 2021-10-29
    Description: The p53 tumor suppressor protein, known to be critically important in several processes including cell-cycle arrest and apoptosis, is highly regulated by multiple mechanisms, most certifiably the Murine Double Minute 2–Murine Double Minute X (MDM2–MDMX) heterodimer. The role of MDM2–MDMX in cell-cycle regulation through inhibition of p53 has been well established. Here we report that in cells either lacking p53 or expressing certain tumor-derived mutant forms of p53, loss of endogenous MDM2 or MDMX, or inhibition of E3 ligase activity of the heterocomplex, causes cell-cycle arrest. This arrest is correlated with a reduction in E2F1, E2F3, and p73 levels. Remarkably, direct ablation of endogenous p73 produces a similar effect on the cell cycle and the expression of certain E2F family members at both protein and messenger RNA levels. These data suggest that MDM2 and MDMX, working at least in part as a heterocomplex, may play a p53-independent role in maintaining cell-cycle progression by promoting the activity of E2F family members as well as p73, making them a potential target of interest in cancers lacking wild-type p53.
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  • 38
    Publication Date: 2021-10-20
    Description: To form synaptic connections and store information, neurons continuously remodel their proteomes. The impressive length of dendrites and axons imposes logistical challenges to maintain synaptic proteins at locations remote from the transcription source (the nucleus). The discovery of thousands of messenger RNAs (mRNAs) near synapses suggested that neurons overcome distance and gain autonomy by producing proteins locally. It is not generally known, however, if, how, and when localized mRNAs are translated into protein. To investigate the translational landscape in neuronal subregions, we performed simultaneous RNA sequencing (RNA-seq) and ribosome sequencing (Ribo-seq) from microdissected rodent brain slices to identify and quantify the transcriptome and translatome in cell bodies (somata) as well as dendrites and axons (neuropil). Thousands of transcripts were differentially translated between somatic and synaptic regions, with many scaffold and signaling molecules displaying increased translation levels in the neuropil. Most translational changes between compartments could be accounted for by differences in RNA abundance. Pervasive translational regulation was observed in both somata and neuropil influenced by specific mRNA features (e.g., untranslated region [UTR] length, RNA-binding protein [RBP] motifs, and upstream open reading frames [uORFs]). For over 800 mRNAs, the dominant source of translation was the neuropil. We constructed a searchable and interactive database for exploring mRNA transcripts and their translation levels in the somata and neuropil [MPI Brain Research, The mRNA translation landscape in the synaptic neuropil. https://public.brain.mpg.de/dashapps/localseq/. Accessed 5 October 2021]. Overall, our findings emphasize the substantial contribution of local translation to maintaining synaptic protein levels and indicate that on-site translational control is an important mechanism to control synaptic strength.
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  • 39
    Publication Date: 2021-10-29
    Description: Actions with identical goals can be executed in different ways (gentle, rude, vigorous, etc.), which D. N. Stern called vitality forms [D. N. Stern, Forms of Vitality Exploring Dynamic Experience in Psychology, Arts, Psychotherapy, and Development (2010)]. Vitality forms express the agent’s attitudes toward others. In a series of fMRI studies, we found that the dorso-central insula (DCI) is the region that is selectively active during both vitality form observation and execution. In one previous experiment, however, the middle cingulate gyrus also exhibited activation. In the present study, in order to assess the role of the cingulate cortex in vitality form processing, we adopted a classical vitality form paradigm, but making the control condition devoid of vitality forms using jerky movements. Participants performed two different tasks: Observation of actions performed gently or rudely and execution of the same actions. The results showed that in addition to the insula, the middle cingulate cortex (MCC) was strongly activated during both action observation and execution. Using a voxel-based analysis, voxels showing a similar trend of the blood-oxygen-level-dependent (BOLD) signal in both action observation and execution were found in the DCI and in the MCC. Finally, using a multifiber tractography analysis, we showed that the active sites in MCC and DCI are reciprocally connected.
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  • 40
    Publication Date: 2021-10-29
    Description: Yersinia pestis causes human plague and colonizes both a mammalian host and a flea vector during its transmission cycle. A key barrier to bacterial infection is the host’s ability to actively sequester key biometals (e.g., iron, zinc, and manganese) required for bacterial growth. This is referred to as nutritional immunity. Mechanisms to overcome nutritional immunity are essential virulence factors for bacterial pathogens. Y. pestis produces an iron-scavenging siderophore called yersiniabactin (Ybt) that is required to overcome iron-mediated nutritional immunity and cause lethal infection. Recently, Ybt has been shown to bind to zinc, and in the absence of the zinc transporter ZnuABC, Ybt improves Y. pestis growth in zinc-limited medium. These data suggest that, in addition to iron acquisition, Ybt may also contribute to overcoming zinc-mediated nutritional immunity. To test this hypothesis, we used a mouse model defective in iron-mediated nutritional immunity to demonstrate that Ybt contributes to virulence in an iron-independent manner. Furthermore, using a combination of bacterial mutants and mice defective in zinc-mediated nutritional immunity, we identified calprotectin as the primary barrier for Y. pestis to acquire zinc during infection and that Y. pestis uses Ybt to compete with calprotectin for zinc. Finally, we discovered that Y. pestis encounters zinc limitation within the flea midgut, and Ybt contributes to overcoming this limitation. Together, these results demonstrate that Ybt is a bona fide zinc acquisition mechanism used by Y. pestis to surmount zinc limitation during the infection of both the mammalian and insect hosts.
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  • 41
    Publication Date: 2021-10-29
    Description: SARS-CoV-2 spillback from humans into domestic and wild animals has been well documented, and an accumulating number of studies illustrate that human-to-animal transmission is widespread in cats, mink, deer, and other species. Experimental inoculations of cats, mink, and ferrets have perpetuated transmission cycles. We sequenced full genomes of Vero cell–expanded SARS-CoV-2 inoculum and viruses recovered from cats (n = 6), dogs (n = 3), hamsters (n = 3), and a ferret (n = 1) following experimental exposure. Five nonsynonymous changes relative to the USA-WA1/2020 prototype strain were near fixation in the stock used for inoculation but had reverted to wild-type sequences at these sites in dogs, cats, and hamsters within 1- to 3-d postexposure. A total of 14 emergent variants (six in nonstructural genes, six in spike, and one each in orf8 and nucleocapsid) were detected in viruses recovered from animals. This included substitutions in spike residues H69, N501, and D614, which also vary in human lineages of concern. Even though a live virus was not cultured from dogs, substitutions in replicase genes were detected in amplified sequences. The rapid selection of SARS-CoV-2 variants in vitro and in vivo reveals residues with functional significance during host switching. These observations also illustrate the potential for spillback from animal hosts to accelerate the evolution of new viral lineages, findings of particular concern for dogs and cats living in households with COVID-19 patients. More generally, this glimpse into viral host switching reveals the unrealized rapidity and plasticity of viral evolution in experimental animal model systems.
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  • 42
    Publication Date: 2021-10-29
    Description: Accurate characterization of the time courses of blood-oxygen-level–dependent (BOLD) signal changes is crucial for the analysis and interpretation of functional MRI data. While several studies have shown that white matter (WM) exhibits distinct BOLD responses evoked by tasks, there have been no comprehensive investigations into the time courses of spontaneous signal fluctuations in WM. We measured the power spectra of the resting-state time courses in a set of regions within WM identified as showing synchronous signals using independent components analysis. In each component, a clear separation between voxels into two categories was evident, based on their power spectra: one group exhibited a single peak, and the other had an additional peak at a higher frequency. Their groupings are location specific, and their distributions reflect unique neurovascular and anatomical configurations. Importantly, the two categories of voxels differed in their engagement in functional integration, revealed by differences in the number of interregional connections based on the two categories separately. Taken together, these findings suggest WM signals are heterogeneous in nature and depend on local structural-vascular-functional associations.
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  • 43
    Publication Date: 2021-10-20
    Description: The East Asian summer monsoon and the precipitation it brings are relevant for millions of people. Because of the monsoon’s importance, there has been a substantial amount of work attempting to describe the driving mechanisms behind its past variability. However, discrepancies exist, with speleothem-based East Asian monsoon reconstructions differing from those based on loess records from the Chinese Loess Plateau during the late Quaternary. The periodicity of wet and dry phases experienced by desert areas that lie on the periphery of the East Asian monsoon’s influence offer another independent view of monsoonal variability. Here, we provide environmental records based on magnetic parameters for the last 3 million years from the Tengger Desert, China, one such marginal arid region. Our results reveal wet–dry cycles at a dominant frequency of 405 kiloyears, with drier intervals corresponding to eccentricity minima. These findings are consistent with previous reconstructions of East Asian summer and North African summer monsoon precipitation variability. Our records emphasize the dominant role of eccentricity in forcing East Asian monsoonal precipitation as well as monsoonal-derived environmental fluctuations experienced in peripheral desert areas. These results challenge the traditional view that high-latitude ice sheets are the primary driver of East Asian monsoon precipitation during the Quaternary based on Chinese loess records.
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  • 44
    Publication Date: 2023-10-26
    Description: The Tierra Blanca Joven (TBJ) eruption from Ilopango volcano deposited thick ash over much of El Salvador when it was inhabited by the Maya, and rendered all areas within at least 80 km of the volcano uninhabitable for years to decades after the eruption. Nonetheless, the more widespread environmental and climatic impacts of this large eruption are not well known because the eruption magnitude and date are not well constrained. In this multifaceted study we have resolved the date of the eruption to 431 ± 2 CE by identifying the ash layer in a well-dated, high-resolution Greenland ice-core record that is 〉7,000 km from Ilopango; and calculated that between 37 and 82 km3 of magma was dispersed from an eruption coignimbrite column that rose to ∼45 km by modeling the deposit thickness using state-of-the-art tephra dispersal methods. Sulfate records from an array of ice cores suggest stratospheric injection of 14 ± 2 Tg S associated with the TBJ eruption, exceeding those of the historic eruption of Pinatubo in 1991. Based on these estimates it is likely that the TBJ eruption produced a cooling of around 0.5 °C for a few years after the eruption. The modeled dispersal and higher sulfate concentrations recorded in Antarctic ice cores imply that the cooling would have been more pronounced in the Southern Hemisphere. The new date confirms the eruption occurred within the Early Classic phase when Maya expanded across Central America.
    Description: Published
    Description: 26061-26068
    Description: 1V. Storia eruttiva
    Description: JCR Journal
    Keywords: Maya; eruption dispersal; large volcanic eruptions; radiocarbon; sulfate
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 45
    Publication Date: 2023-02-08
    Description: Habitat heterogeneity and species diversity are often linked. On the deep seafloor, sediment variability and hard-substrate availability influence geographic patterns of species richness and turnover. The assumption of a generally homogeneous, sedimented abyssal seafloor is at odds with the fact that the faunal diversity in some abyssal regions exceeds that of shallow-water environments. Here we show, using a ground-truthed analysis of multibeam sonar data, that the deep seafloor may be much rockier than previously assumed. A combination of bathymetry data, ruggedness, and backscatter from a trans-Atlantic corridor along the Vema Fracture Zone, covering crustal ages from 0 to 100 Ma, show rock exposures occurring at all crustal ages. Extrapolating to the whole Atlantic, over 260,000 km2 of rock habitats potentially occur along Atlantic fracture zones alone, significantly increasing our knowledge about abyssal habitat heterogeneity. This implies that sampling campaigns need to be considerably more sophisticated than at present to capture the full deep-sea habitat heterogeneity and biodiversity.
    Type: Article , PeerReviewed
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  • 46
    Publication Date: 2023-02-08
    Description: The impact of the ongoing anthropogenic warming on the Arctic Ocean sea ice is ascertained and closely monitored. However, its long-term fate remains an open question as its natural variability on centennial to millennial timescales is not well documented. Here, we use marine sedimentary records to reconstruct Arctic sea-ice fluctuations. Cores collected along the Lomonosov Ridge that extends across the Arctic Ocean from northern Greenland to the Laptev Sea were radiocarbon dated and analyzed for their micropaleontological and palynological contents, both bearing information on the past sea-ice cover. Results demonstrate that multiyear pack ice remained a robust feature of the western and central Lomonosov Ridge and that perennial sea ice remained present throughout the present interglacial, even during the climate optimum of the middle Holocene that globally peaked ∼6,500 y ago. In contradistinction, the southeastern Lomonosov Ridge area experienced seasonally sea-ice-free conditions, at least, sporadically, until about 4,000 y ago. They were marked by relatively high phytoplanktonic productivity and organic carbon fluxes at the seafloor resulting in low biogenic carbonate preservation. These results point to contrasted west–east surface ocean conditions in the Arctic Ocean, not unlike those of the Arctic dipole linked to the recent loss of Arctic sea ice. Hence, our data suggest that seasonally ice-free conditions in the southeastern Arctic Ocean with a dominant Arctic dipolar pattern, may be a recurrent feature under “warm world” climate.
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  • 47
    Publication Date: 2023-02-08
    Description: A fundamental problem for the evolution of pregnancy, the most specialized form of parental investment among vertebrates, is the rejection of the nonself-embryo. Mammals achieve immunological tolerance by down-regulating both major histocompatibility complex pathways (MHC I and II). Although pregnancy has evolved multiple times independently among vertebrates, knowledge of associated immune system adjustments is restricted to mammals. All of them (except monotremata) display full internal pregnancy, making evolutionary reconstructions within the class mammalia meaningless. Here, we study the seahorse and pipefish family (syngnathids) that have evolved male pregnancy across a gradient from external oviparity to internal gestation. We assess how immunological tolerance is achieved by reconstruction of the immune gene repertoire in a comprehensive sample of 12 seahorse and pipefish genomes along the “male pregnancy” gradient together with expression patterns of key immune and pregnancy genes in reproductive tissues. We found that the evolution of pregnancy coincided with a modification of the adaptive immune system. Divergent genomic rearrangements of the MHC II pathway among fully pregnant species were identified in both genera of the syngnathids: The pipefishes (Syngnathus) displayed loss of several genes of the MHC II pathway while seahorses (Hippocampus) featured a highly divergent invariant chain (CD74). Our findings suggest that a trade-off between immunological tolerance and embryo rejection accompanied the evolution of unique male pregnancy. That pipefishes survive in an ocean of microbes without one arm of the adaptive immune defense suggests a high degree of immunological flexibility among vertebrates, which may advance our understanding of immune-deficiency diseases.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 48
    Publication Date: 2023-02-08
    Description: Assessment of the global budget of the greenhouse gas nitrous oxide ([Formula: see text]O) is limited by poor knowledge of the oceanic [Formula: see text]O flux to the atmosphere, of which the magnitude, spatial distribution, and temporal variability remain highly uncertain. Here, we reconstruct climatological [Formula: see text]O emissions from the ocean by training a supervised learning algorithm with over 158,000 [Formula: see text]O measurements from the surface ocean-the largest synthesis to date. The reconstruction captures observed latitudinal gradients and coastal hot spots of [Formula: see text]O flux and reveals a vigorous global seasonal cycle. We estimate an annual mean [Formula: see text]O flux of 4.2 ± 1.0 Tg N[Formula: see text], 64% of which occurs in the tropics, and 20% in coastal upwelling systems that occupy less than 3% of the ocean area. This [Formula: see text]O flux ranges from a low of 3.3 ± 1.3 Tg N[Formula: see text] in the boreal spring to a high of 5.5 ± 2.0 Tg N[Formula: see text] in the boreal summer. Much of the seasonal variations in global [Formula: see text]O emissions can be traced to seasonal upwelling in the tropical ocean and winter mixing in the Southern Ocean. The dominant contribution to seasonality by productive, low-oxygen tropical upwelling systems (〉75%) suggests a sensitivity of the global [Formula: see text]O flux to El Niño-Southern Oscillation and anthropogenic stratification of the low latitude ocean. This ocean flux estimate is consistent with the range adopted by the Intergovernmental Panel on Climate Change, but reduces its uncertainty by more than fivefold, enabling more precise determination of other terms in the atmospheric [Formula: see text]O budget.
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  • 49
    Publication Date: 2023-02-08
    Description: Climate-driven depletion of ocean oxygen strongly impacts the global cycles of carbon and nutrients as well as the survival of many animal species. One of the main uncertainties in predicting changes to marine oxygen levels is the regulation of the biological respiration demand associated with the biological pump. Derived from the Redfield ratio, the molar ratio of oxygen to organic carbon consumed during respiration (i.e., the respiration quotient, r−O2:C) is consistently assumed constant but rarely, if ever, measured. Using a prognostic Earth system model, we show that a 0.1 increase in the respiration quotient from 1.0 leads to a 2.3% decline in global oxygen, a large expansion of low-oxygen zones, additional water column denitrification of 38 Tg N/y, and the loss of fixed nitrogen and carbon production in the ocean. We then present direct chemical measurements of r−O2:C using a Pacific Ocean meridional transect crossing all major surface biome types. The observed r−O2:C has a positive correlation with temperature, and regional mean values differ significantly from Redfield proportions. Finally, an independent global inverse model analysis constrained with nutrients, oxygen, and carbon concentrations supports a positive temperature dependence of r−O2:C in exported organic matter. We provide evidence against the common assumption of a static biological link between the respiration of organic carbon and the consumption of oxygen. Furthermore, the model simulations suggest that a changing respiration quotient will impact multiple biogeochemical cycles and that future warming can lead to more intense deoxygenation than previously anticipated.
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  • 50
    Publication Date: 2023-02-08
    Description: The ocean is our planet’s largest life-support system. It stabilizes climate; stores carbon; produces oxygen; nurtures biodiversity; directly supports human well-being through food, mineral, and energy resources; and provides cultural and recreational services. The value of the ocean economy speaks to its importance: The Organization for Economic Cooperation and Development (OECD) estimates that by 2030, $3 trillion USD will be generated annually from ocean sectors such as transportation, fishing, tourism, and energy (1). Unsustainable resource extraction, pollution, climate change, and habitat destruction are on the rise and affecting many parts of the world’s oceans (2). The ocean is rapidly changing, and yet the ways in which these changes will play out are not yet clear.
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  • 51
    Electronic Resource
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4455-4461 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intermolecular potentials for Ar and Kr interacting with HBr are obtained by least-squares fitting of potential parameters to data obtained from the molecular-beam microwave spectra of the Ar–HBr and Kr–HBr van der Waals complexes. The equilibrium geometry is linear Rg–H–Br in each case, but there are substantial secondary minima at the linear Rg–Br–H geometries; for Ar–HBr, the secondary minimum is only about 5 cm−1 shallower than the primary minimum. This potential feature is found to explain the anomalous H/D isotope effects in centrifugal distortion constants that have been observed for the Rg–HBr complexes. It is predicted that Ar–HBr will have a very low-energy bending state, only 11 cm−1 above the ground state, arising from the secondary minimum.
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  • 52
    Electronic Resource
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4477-4484 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Excited rotational level dependence of the external magnetic field effects both on intensity and on decay of fluorescence of pyrazine vapor has been carefully examined for the zero-point vibrational level in S1 with a field strength of 0–170 G. The magnetic quenching of the slow fluorescence becomes more effective with increasing rotational quantum number J' of the excited level, and the field strength at which the amount of fluorescence quenching becomes one-half of the total amount of quenching at the saturated fields is roughly proportional to (2J'+1)−1. The magnetic quenching is also found to depend on K' of the excited level. The rotational level dependence of the magnetic quenching of the slow fluorescence is related to a difference in the number of the triplet levels coupled to the optically excited singlet rovibronic level, based on the spin decoupling mechanism of the singlet–triplet mixed level.
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  • 53
    Electronic Resource
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4499-4503 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have observed many collision-induced-dipole (CID) absorption bands arising from the transitions between quasimolecular ground and high-lying (n≤10) states in the strontium–rare-gas systems. For each absorption band, we have measured the energy shift of the absorption peak from the energy of the correlating atomic forbidden transition and the effective oscillator strength per unit perturber density fCID/Np. The shift is roughly proportional to the electron scattering length L0 for each rare-gas atom, whereas the fCID/Np is roughly proportional to L20. The shift decreases in general as the principal quantum number n increases, and increases as one goes from the s state to the d state, and to the degenerate manifold state with l≥3. These general features of the shift and fCID/Np are consistent with the predictions by a simple Fermi-potential model, suggesting the important role of the interaction between a Rydberg electron and a rare-gas atom in the CID absorption processes.
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  • 54
    Electronic Resource
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4582-4586 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ν2 band of the silylene SiH2 molecule in X˜ 1 A1 was observed for the first time in the gas phase by using infrared diode laser kinetic spectroscopy. Silylene molecules were generated by the photolysis of phenylsilane at 193 nm. The observed spectrum was analyzed to determine the rotational and centrifugal distortion constants in the ground and v2 =1 states and the band origin ν0 =998.6241(3) cm−1 with one standard deviation in parentheses. The significance of the derived parameters is discussed in detail.
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  • 55
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4597-4602 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using translational spectroscopy we have studied the d 1Πg and C 3 Πg Rydberg states of O2 . The dissociation of the vibrational levels v=0–2 to all energetically accessible dissociation limits has been followed. The dissociation pathways directly reflect the predissociation mechanisms involved. For the d 1 Πg Rydberg state competition between predissociation by a 3 Πg valence state, due to singlet–triplet mixing, and by a 1 Πg valence state has been observed. Using the Fermi golden rule the observed vibrationally dependent competition is reproduced, corroborating the positions of the lower 1 Πg and 3 Πg valence state curves and yielding various coupling strengths.
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  • 56
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4636-4642 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally inelastic scattering of carbon dioxide by translationally hot H, D, and Cl atoms was studied by time-resolved diode laser absorption. The high J rotational distribution falls off quite rapidly between J=60 and J=80. D atom collisions have roughly twice the excitation cross section versus H atom collisions, with the H*/D* ratio decreasing with increasing J. These results are consistent with a constraint on the total reagent orbital angular momentum available for rotational excitation. Transient Doppler profiles measured immediately after hot atom/CO2 collisions indicate that CO2 molecules excited to high J levels have a larger recoil velocity than molecules excited to lower J levels. This result is consistent with predictions based on a simple model which treats the CO2 potential as a hard shell ellipsoid.
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  • 57
    Electronic Resource
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4643-4650 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The molecular dynamics simulation method is extended to study a model of diffusion-controlled reactions. This allows a molecular description of solvent at an equal footing of reactants. Nondiffusional dynamic behavior of reactive molecules is found at short times. It enhances the rate of reactive encounter in comparison to the prediction of Smoluchowski theory. The model studied in this work can be regarded as a theoretical prototype of fluorescence quenching. In this context it is shown that the nondiffusional dynamics is mainly responsible for the discrepancy between Stern–Volmer plots measured in a continuous excitation experiment or obtained by integrating the time resolved fluorescence intensity. The other aspects such as the long-time behavior of survival probability, solvent effect as well as competing effect from finite concentration of one reactive species are also studied in some detail.
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  • 58
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4714-4727 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of neutral iron clusters Fe7–27 with water are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. Reaction products are detected via laser ionization and time-of-flight mass spectrometry. The reactions of room-temperature clusters with H2O show adsorbate decomposition and hydrogen desorption, as do the reactions with D2O at elevated temperatures. The room-temperature reaction with D2O appears not to involve any decomposition, and is at equilibrium under the conditions of these experiments. The dependence of reaction extent on D2O pressure yields equilibrium constants for the addition of the first and second D2O molecules. The analysis is complicated by the presence of two-photon ionization processes that are treated quantitatively with a rate-equation model. This treatment also yields estimates for cluster photoabsorption cross sections, which are found to be approximately linear in cluster size, having a magnitude of 2.3×10−17 cm2 per iron atom. From the derived equilibrium constants and estimated adsorption entropies, approximate D2O–cluster binding energies are determined. They range from 0.42 to 0.59 eV, and their dependence on cluster size shows a remarkable similarity to the dependence of the rate constants for reaction of iron clusters with H2. The implications of this similarity are discussed.
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  • 59
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4738-4744 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Relativistic effect Erel upon the total electronic energy of an atom is discussed with particular reference to obtaining the nonrelativistic total energy Eexact from the experimental total energy. Numerical values of this effect obtained by various authors by different nonempirical methods are compared for neutral atoms of rare-gas elements. It is shown that methods either of a Hartree–Fock-type or of a Dirac–Hartree–Fock-type give much the same Erel value for He through Ar. It is pointed out that Erel calculated with Hartree–Fock wave functions is not adequate for use in obtaining Eexact from the experimental total energy and that the Erel value calculated with wave functions including electron correlation should work well, although an actual demonstration can be done only for two-electron systems for lack of data. A semiempirical formula is therefore proposed, which is useful for least-squares fit of experimental total energies of isoelectronic series of atoms to extract nonrelativistic total energies along with the relativistic effect. From nonrelativistic energies thus derived, semiempirical values of correlation energies of atoms are obtained. The results thus obtained are in reasonable agreement with correlation energies derived by Clementi along somewhat different lines. The power series expansion in Z of the fitted formula for the He series shows that numerical values of expansion coefficients agree reasonably well with the corresponding values obtained by accurate relativistic and nonrelativistic Z expansion-type calculations.
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  • 60
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4785-4792 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and low-lying states of CuF2 and CuCl2 have been studied using the single and doubles configuration interaction (SDCI) and coupled pair functional (CPF) methods in a large Gaussian basis set including up to g-type functions. The results include a determination of the bond distances for all the ligand field states (the three states with one hole in the 3d shell) and charge transfer (CT) states, and force constants for the ligand field states. Overall the SDCI (with or without the Davidson correction for higher excitations) and CPF results are in good agreement. The CPF results, which we believe to be the most accurate, can be summarized as follows. The computed value for the asymmetric stretching frequency in the 2Σ+g ground state of CuF2 is 740 cm−1, compared to the experimental value of 765 cm−1. The d–d transition energies for CuF2 are 2500 and 10 800 cm−1. The two lowest charge transfer states are at 31 200 and 33 900 cm−1, respectively. CuCl2 has been found to have a ground state which is an almost equal mixture of 2Σ+g and 2Πg when an estimate of the spin–orbit interaction is included. Three d–d transitions are found: 1600 cm−1 ( J=1/2), 7500 cm−1 (J=3/2), and 9700 cm−1 (J=5/2). The lowest charge transfer states have been computed to lie at 16 700 and 19 600 cm−1. Two bands have been found in the gas-phase spectrum of CuCl2 at 9000 and 19 000 cm−1, in good agreement with the present results. The computed f value for the CT band is 0.017, to be compared to the experimental value of 0.014.
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  • 61
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4909-4911 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation equilibrium AlBr=Al+Br was studied by effusion beam mass spectrometry over the range 1970 to 2260 K and the dissociation energy D00(AlBr) was derived as 4.41±0.06 eV. This value is in general agreement with other fragmentary thermochemical results, but it is lower than a value derived from a short extrapolation of vibrational levels in the excited 1π state, doubtless because of a potential maximum of about 0.22 eV in that state. A Birge–Sponer extrapolation of the ground state vibrational levels, when corrected for degree of ionicity, yields a D00 value in close accord with the experimental result, but an electrostatic model calculation falls short by 0.45 eV.
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  • 62
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4920-4925 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new scheme of experimental criterion is proposed for estimating the carrier mobility and main-chain conformation of an electroactive polymer chain by the technique of transient electric birefringence (TEB). The rise response Δnr (t) and the decay response Δnd (t) in the TEB satisfy in the Kerr regime (i) Δnr (t)=Δnst -Δnd (t) for a polymer molecule of arbitrary conformation with an induced dipole moment alone arising from carriers highly mobile along the main chain, (ii) Δnr (t)=Δnst -(3/2)Δnd (t/3)+(1/2)Δnd (t) for a rodlike polymer molecule with a permanent dipole moment alone originating from carriers with low mobility, and (iii) Δnr(t)=Δnst-2Δnd (t/2)+Δnd (t) for a random-coiled polymer with a permanent dipole moment alone due to low-mobility carriers, where Δnst =limt→∞ Δnr(t) . Then, comparison of the TEB rise and decay responses gives us information on the carrier mobility and main-chain conformation. This criterion is valid also for polydispersed polymer samples. By applying the criterion to the exemplifying data of the TEB responses for soluble urethane-substituted polydiacetylenes, it is found that the polydiacetylene molecules take a random-coil conformation with a permanent dipole moment in a good solvent, while in a poor solvent the carrier mobility depends crucially on the solvent condition.
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  • 63
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4942-4948 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rate of catalytic carbon monoxide oxidation on a Pt(100) single crystal surface under isothermal, low-pressure conditions exhibits for certain ranges of parameters (O2 and CO partial pressures, temperature) sustained temporal oscillations whose mechanism had been explored in previous work. Coupling between reaction and diffusion leads to spatial pattern formation as manifested by patches with different work function on the intrinsically homogeneous surface. Imaging is performed by means of the novel technique of scanning photoemission microscopy. Typically, nuclei with dimensions of a few microns, as determined by the instrumental resolution, are formed spontaneously and expand with sharp fronts and velocities of about 0.5 mm/min (at 480 K) up to sizes ≥1 mm. Waves with even more extended fronts propagating with somewhat higher velocities across the sample surface are responsible for the occurrence of large amplitude temporal oscillations of the integral reaction rate.
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  • 64
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4338-4345 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Carbon K edge near edge x-ray absorption fine structure (NEXAFS) spectra of adsorbed acetonitrile and reactive intermediates derived therefrom on Ag(110) were studied in order to determine their orientation and bonding to the surface. For both the multilayer and the adsorbed monolayer there is excellent agreement between the spectra of the adsorbed acetonitrile and the gas phase species. For the multilayer coverage the orientation of the acetonitrile molecules is not completely random; the molecules prefer an orientation with their bond axes closer to parallel than perpendicular to the surface. At monolayer coverage acetonitrile bound to the surface is unrehybridized. It is bound with its linear molecular skeleton parallel to the surface and randomly oriented azimuthally. Annealing molecular CH3 CN to 325 K in the presence of coadsorbed atomic oxygen produces a linear CH2 CN surface intermediate, the molecular axis of which is parallel to the surface and perpendicular to the close-packed direction. The C–C and C–N bond lengths are essentially unperturbed from their values in gas phase CH3 CN. Bonding appears to occur between the methylene carbon and the metal, as expected. A broadening of the π* resonance for transition into the π* orbital perpendicular to the surface is interpreted in terms of π-donor bonding.
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  • 65
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4265-4272 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Attenuated total reflectance (ATR) from principal faces of orthorhombic (and higher symmetry) crystals in spectral regions including strongly polar optic modes is considered. A detailed, quantitative comparison between experimental and calculated ATR infrared (IR) spectra of orthorhombic sodium nitroprusside dihydrate (SNP), Na2[Fe(CN)5NO]⋅2H2O, single crystals in the 500–3800 cm−1 region is presented. Calculated ATR spectra are generated employing the dielectric tensor of SNP obtained from Kramers–Kronig analysis of external reflectance data. The dependence of intensities, profiles, and locations of bands in the spectra upon the relative refractive index of the sample against the ATR prism and the polarization state of the reflected IR beam will be discussed. Both theoretical and experimental evidence of the coupling of the TM-polarized evanescent wave with strong in-plane-polarized optic modes at about their LO frequencies will be given with reference to the case of the strong NO stretching mode of SNP. Finally, the computed and experimental ATR results in SNP single crystals will be critically compared with powder ATR data and reported polarized IR transmittance spectra of SNP single crystals plates.
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  • 66
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    The Journal of Chemical Physics 91 (1989), S. 4299-4306 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: X-ray diffraction data for liquid n-butane near the triple and boiling points are presented. The data are analyzed using scattering factors for −CH3 and −CH2 groups which permits extraction of structure and distribution functions for carbon sites. An intramolecular carbon–carbon distance at approximately 3.1 A(ring) is assigned to a short end-to-end distance in a near-gauche conformation, which is in agreement with the electron diffraction results for gaseous n-butane. The intermolecular pair distribution functions show a large difference in the liquid structure for the low and high temperature states, similar to that found in liquid propane.
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    The Journal of Chemical Physics 91 (1989), S. 4330-4337 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report studies of the kinetics of thermal desorption of In from Ga-stabilized GaAs(100) in ultrahigh vacuum. The relative coverage of In was monitored by x-ray photoelectron spectroscopy (XPS), while the substrate temperature was accurately measured using infrared laser interferometric thermometry. The In was deposited on GaAs by dosing, at room temperature, to saturated monolayer coverage with trimethylindium, yielding In ∼2×1014 cm−2. Subsequent heating to 400 °C desorbs all hydrocarbon species without affecting the In coverage. Further heating leads to first-order desorption of In, as confirmed by exponential decay of In(3d 5/2) XPS signal in isothermal desorption experiments at 473 and 503 °C. From temperature programmed desorption studies through the range 450 to 530 °C, differentiation of the In coverage vs time yielded desorption rates from which Arrhenius parameters were extracted. The unit-weighted average values of the preexponential factor and the desorption energy, obtained from three experiments with heating rates from 0.6 to 3.2 °C/min, are: log10A(s−1)=12.2±0.5 and Ed=53.5±1.2 kcal/mol, where the uncertainties are 1σ sample standard deviations. The Arrhenius parameters of desorption of In from Ga-stabilized GaAs(100) are found to be similar to those of vaporization of pure In. This is consistent with the known tendency of In to form islands on GaAs surfaces, but could also reflect the similarity of the local environments of an In atom adsorbed on an In island and an In atom adsorbed on a Ga-terminated surface. The observed first-order kinetics and the constancy of the In(3d 5/2) XPS signal up to the temperature where desorption occurs indicates that if islands are formed they are small (〈10 A(ring) diameter) at the low coverage used here.
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  • 68
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    The Journal of Chemical Physics 91 (1989), S. 4360-4368 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Cooperative diffusion D of polyacrylamide gels in water was investigated by quasielastic light scattering both under the isotropically swollen state and under the uniaxially stretched and swollen state. The concentration (Ce) dependence of D for the isotropic gels having equilibrium degrees of swelling was measured by systematically changing crosslinking density of gels. The results yielded D=(3.4±0.5)×10−6 C0.76±0.03e cm2/s, in accord with a scaling prediction. For each of the gels prepared and having a given Ce, the uniaxial stretching was applied, and the values D were investigated as a function of extension ratio α and direction β with respect to the stretching direction. The results gave D (α,Ce)=(3.4±0.5)×10−6 αx Cye cm2/s, where x=2/3 for β=0° and −1/4 for β=90°, and y=0.76, independent of α and β. The results clearly indicate that upon increasing α, D increases and decreases, respectively, in the direction parallel and perpendicular to the stretching direction, implying suppression and enhancement of the concentration fluctuations as a result of the network being stiffened and softened in the respective directions.
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  • 69
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    The Journal of Chemical Physics 91 (1989), S. 4387-4400 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The discrete chain representation of multiple scattering theory of the concentration dependence of the hydrodynamics of polymer solutions is applied to the calculation of the leading concentration correction to the dynamic structure factor S(k,t) and its first and second cumulants of individual labeled Gaussian chains in theta solutions at nonzero concentrations. Contributions are separated into those from overall translational and internal chain motions as well as couplings between different internal modes and between translation and internal modes, coupling that are introduced by interchain hydrodynamic interactions. The separate contributions are analyzed as a function of k and of t in order to isolate regions where certain contributions are dominant. As expected, short times and larger k tend to favor contributions from internal chain dynamics, while longer times and smaller k make concentration dependent translational effects predominate. Computations for shorter chains are extrapolated to provide the asymptotic long chain behavior.
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  • 70
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    The Journal of Chemical Physics 91 (1989), S. 4418-4419 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The interconversion tunneling frequencies for (HCl)2 and (DCl)2 are obtained from near-infrared absorption spectra of the H(D)Cl stretching transitions, to spectroscopic precision for the mixed 35Cl–37Cl dimers. A phenomenological model of the interconversion process explains several experimental observations, and provides good estimates of the splittings expected for the 35Cl–35Cl and 37Cl–37Cl species.
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  • 71
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    The Journal of Chemical Physics 91 (1989), S. 4425-4426 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation energy of boron hydride has been calculated ab initio using a very accurate theoretical model, with an expected accuracy of ±0.2 kcal mol−1. The values of 85.7 and 82.4 kcal mol−1 for De and D0, respectively, confirm the conclusion of Curtiss and Pople [J. Chem. Phys. 90, 2522 (1989)], that the predissociation limit of 82.6±0.4 kcal mol−1 might be close to the actual dissociation energy, and that an earlier proposed De of 78.9 kcal mol−1 is in error.
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  • 72
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    The Journal of Chemical Physics 91 (1989), S. 3525-3531 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultrafast photodynamics of four-coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocycle. However, the transient absorption spectrum of this ligand-field electronic excited state continues to evolve and reaches the steady-state form only after about 20 ps. This spectral behavior and the attendant complex kinetics can be modeled phenomenologically in terms of a broad distribution of the (d,d) excited states evolving to a narrower distribution. The dynamics are associated with vibrational relaxation. Intramolecular and intermolecular contributions to this process are considered.
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  • 73
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    The Journal of Chemical Physics 91 (1989), S. 3571-3576 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for the calculation of bound state eigenvalues and eigenfunctions of the Schrödinger equation is presented. The Fourier grid Hamiltonian method is derived from the discrete Fourier transform algorithm. Its implementation and use is extremely simple, requiring the evaluation of the potential only at certain grid points and yielding directly the amplitude of the eigenfunctions at the same grid points.
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  • 74
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    Notes: The evaluation of free energy differences using the perturbation method or thermodynamic integration method requires special caution if multiple rotational isomeric states may exist in the system under investigation. In this article a recently suggested procedure to properly treat rotational isomeric states is illustrated with a molecular dynamics simulation of an aqueous solution of uncomplexed 18-crown-6 crown ether, as an example of a system in which large numbers of isomeric states may exist. By using very long molecular dynamics simulations, thermodynamic perturbation methods and symmetry arguments, the free energy of host organization into the conformation required to form the complex with K+ is estimated to be 2.6 kJ mol−1. It has also been found that the lowest energy conformations of 18-crown-6 in vacuo do not necessarily correspond to the most populated conformations in aqueous solution.
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  • 75
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    The Journal of Chemical Physics 91 (1989), S. 3689-3699 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Pulsed neutron diffraction measurements have been made on liquid 1,2-dichloroethane-d4 (DCE). The wide momentum-transfer range (∼0.3–50 A(ring)−1) available has been used to further refine previously measured molecular structure parameters as well as to test the validity of the inelasticity corrections applied. A measurement using chlorine isotopes on a steady (reactor) source served to partially separate the chlorine–chlorine and the chlorine–carbon plus chlorine–deuterium correlations. The isotopic difference curves were then analyzed and the relevant features of the distribution of internal dihedral angles [P(τ)] obtained by adequate inversion of the experimental difference–functions. The intermolecular pair correlation function was then derived and both sets of functions (from pulsed and steady sources) are compared and tentatively assigned.
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  • 76
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    The Journal of Chemical Physics 91 (1989), S. 3721-3723 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics computer simulations are used to investigate the shape of the mean-squared end-to-end distance distribution function in the three regimes: (1) excluded volume; (2) θ; (3) collapsed. It is found that Mazur's function fits regime (1), that the Gaussian function fits regime (2), and that neither of these appear to describe the collapsed state. The implications of these results to theories of ring formation during polymerization is pointed out.
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  • 77
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    The Journal of Chemical Physics 91 (1989), S. 5154-5159 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a purely ab initio minimum energy path for the trans-tunneling motion in the HF dimer, the energy levels for the K-type rotation and trans-tunneling motion for (HF)2 and (DF)2 are calculated with a one-dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation-tunneling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate B¯ values. The energy levels we obtain are in close agreement with experiment; for example, the K=0 tunneling splitting in (HF)2 is calculated as 0.65 cm−1 compared to the experimental value of 0.658 69 cm−1. As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual K dependence of the rotational energies resulting from the quasilinear behavior, and that the trans-tunneling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the ΔK=0 and ±1 subbands in the tunneling spectra of (HF)2 and (DF)2, many of which have not yet been observed.
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  • 78
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    The Journal of Chemical Physics 91 (1989), S. 5139-5153 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air-cooled and water-cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen-cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler-limited, high sensitivity spectroscopy.The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pattern due to spin statistical weights. Agreement of vibrational band patterns and the rotational constants with ab initio values gives supporting evidence. The anomaly of the spectrum is at least partly ascribed to the small energy difference between the classical and nonclassical structures and possible rearrangement between them, the idea used by organic chemists over the years in wet chemistry. Systematic splittings with the intensity ratio of 2:1 have been noticed in some parts of the spectrum indicating that the protons tunnel between the apex and the two end equilibrium positions of the bridged structure. Using a simplified internal rotation model proposed by Hougen, the barrier height of the tunneling has been estimated. Chemical kinetics in plasmas related to C2H+3 is also discussed.We conclude that (1) the nonclassical structure is lower in energy than the classical structure, and (2) the apex proton and the two end protons exchange their positions with a measurable time scale.
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  • 79
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    The Journal of Chemical Physics 91 (1989), S. 5164-5169 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The inelastic neutron scattering (INS) spectra of some ammonium halides and ammonium nitrate are analyzed in terms of the internal vibrations of the NH+4 ion convoluted with multiphonon lattice modes up to the eighth term. Due to the low mass of this ion, most of the intensity of the internal-mode region of the INS spectra is in the "phonon wings,'' differences between the salts in this spectral region being mainly due to differences in their lattice-mode spectra. Refinement of the lattice Debye–Waller factor is included in the profile refinement of interatomic force constants.
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  • 80
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    The Journal of Chemical Physics 91 (1989), S. 5160-5163 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A new upper bound for the dissociation energy of acetylene, D00(HCC–H) =529.89(±0.01) kJ/mol, has been determined by Stark anticrossing spectroscopy. The zero-pressure extrapolated (unimolecular) decay rates of levels of S1 (A˜ 1Au) v'3=2 and 3 (quanta of the trans-bending vibration) increase upon application of an electric field of 113 kV/cm. We attribute this increase in molecular decay rate to predissociation rather than any other Stark-induced nonradiative or radiative phenomenon. The lowest level (v'3 =2, J'=2, K'=1) from which we have observed an increase in decay rate (i.e., predissociation) has an internal energy of 44 295.65 cm−1 relative to v=0, J=0 of S0 (X˜ 1∑+g). This corresponds to a value about 24 kJ/mol lower then the consensus upper bound result from four different recent experimental determinations (including one of ours) as well as current ab initio results. The new value agrees, however, with earlier work and with recent modeling studies of acetylene pyrolysis kinetics.
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  • 81
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    The Journal of Chemical Physics 91 (1989), S. 3792-3793 
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  • 82
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    The Journal of Chemical Physics 91 (1989), S. 2776-2778 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron paramagnetic resonance (EPR) studies performed at room tempertaure in Mn2+ doped Cs2NaYCl6 single crystals (elpasolite) showed that the Mn2+ ion occupies a tetragonal symmetry site in the cubic crystalline lattice. In contrast with previous results on elpasolite, where trivalent rare earth impurities occupy Oh sites, and divalent europium impurities occupy trigonal sites, this work reports, the first evidence of an impurity in a tetragonal symmetry site in this host with principal axes along 〈100〉 crystallographic directions. The unusually large total splitting of the EPR spectrum (∼10 000×10−4 cm−1) is discussed in order to establish a tentative model for the site.
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  • 83
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    The Journal of Chemical Physics 91 (1989), S. 3596-3602 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Novel rigorous upper and lower bounds, at primitive level, to general electron-repulsion integrals (ERIs) involving Gaussian basis sets have been derived and interconnections with the earlier works in the literature are brought out. New optimal strategies for a preemptive elimination of insignificant ERIs at atom and contraction levels are discussed and tested, resulting in a significant reduction in CPU time. Similar analysis is carried out for the computation of the molecular electrostatic potential for the first time in the literature, leading to a marked savings in computer time.
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  • 84
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    The Journal of Chemical Physics 91 (1989), S. 5185-5200 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Multiphoton ionization spectra have been obtained and analyzed for excitation in the 215–360 nm region from the X 3Σ−g, a 1Δg, and b 1Σ+g states of O2. The 0–0 band of the C 1Πg state is reported for the first time. Measurements of other vibrational bands terminating in the C 3Πg and d 1Πg states are in good agreement with determinations by other groups. Several vibrational levels (v'=0–5) of the 3dπg Rydberg complex have been assigned on the basis of (1) an analysis of the spin–orbit couplings between the (Λ,S) basis-set states, (2) spectral simulation, and (3) the behavior of the states when the excitation radiation is changed from linear to circular polarization.
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  • 85
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    The Journal of Chemical Physics 91 (1989), S. 5201-5207 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Diode laser transient absorption/gain spectroscopy is used to monitor time-dependent populations of high rotational levels in OD (v=0) produced in the reaction of O(1D)+D2. Pure rotational transitions on species with large dipole moments offer good sensitivity, full state resolution and μs time resolution in the present apparatus. Measured nascent populations of OD in the four highest rotational levels thermodynamically accessible in this reaction are in reasonable agreement with the reported results of earlier laser-induced-fluorescence measurements, in which corrections for transition moments and predissociation introduce increasing uncertainties at high rotational levels. The relaxation kinetics of the highest rotational levels are not hopelessly complex, and evidence is presented for strong, but not complete propensity for conservation of Λ doublet symmetry during rotational relaxation.
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  • 86
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    The Journal of Chemical Physics 91 (1989), S. 3647-3661 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A consistent simulation of ionic or strongly polar solutes in polar solvents presents a major challenge from both fundamental and practical aspects. The frequently used method of periodic boundary conditions (PBC) does not correctly take into account the symmetry of the solute field. Instead of using PBC, it is natural to model this type of system as a sphere (with the solute at the origin), but the boundary conditions to be used in such a model are not obvious. Early calculations performed with our surface constrained soft sphere dipoles (SCSSD) model indicated that the dipoles near the surface of the sphere will show unusual orientational preferences (they will overpolarize) unless a corrective force is included in the model, and thus we implemented polarization constraints in this spherical model of polar solutions. More recent approaches that treated the surface with stochastic dynamics, but did not take into account the surface polarization effects, were also found to exhibit these nonphysical orientational preferences. The present work develops a surface constrained all-atom solvent (SCAAS) model in order to consistently treat the surface polarization effects in all-atom molecular dynamics simulations. The SCAAS model, which was presented in a preliminary way in previous works, introduces surface constraints as boundary conditions in order to make the necessarily finite system behave as if it was part of an infinite system. The performance of the model with regard to various properties of bulk water is examined by comparing its results to those obtained by PBC simulations. The results obtained from SCAAS models of different sizes are found to be similar to each other and to the corresponding PBC results. The performance of the model in simulations of solvated ions is emphasized and a comparison of the results obtained with spheres of different sizes demonstrates that the model does not possess significant size dependence. This indicates that the model can be used with a relatively small number of solvent molecules for convergent simulation of structure, energetics, and dynamics of polar solutions. The much simpler fixedcenter Langevin dipoles (FCLD) model is also examined and found to provide a powerful tool for estimating solvation free energies. Finally, a preliminary study of the dielectric properties of the SCAAS model is reported and the potential of this model for exploring the correct implementation of the solvent reaction field is discussed.
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    The Journal of Chemical Physics 91 (1989), S. 3700-3706 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The first-order phase transition of the ferromagnetic Ising model driven by the magnetic field at temperatures below criticality is studied by Monte Carlo methods for a two-dimensional thin film geometry, L×M with two free boundaries of length M(very-much-greater-than)L, at which boundary fields act. This model study is relevant, in particular, for phase transitions in monolayers adsorbed at stepped surfaces. While in the bulk geometry (L→∞) this transition occurs for zero field in the present model, with the system "jumping'' from a state with uniformly positive magnetization to a state with uniformly negative magnetization, in the thin film geometry the transition occurs at a critical field H*∼L−1, and the two states between which the transition occurs are characterized by strongly nonuniform magnetization profiles across the film. These findings are in agreement with the scaling theory of Fisher and Nakanishi.
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  • 88
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    The Journal of Chemical Physics 91 (1989), S. 3724-3728 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A simplified stability condition for the Hartree–Fock (HF) solution giving the self-consistent field crystal orbitals (SCF-CO) of the infinite one-dimensional system is derived. Since the present formulation, particularly for the systems having nearly or entirely degenerated highest occupied and the lowest unoccupied COs, contains only two physical parameters, that is, the density of states and the Coulomb repulsion integrals both at the Fermi level, it is tractable to examine the stability of the HF solutions of such polymers as well as the ordinary molecular systems. An example of its application to metallic trans-type polyacetylene is also shown.
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  • 89
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    The Journal of Chemical Physics 91 (1989), S. 3744-3761 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: An ensemble of distributed donor molecules that undergoes rotational transitions into a discrete excimer state has been analyzed, in an attempt to model distributed electronic relaxation and nonexponential fluorescence of aromatic polymers in presence of rotational sampling processes. In case of irreversible trapping, the donor survival function has been formulated in terms of the one-sided Laplace transform and specified for a modified Gaussian distribution to yield a closed-form expression for the donor decay. The formalism permits a time-dependent rate function to be derived that makes possible the construction of the excimer excitation function by means of the convolution theorem. In case of reversible constraints, a generalized treatment based upon time-dependent transition and transform matrices has been given which applies a perturbation technique for approximately solving the system of nonautonomous differential equations in the time domain. In the limit of weak coupling, the method develops approximate Mth order expressions (M=2,4,6, and 8) to the fluorescence response functions of donor and excimer. The perturbational solutions are well behaved up to relatively long time scales and they prove useful for providing the typical nonexponential time behavior of such a system affected by dispersion and dissociation. The physical restriction of this mathematical analysis (weak reversibility) has been addressed and the implications of distributed event times in future analyses of polymer fluorescence have been discussed.
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  • 90
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    The Journal of Chemical Physics 91 (1989), S. 2771-2775 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: 5,6-Dihydro-2H-thiopyran, CH2CH2CH=CHCH2S, has been synthesized and its far-infrared and Raman spectra recorded. Two series of sharp bands were observed originating from 139 and 235 cm−1 in the infrared spectrum for the out-of-plane ring-bending and the ring-twisting vibrations, respectively. A detailed energy level diagram including numerous excited states was determined for the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits more than two dozen observed transitions, was calculated to be V=2.431×104 x41 −0.383×104x21 +2.258×104x42 −1.966×104 x22 +1.026×105x21 x22 , where x1 is the ring-bending coordinate and x2 is the ring-twisting coordinate. The minimum energy on the potential surface corresponds to a twisting angle of 37.8° (the half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 4130 cm−1 above the twisted conformation on the potential energy surface. The results are compared to analogous molecules and to molecular mechanics calculations.
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  • 91
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    The Journal of Chemical Physics 91 (1989), S. 2808-2813 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Photoabsorption cross sections of thiirane and thietane vapors have been studied in the 110–240 nm region using synchrotron radiation. A number of peaks from thiirane and thietane are arranged into four and five Rydberg series, respectively, converging to the first ionization potential. Many Rydberg peaks from both molecules are observed to possess vibronic structures. The vibrational progressions from thiirane are assigned to the CH2 wagging (ν4) and CS symmetry stretching (ν5) modes. For thietane, the CH2 wagging (ν5) and CS symmetry stretching (ν7) modes of the excited states are found to be active. Molecular geometries and vibrational frequencies for the excited states are discussed on the basis of ab initio calculations.
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  • 92
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    The Journal of Chemical Physics 91 (1989), S. 2840-2847 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Exact close-coupling calculations are used to evaluate the effectiveness of the coupled states approximation in the interpretation of low energy, noble gas methane collisions. The effect of the higher order angle dependent terms of the potential on the inelastic rotational transition was investigated. Calculations using only open channel basis functions were clearly different from converged calculations, but the differences were small enough to be negligible in comparison with experiment. It was shown that open channel coupled states calculations are of sufficient accuracy to evaluate the agreement of the potential model with experiment. The use of the coupled states approximation does not fully account for discrepancies between theory and experiment observed earlier.
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  • 93
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    The Journal of Chemical Physics 91 (1989), S. 2892-2897 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A modified general model for radiationless energy transfer is examined. The model does not rely on selection criteria and is intended to apply over the range of diffusion and resonance energy transfer parameters normally encountered in quenching experiments. Experimental data is compared with theoretical predictions and found to confirm the model's validity for a wide range of physical conditions.
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    The Journal of Chemical Physics 91 (1989), S. 2912-2921 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of aluminum clusters, Al+n (n=7–17), has been studied over a broad energy range (1.88–6.99 eV). Measurements of the lifetimes of the photoexcited clusters are described. Dissociation energies have been determined by comparing the measured lifetimes with the predictions of a simple RRKM model. The dissociation energies show an overall increase with cluster size, but there are substantial oscillations around n=7–8 and n=13–15. Cluster cohesive energies are derived from these results and from previous measurements of the dissociation energies of the smaller clusters. The cohesive energies of the larger clusters (n〉6) are in good agreement with the predictions of a simple model based on the bulk cohesive energy and the cluster surface energy. However, the cohesive energies are substantially larger than the results of recent ab initio calculations. The photodissociation spectrum of Al+8 has been measured and shows a broad absorption feature with a maximum ∼470 nm.
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  • 95
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2102-2107 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The microwave spectrum of dichlorosilylene SiCl2 has been observed to characterize this molecule of chemical interest. The molecule was generated by the thermal reaction between silicon powder and tetrachlorosilane at about 1000 °C. The rotational constants and the centrifugal distortion constants were determined for the three isotopic species Si35Cl2, Si35Cl37Cl, and Si37Cl2. The nuclear quadrupole coupling constants were determined from triplet hyperfine splittings observed for several transitions. The asymmetry of the Cl nuclear quadrupole coupling tensor was found to be very large and was accounted for by π electron backdonation from Cl to Si.
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  • 96
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2971-2979 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures and stabilities of N2 and its 15 possible first-row isoelectronic analogs (CO, BF, BeNe, NO+, CF+, BNe+, O2+2, NF2+, CNe2+, OF3+, NNe3+, ONe4+, F4+2, FNe5+, and Ne6+2) have been examined using ab initio molecular orbital theory. Equilibrium structures have been obtained at a variety of levels of theory including MP3/6-311G(d) and ST4CCD/6-311+G(2df ) and dissociation energies determined at the MP4/6-311+G(3d2f ) level. Full potential energy curves for dissociation, including dissociation barriers, have been obtained at the CASSCF/6-311G(d) level. Spectroscopic constants have also been determined at this level. For the neutral and monocation analogs of N2, the calculated equilibrium geometries, dissociation energies, and spectroscopic constants are in good agreement with the experimental values. The dication analogs of N2, namely O2+2, NF2+, and CNe2+, are all found to be kinetically stable species lying in deep potential wells. In particular, the hitherto unobserved NF2+ dication is predicted to have a short equilibrium bond length (1.102 A(ring)) and a large barrier (445 kJ mol−1) to dissociation to N++F+. Thus NF2+ should be experimentally accessible in the gas phase. The (experimentally known) O2+2 dication is predicted to contain the shortest bond between any two heavy atoms, our best estimate of the bond length being 1.052 A(ring). The first excited state (A 3Σ+u) of O2+2 is predicted to be unbound, and observed metastable decomposition processes are reinterpreted in terms of the ground-state (X 1Σ+g) potential surface. In agreement with previous theoretical studies, we find that CNe2+ is a kinetically stable species, albeit with a relatively long C–Ne bond length. The OF3+ trication is calculated to have a relatively short bond but lies in a well of depth only 23 kJ mol−1. The potential energy curves of the other highly charged species are found to be purely repulsive.
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  • 97
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3002-3011 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relative efficacy of using low order trucations with large reference space vs high order methods with small reference space is tested by comparing quasidegenerate many-body perturbation theory (QDMBPT) calculations of potential curves for the five lowest electronic states of LiH with the multireference coupled cluster calculations of Ben-Shlomo and Kaldor [J. Chem. Phys. 89, 956 (1988)]. The infinite order coupled cluster calculations use two configurational reference spaces involving the 2σ, 3σ, and 1π orbitals, while the QDMBPT computations are truncated at either second or third orders and employ the full active reference space formed either from the 2σ, 3σ, and 1π or from the 2σ, 3σ, 4σ, and 1π orbitals. This gives us the opportunity of testing the dependence of QDMBPT computations on the size of reference space, the available freedom in choosing valence orbitals and orbital energies, and the order of truncation. Second order, four valence orbital space QDMBPT calculations provide good agreement with the repulsive portion of the coupled cluster potentials, but yield a separated atom limit that is too high and that therefore distorts the remainder of the potential. Third order improves the separated atom limit considerably, providing good agreement with the coupled cluster calculations. The "full chemical'' five orbital reference space, on the other hand, yields very good agreement with coupled cluster potentials when using only the simpler second order QDMBPT calculations, and third order corrections in this case are very small but generally improve agreement with coupled cluster potentials. The five orbital reference space calculations are quite insensitive to a wide range of different choices of valence orbitals and orbital energies, demonstrating a robustness to the QDMBPT formalism used.
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  • 98
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3045-3055 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The hypernetted-chain (HNC) approximation is solved for fluids of dipolar hard ellipsoids and the structural, thermodynamic, and dielectric properties of the isotropic phase are discussed in some detail. Both prolate and oblate particles are considered and the isotropic–nematic transition is investigated using density functional theory (DFT). For fluids of prolate particles dipolar forces are found to have a significant effect upon the isotropic–nematic transition which occurs at lower densities as the dipole moment is increased. For the oblate case the dipolar interactions have only a very small, if any, influence upon the isotropic–nematic transition density. For both prolate and oblate particles the present HNC/DFT calculations do not predict ferroelectric nematic phases.
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  • 99
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5313-5315 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ν3 (O–Si stretch) fundamental bands of the HOSi+ and DOSi+ molecular ions in the 9 μm region have been detected for the first time, using a tunable infrared diode laser spectrometer and a hollow cathode discharge cell. Analysis of the results yielded accurate values for the molecular rotational and centrifugal distortion parameters, as well as for the band origins, which are 1127.009 cm−1 for HOSi+ and 1103.112 cm−1 for DOSi+ . The ground vibrational state parameters are in excellent agreement with those determined from the ν1 bands of the two isotopes.
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  • 100
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5324-5328 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Fragment ions from N2 in the states of (K)−1(nl)1 have been measured at 54.7° with respect to the polarization direction of the incident photon by using a time-of-flight (TOF) mass spectrometer. Simulation fitting calculation to the measured TOF spectra gave distributions of the kinetic energy for these ions. Ion yield spectra observed showed that no singly charged molecular ion was produced from the states of (K)−1(nl)1 in contrast to that of (K)−1(1πg)1. These results are discussed in connection to the dissociation pathway of N2 in these states.
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