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  • Atomic, Molecular and Optical Physics  (16)
  • 1
    Electronic Resource
    Electronic Resource
    Proton transfers in hydrogen-bonded systems V. Analysis of electronic redistributions in (N2H7)+See references [1-5] for previous papers in this series. (1981)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 20 (1981), S. 221-229 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The transfer of the central proton between the two NH3 units of (H3NHNH3)+ is studied using the 4-31 G basis set within the ab-initio Hartree-Fock formalism. Electron density difference maps are constructed which clearly indicate electronic redistributions which accompany the half-transfer of the proton from its equilibrium position (NH—N) to the midpoint of the hydrogen bond (N—H—N). The overall loss of electronic charge from the proton-accepting molecule originates in three distinct regions of space, while a density buildup of smaller magnitude is observed in a characteristic region centered about the N nucleus. Similar regions are noted for the proton-donating molecule, although the changes are reversed in sign. A partitioning of the total density changes into contributions from the various molecular orbitals demonstrates that the a1 orbitals are associated with density shifts along the H-bond axis. Changes in the N lone pairs are attributed chiefly to the (5a1,6a1) pair and are somewhat attenuated by opposite shifts involving the (3a1. 4a1) pair. Orbitals of e symmetry lead to polarizations of the NH bonds and density shifts perpendicular to the H-bond axis.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560200719
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  • 2
    Electronic Resource
    Electronic Resource
    Energetics and electronic rearrangements of proton transfer in (H3NHOH2)+ (1983)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 753-764 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Proton transfer between N and O in the hydrogen-bonded system (H3NHOH2)+ is studied by ab initio molecular orbital methods. Potential energy curves are calculated at the hartree-Fock level using the 4-31G basis set for hydrogen bond lengths R(NO) varying from the equilibrium value of 2.664 to 3.10 Å. Short hydrogen bonds are associated with asymmetric single-well potentials in which the minimum corresponds to the NH—O configuration. For longer R(NO) separations, the potential is of double-well form, including both N—HO and NH—O as minima. It is found that the height of the energy barrier to proton transfer is sensitive to both stretches and bends of the hydrogen bond. Continuous changes in the electron density are monitored at various stages of proton transfer via density difference maps and Mulliken population analyses. The initial loss of density from the proton-accepting molecule during the first half of the transfer is accelerated during the second half. A correlation is drawn between the energetics of transfer in a number of systems and the net charge lost by the proton-acceptor group.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560230244
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  • 3
    Electronic Resource
    Electronic Resource
    Energetics of proton transfer between carbon atoms (H3CH — CH3)- (1986)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 285-292 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations were carried out to study the potential energy surface of (H3C—H—CH3)-. The 6-31G* basis set is supplemented by a set of diffuse p functions on both C and H (with a range of exponents for the latter). The binding energy of CH4 and CH3- to form the (H3CH—CH3)- complex is about 2 kcal/mol, much smaller than for comparable ionic H-bonded systems involving O or N atoms. Nearly half of this interaction energy is due to correlation effects, computed at second and third orders of Møller-Plesset perturbation theory. Correlation is also responsible for substantial reductions in the energy barrier to proton transfer within the complex. This barrier is computed to be 13-15 kcal/mol at the MP3 level, depending upon the exponent used for the H p functions.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560290218
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  • 4
    Electronic Resource
    Electronic Resource
    Ab Initio investigation of the structure of hydrogen halide-amine complexes in the gas phase and in a polarizable medium (1987)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 47-56 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The geometries of all 12 complexes in which HF, HCl, or HBr is paired with NH3, NMeH2, NMe2H, or NMe3 are optimized with the MINI-1 basis set. As the basicity of the amine is increased via progressive methylation, or as the proton affinity of the halide is diminished, the proton equilibrium position shifts toward the nitrogen, but in no case is this shift far enough to classify the complex as an ion pair. When the effects of a polarizable medium are included via the SCRF formalism, the shift of the proton toward the nitrogen is enhanced by increases in the solute-solvent interaction such that relatively modest coupling leads to complexes of ion-pair type. In all cases, complexes containing HBr are the most sensitive to either the basicity of the amine or the influence of the medium whereas the HF analogs are affected very little.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560320809
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  • 5
    Electronic Resource
    Electronic Resource
    Effects of external ions upon proton transfer reactions: H-bonded systems containing HCOOH (1988)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 34 (1988), S. 137-147 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations reveal that a cation, in a position equidistant from the two H-bonding oxygen atoms of (HC(OH)O · H · OH2)+, acts to push the proton from HCOOH toward OH2 if the ion is located near the  - OH or C—H group of HCOOH. In contrast, rotation of the cation out of the HCOOH plane, while maintaining its position relative to the two H-bonding atoms, leads to a potential which is little different than that observed in the absence of an ion. These findings are readily explained on the basis of ion-induced internal polarization of the subunit to which the ion is closer. Replacement of OH2 by HCOOH suggests that polarization of the more distant subunit can also be a factor.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560340713
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  • 6
    Electronic Resource
    Electronic Resource
    Factors influencing proton positions in biomolecules (1986)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 817-827 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of quantum mechanical calculations are presented that suggest a number of mechanisms whereby protons may be shifted from one group to another along an H bond. The first factor to be considered is a stretching of the bond that drastically raises the energy barrier to transfer. It is possible to predict barriers for an arbitrary system based only on results for a simple system and knowledge of the relevant bond length in the isolated subsystems. Factors that increase the intrinsic basicity of the B group in A-H-B lead not only to a lowering of the energy of the A-HB state relative to AH-B but also to a reduction in the barrier to transfer of the proton from A to B. Ions in the vicinity of the H bond exert a powerful influence and can shift the proton to the less basic group across a gradient of several pK units. Rather than shielding the proton from the external ion, the H bond acts instead to amplify the effects of the electric field. Reorientation of the A and B groups relative to one another, i.e., bends of the H bond, also produce surprisingly large changes in the relative energies of the AH-B and A-HB states. Such bends are capable of pushing the proton across to the normally less basic group, providing a mechanism of coupling conformational changes to proton ‘pumping’ activity. It is found that the high and low pH states of a given H bond can have dramatically differnt relative populations of the AH-B and A-HB configurations. These observations are explained in terms of fundamental concepts involving electrostatic interaction energies.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560290420
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  • 7
    Electronic Resource
    Electronic Resource
    The basis set dependence of structures and energies of various states of cyclodisiloxane (1986)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 1191-1208 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several common basis sets, ranging from minimal to double-zeta, are applied to study the neutral singlet and triplet as well as positive- and negative-ion doublet states of cyclodisiloxane. The effect of d-polarization function exponents on the equilibrium geometries and energies is analyzed. The d-type functions seem to be essential in the basis set of silicon, whereas their presence on oxygen is less critical. The optimum exponents (with respect to SCF energy) are determined to be 0.45 for Si and 0.60 for O, very close to those recommended for the 6-31G** basis set. The best structural predictions are obtained with the 6-31G(2d, p) basis set, which contains two sets of d functions on the heavy atoms. The predicted Si—O bond length is 166 pm; the Si—Si and O—O distances are 237 and 232 pm, respectively, which correspond to an O - Si—O angle of 88.6°. The ground state is found to be a singlet. All higher states have longer Si—O bonds and Si - Si distances, whereas O - O distances are shorter. The energy separation between the singlet and other states is modified by electron correlation (MP treatment) by only a few kcal/mol.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560290518
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  • 8
    Electronic Resource
    Electronic Resource
    Ab initio study of proton transfers including effects of electron correlation (1983)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 739-751 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Proton transfers in a number of systems are investigated using ab initio molecular orbital methods. Calculations are carried out with several different basis sets ranging in size from 4-31G to 6-311G**. Electron correlation is included using Møller-Plesset (MP) perturbation theory to second and third orders. Enlargements of the basis set invariably lead to higher energy barriers to proton transfer, while substantial reductions result from inclusion of correlation effects. Application to (HOHOH)- of third-order MP theory with a triple-valence basis set augmented by polarization functions on oxygens and the central proton, denoted MP3/6-311G*(*), leads to excellent agreement with the results of Roos et al. whose calculations involved an extensive CI treatment with a large basis set. For equivalent hydrogen bond lengths, the transfer barrier in the cation (H2OHOH2)+ is nearly identical to that for the (HOHOH)- anion while the barrier in (H3NHNH3)+ is somewhat smaller. The reduction of the SCF barrier height resulting from inclusion of correlation is greater for (O2H3)- than for the above cations. The lowest energy structure of (O2H5)+ contains a symmetric hydrogen bond in which the proton is located midway between the two oxygens whereas asymmetric H bonds are found in the equilibrium geometries of (N2H7)+ and (S2H5)+. The difference in energy between the symmetric and asymmetric configurations of (O2H3)- is extremely small.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560230243
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  • 9
    Electronic Resource
    Electronic Resource
    Interactions involving aromatic systems: Benzene + acetylene (1984)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 201-208 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Various configurations of the system composed of benzene and acetylene are investigated in an effort to identify the most stable. The total interaction energy is computed as the sum of the dispersion energy ED and ΔESCF, the interaction energy calculated by ab initio molecular orbital methods. Pairwise summation schemes of both the atom-atom and bond-bond types are used to evaluate ED. The most stable structure is found to be that in which the acetylene approaches benzene from above and is oriented such that it lies directly along the C6 axis of benzene. Although the contribution of dispersion to the total interaction energy is dominant, the smaller electrostatic component plays a crucial role in controlling the relative orientations of the two molecules. Indeed, it is possible to select the most stable configuration solely on grounds of the most favorable quadrupole-quadrupole interaction. The likelihood of observation of this configuration is further confirmed by consideration of basis set extension and entropic effects.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560260721
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  • 10
    Electronic Resource
    Electronic Resource
    Effect of proton transfer on neighboring hydrogen-bond strength (1991)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 40 (1991), S. 37-48 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The transfer of a proton from A to B in AH+‥B‥C to form A‥+HB‥C is expected to enhance the binding energy of C to the remainder of the H-bonded chain since C is now adjacent to a charged ion rather than to a neutral molecule. But since the actual proton motion between A and B is typically only 1 Å or so, the interaction should be considerably less than that in isolated +HB‥C. Ab initio calculations with a variety of basis sets are used to quantitatively measure the enhancement of this binding energy. Systems investigated include the homotrimers of water and of ammonia, ammonium-formate-water, and formic acid-imidazole-water.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560400709
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