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  • 1
    Electronic Resource
    Electronic Resource
    The structure and stability of Sb4H+ clusters: The importance of nonclassical structures (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 2258-2264 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and relative stabilities of the different Sb4H+ clusters were investigated by means of high level ab initio calculations. For this purpose we have developed a split valence and an extended basis set for the treatment of Sb-containing compounds to be used with different effective core potentials available in the literature. The split-valence basis set reported seems to reproduce nicely the geometries and vibrational frequencies of different Sb-containing compounds, provided that electron correlation effects are included at the MP2 level. When the extended basis set is used, within the framework of the G2(ECP) theory, the atomization enthalpies of antimony derivatives are reproduced within ±3 kcal/mol. A systematic study of the Sb4H+ potential energy surface (PES) using these basis sets, showed that the global minimum is the result of the side protonation of the Sb4 tetrahedral molecule. In this species the hydrogen is covalently attached to two Sb atoms through the formation of a three-centered delocalized bonding orbital similar to the one responsible for the stability of analogous P4H+ and As4H+ side-protonated species. This "nonclassical" structure is estimated to be 26 kcal/mol more stable than the classical corner-protonated form. The stability of these "nonclassical" structures increases as one descends in the group. Concomitantly, the gas-phase basicity also increases from P4 to Sb4, the latter being about 20 kcal/mol more basic than P4 and about 14.5 kcal/mol more basic than As4. In contrast with P4 and As4, the body- centered protonated species of Sb4 is predicted to be a local minima of the PES. Similar trends have been found for BiH4+ species. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480824
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  • 2
    Electronic Resource
    Electronic Resource
    Very strong hydrogen bonds in neutral molecules: The phosphinic acid dimers (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 2685-2693 
    Details
    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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  • 3
    Electronic Resource
    Electronic Resource
    Density functional theory study on ethanol dimers and cyclic ethanol trimers (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3855-3861 
    Details
    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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  • 4
    Electronic Resource
    Electronic Resource
    High level ab initio and density functional theory studies on methanol–water dimers and cyclic methanol(water)2 trimer (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 139-150 
    Details
    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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  • 5
    Electronic Resource
    Electronic Resource
    Structure and charge distribution of some alkynoyl cations (1979)
    Springer
    Theoretical chemistry accounts 53 (1979), S. 337-343 
    Details
    ISSN: 1432-2234
    Keywords: Population analysis of alkynoyl cations ; Alkynoyl cations ; Alkynoyl fluorides
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We have performedab initio calculations to determine the structure and charge distribution of some alkynoyl cations and their parent alkynoyl fluorides. We have used Mulliken population analysis and a new technique developed by Yáñez, Stewart and Pople. Our results indicate that the mesomeric form O+≡Cα−Cβ≡Cγ−R is one of the most important contributors to the structure of these cations, in agreement with experimental conclusions. We have also found that the participation of mesomeric form O=Cα=Cβ=C+ γ-R is not negligible and increases with γ-substitution. In the 3-phenylpropynoyl cation substantial delocalization of charge into the phenyl group occurs. Calculations from YSP population analysis are in good agreement with experimental evidence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00555693
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  • 6
    Electronic Resource
    Electronic Resource
    Enhanced Li+ binding energies of some azines: a molecular orbital study (1990)
    Springer
    Theoretical chemistry accounts 77 (1990), S. 1-15 
    Details
    ISSN: 1432-2234
    Keywords: Azines ; Li+ binding energies ; Intrinsic basicities
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Hartree-Fock calculations with the 6–31G* basis have been performed to investigate the structure and Li+ binding energies of the complexes between Li+ and pyridine, diazines, triazines and tetrazines. Structures have been fully optimized at the 3–21G level. As for azole-Li+ and methyldiazole-Li+ complexes, a topological analysis of the Laplacian of the electronic charge density reveals that the azine-Li+ is a typical closed-shell interaction and that the stabilization of the complex is mainly electrostatic. BSSE is quite significant, specially for Li+-bridging complexes. The correlation between calculated Li+ binding energies and proton affinities follows two different linear relationships, one for those cases where Li+ is singly coordinated and a different one for those cases in which an additional three-membered ring is formed. The enhanced stability of these particular conformations explains why while polyazines are less basic than pyridine when the reference acid is a proton; pyridazine and 1,2,4 triazine are more basic than pyridine when the reference acid is Li+. The effect on Li+ binding energies of systematic nitrogen substitution roughly follows an additive model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01114648
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  • 7
    Electronic Resource
    Electronic Resource
    Counterpoise estimates of the BSSE in the evaluation of protonation energies (1988)
    Springer
    Theoretical chemistry accounts 73 (1988), S. 307-316 
    Details
    ISSN: 1432-2234
    Keywords: BSSE ; Protonation energies
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Counterpoise estimates of the BSSE in the evaluation of protonation energies have been calculated for basis sets ranging from minimal to split-valence plus polarization quality. Three-, five- and six-membered-ring heterocycles have been chosen as suitable model compounds for this study. Counterpoise corrections are significant, at the minimal basis set and 3–21G levels, when considering both, absolute and relative protonation energies and depend on the nature of the centre which undergoes protonation. In general, second- and third-order counterpoise corrections to the protonation energies are comparable to the corresponding SCF values. BSSE depend not only on the size of the basis sets but also on their quality. The presence in the basis of quite diffuse functions (either sp or d) leads to lower protonation energies and greater BSSE. Relative protonation energies are not substantially affected by BSSE or correlation effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00527417
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  • 8
    Electronic Resource
    Electronic Resource
    Influence of polarization functions on molecular electrostatic potentials (1978)
    Springer
    Theoretical chemistry accounts 47 (1978), S. 263-273 
    Details
    ISSN: 1432-2234
    Keywords: Polarization functions ; Molecular electrostatic potentials
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We study the effects of d-polarization functions, centered on the heavy atoms, on the SCF molecular electrostatic potentials calculated for some molecules. The positions and energies of the minima found are very sensitive to the inclusion of polarization functions in the basis set used. The variations depend on the heavy atom involved and on the possible anisotropy of its charge distribution. These variations are particularly important for second-row atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00549256
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  • 9
    Electronic Resource
    Electronic Resource
    Structure of 1,2,6-thiadiazine 1,1-dioxides (1990)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 3 (1990), S. 470-476 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Ab initio theoretical calculations were carried out on the three tautomers, NH, OH and CH, of 1,2,6-thiadiazine 1,1-dioxides. Different basis sets were employed in order to obtain an adequate description of these cyclic sulphamide derivatives. A coherent picture is obtained which includes relative stability of tautomers (NH 〉 CH 〉 OH), non-planarity of NH and CH tautomers and electronic distribution (Boys' localized orbitals).
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610030709
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  • 10
    Electronic Resource
    Electronic Resource
    The geometry of pyrazole: A test for ab initio calculations (1995)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 16 (1995), S. 263-272 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Ab initio calculations on the structure of pyrazole have been carried out at different levels of accuracy. At the Hartree-Fock (HF) level, the performance of several basis sets, namely 3-21G, 6-31G, 6-31G**, and 6-311G** was investigated. The influence of electron correlation effects also was studied by carrying out geometry optimizations at the MP2, MP4, and QCISD levels. The performance of a density functional method also was evaluated. We have also investigated the possible influence of the frozen core approximation on the final optimized geometry. Three different statistical analyses were considered in determining which geometry is closest to the experimental microwave geometry - namely Paul Curtin's diagrams, cluster analysis, and multidimensional scaling. From these analyses, we conclude that there is no asymptotic approach to the experimental geometry by increasing the quality of the theoretical model, although, as expected, the more reliable structures are those obtained at the MP2, MP4, and QCISD levels, as well as those obtained by the B3LYP density functional method. We have also found that the values of the rotational constants are a tight criterion to define the quality of a molecular geometry. © 1995 by John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540160302
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