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Separation of enantiomers by ultraviolet laser pulses in H2POSH: π pulses versus adiabatic transitions (2001)

González, Leticia ; Kröner, Dominik

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 115 (2001), S. 2519-2529

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Different strategies to separate enantiomers from a racemate using analytical laser pulses in the ultraviolet frequency domain are proposed for the prototype model system H2POSH. Wave-packet propagations on ab initio ground- and electronic-excited state potentials show that it is possible to produce 100% of enantiomeric excess in a sub-picosecond time scale using a sequence of π and half-π pulses. Alternatively, the previous transitions can be substituted by adiabatic counterparts, using chirped laser pulses and a half-STIRAP (stimulated Raman adiabatic passage) method which only transfers half of the population between appropriate levels. Such an overall adiabatic mechanism gains stability concerning the pulse areas and frequencies at the expense of introducing new control variables, like the chirp and time delay. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1384871

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Density functional theory study on ethanol dimers and cyclic ethanol trimers (1999)

González, Leticia ; Mó, Otilia ; Yáñez, Manuel

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 111 (1999), S. 3855-3861

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The structure and the relative stability of the ethanol dimer and the cyclic ethanol trimer were studied using density functional theory methods. The geometries of the different dimers and trimers were optimized at the B3LYP/6-311+G(d,p) level of theory, while the final energies were obtained at the B3LYP/6-311+G(3df,2p) level. Four different (ethanol)2 complexes were found to be local minima of the potential energy surface, the global minimum being that in which both monomers exhibit a trans conformation. The hydrogen bond (HB) in ethanol dimer is slightly stronger than in methanol dimer, reflecting the enhanced intrinsic basicity of ethanol with regards to methanol. The OH donor stretch appears redshifted by 161 cm−1, while the redshifting undergone by the OH acceptor stretch is negligibly small. The relative stability of the trimers is a function of the number of monomers with a gauche conformation, the global minimum being that in which the three monomers have a trans conformation. As for water and methanol trimers, the three HBs in the cyclic ethanol trimer are not strictly equivalent. Consistently, the redshiftings of the OH stretching frequencies are different. Cooperative effects are sizably large, as reflected in the O(centered ellipsis)O distances, the elongation of the OH donor groups, the charge density at the bond critical points, the frequency shiftings of the OH stretches, and the additivity interaction energy. The most significant features of the vibrational spectra of the monomers, the dimers, and the trimers in the 800–1200 cm−1 region are reasonably well reproduced by our calculations.© 1999 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.479689

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High level ab initio and density functional theory studies on methanol–water dimers and cyclic methanol(water)2 trimer (1998)

González, Leticia ; Mó, Otilia ; Yáñez, Manuel

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 109 (1998), S. 139-150

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The methanol-water dimers and the potential energy surface of the cyclic methanol(water)2 trimer have been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies of the different local minima were calculated in the framework of the G2 and G2(MP2) theories. These values were compared with those obtained using the B3LYP/6-311+G(3df,2p) approach. At all the levels of theory considered the most stable conformer of methanol-water heterodimers corresponds to that in which water behaves as a hydrogen bond donor, in agreement with the most recent experimental evidences [P. A. Stockman et al., J. Chem. Phys. 107, 3782 (1997)]. The energy differences between the different conformers of the cyclic methanol(water)2 trimer are rather small, as well as the energy barriers connecting them. The global minimum corresponds to a conformer with the methyl group on one side of the O-O-O plane and the two free OH groups of the water molecules on the other side. Other stationary points associated with a systematic flipping of the methyl group and the free OH groups have been also located. These stationary points, which are transition states or saddle points of higher order, are very close in energy to the global minimum, indicating that the potential energy surface of the methanol(water)2 trimer is very flat and very similar to that reported before for water and methanol trimers. The calculated enthalpies of association are slightly smaller than those estimated for methanol trimers. The most stable trimer has three different O–H donor stretching frequencies, showing that the HB in which the methanol moiety behaves as a HB donor is the weakest one. Cooperative effects are significant. They are reflected in larger frequency shifts, greater lengthening of the O–H bonds, and shorter O(centered ellipsis)O distances than in (water)2 and methanol-water dimers. The estimated additive interaction energy is also significantly large. © 1998 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.476531

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Very strong hydrogen bonds in neutral molecules: The phosphinic acid dimers (1998)

González, Leticia ; Mó, Otilia ; Yáñez, Manuel

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 109 (1998), S. 2685-2693

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Ab initio molecular orbital and density functional theories have been used to study the structures and binding energies of the dimers of phosphinic acid (PA) and its dimethyl derivative (DMPA). For the first compound we have located all possible minima of the potential energy surface, while for the second only the most stable dimer was considered. The geometries were fully optimized at the MP2(full)/6-31+G(d,p) and B3LYP/6-31+G(d,p) levels of theory. The harmonic vibrational frequencies were evaluated at the same levels, while the final energies were obtained using a B3LYP/6-311+G(3df,2p) approach. Both phosphinic acid and its dimethyl derivative form cyclic dimers in the gas phase, where the two monomers are held together by hydrogen bonds (HBs) which are significantly stronger than those found for their carboxylic analogs. The estimated dimerization enthalpies for PA (23.2) and DMPA (23.2 kcal/mol) are the highest reported so far for neutral homodimers in the gas phase and almost twice those measured for formic and acetic acid dimers. For the particular case of DMPA this estimated value is in very good agreement with the experimental one (23.9±6 kcal/mol). As a consequence of the large strength of the HBs, the activation barriers associated with the concerted double proton transfer are also sizably smaller than those predicted for their carboxylic analogs. These barriers become negligibly small when zero point energy (ZPE) corrections are taken into account and therefore tunneling must be very efficient. The calculated harmonic vibrational frequencies for the most stable DMPA dimer are consistent with its experimental infrared (IR) spectrum in the gas phase, which shows a characteristic ABC structure of the ν(OH) band, typically associated with strongly hydrogen bonded complexes. Internal cooperative effects are not negligible in the case of phosphinic acid dimers, and the P=O(centered ellipsis)H hydrogen bonds in the global minimum are about 1.0 kcal/mol stronger than those found in other stable dimers where only one of these linkages exists. © 1998 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.476868

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Bottled Mineral Waters Polluted by Protozoa in Mexico (1981)

RIVERA, FERMÍN ; GALVÁN, MATILDE ; ROBLES, ESPERANZA ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

The @journal of eukaryotic microbiology 28 (1981), S. 0

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ISSN: 1550-7408

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology

Notes: A survey of protozoa polluting bottled mineral water in Mexico was carried out using samples obtained from the three best-selling brands of bottled mineral water in the country. The organisms were concentrated through filtration procedures and subsequently cultured in sterile media. The cultures were observed over four weeks, with identification to the level of genus and species. Most commonly found were the amoebae Naegleria gruberi, Acanthamoeba astronyxis, and Vahlkampfia vahlkampfi (trophic as well as cystic stages) plus one flagellate, Bodomorpha minima. No ciliates were detected. The public health importance of the findings is obvious, since some strains of Naegleria and Acanthamoeba have the potential to cause human disease that may lead to death.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1550-7408.1981.tb02803.x

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High-level ab initio versus DFT calculations on (H2O2)2 and H2O2-H2O complexes as prototypes of multiple hydrogen bond systems (1997)

González, Leticia ; Mó, Otilia ; Yáñez, Manuel

New York, NY [u.a.] : Wiley-Blackwell

Journal of Computational Chemistry 18 (1997), S. 1124-1135

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ISSN: 0192-8651

Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry

Source: Wiley InterScience Backfile Collection 1832-2000

Topics: Chemistry and Pharmacology , Computer Science

Notes: The performance of B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91 density functionals to describe multiple hydrogen bond systems was studied. For this purpose we have chosen the dimers of hydrogen peroxide and the hydrogen peroxide-water complexes. The geometries and vibrational frequencies obtained with a 6-311+G(d,p) basis set were compared with those obtained at the MP2 level using the same basis set expansion. The corresponding dimerization energies were obtained using a 6-311+G(3df,2p) basis set and compared with those obtained using the G2(MP2) theory. Red shiftings of the OH donor stretching frequencies were predicted by all approaches investigated; however, in all cases, the DFT values were sizably larger than the MP2 ones. Similarly, the blue shifting of the torsion of the hydrogen peroxide subunit was larger when evaluated at the DFT level. All functionals reproduced the G2(MP2) relative stabilities of the different local minima quite well. With the exception of the B-LYP and B3-PW91 approaches, all functionals yielded binding energies which deviated from the G2(MP2) values by less than 0.5 kcal/mol, provided that G2-type basis sets were used and that the corresponding BSSE corrections were included. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1124-1135

Additional Material: 1 Ill.

Type of Medium: Electronic Resource

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