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  • 1
    Electronic Resource
    Electronic Resource
    Antiaromaticity in relation to 1,3,5,7-cyclooctatetraene structures (1994)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 273-277 
    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: An isodesmic energy analysis has been carried out at the MP2/6-31G*//HF/3-21G level for the nonplanar ground state (1) of 1,3,5,7-cyclooctateraene and for two planar forms, one having complete π delocalization (2) and the other having alternating single and double bonds (3). 1 is found to have a considerable degree of stabilization, which is attributed to limited π delocalization. The polyene 3 is the more stable of the two planar forms; it is a transition state in the inversion between two possible nonplanar structures. 2 is found to be a triplet at the Hartree-Fock level and is a critical point on an alternate pathway between the two possible arrangements of alternating single and double bonds in 3. Both 2 and 3 have negative isodesmic energies, indicating the presence of stabilizing factors. Our results for 3 show that an “antiaromatic” system need not necessarily show a net destabilization. © 1994 John Wiley & Sons, Inc.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560500404
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  • 2
    Electronic Resource
    Electronic Resource
    Does antiaromaticity imply net destabilization? (1994)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 575-579 
    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: An analysis is presented of the results of earlier ab initio computational studies of cyclobutadiene, cyclooctatetraene, and 1,4-dihydropyrazine. The first and third of these are normally categorized as antiaromatic. All three molecules are polyenes, even when the last two are forced into planar conformations. There is no driving force for extensive π delocalization, even when it would appear to have been facilitated. Calculated isodesmic energies show a net destabilization only in the case of cyclobutadiene, which we attribute to strain and repulsion between the π electrons of the C=C double bonds. The other two molecules have negative isodesmic energies, indicative of net stabilizing effects. We conclude that the concept of antiaromaticity is useful for identifying molecules that resist the apparent opportunity for extensive © delocalization, but that it does not intrinsically imply net destabilization. © 1994 John Wiley & Sons, Inc.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560490503
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  • 3
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulation of liquid nitromethane shocked to 143 kbar (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 621-625 
    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: A molecular dynamics simulation, using a force field established by density functional calculations, was used to investigate structural and vibrational frequency shifts that accompany a compression from 1 bar, 300 K to 143 kbar, 600 K. Under the latter conditions, most pairs of nitromethane molecules were found to be linked by two hydrogen bonds, instead of the one that was observed at 1 bar, 300 K. The computed shifts in the CN and NO, stretching frequencies were in good agreement with the experimental results. © 1995 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560560867
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  • 4
    Electronic Resource
    Electronic Resource
    Density-Functional and ab initio computational studies of palladium clusters (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 263-268 
    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: Nonlocal density-functional and correlated ab initio methods have been used to compute singlet-triplet separations for the Pd atom and Pd2, as well as the dissociation energies of the latter in its ground and first excited states. The results are in good agreement with the available experimental data and with other high-level calculations. Single-point local density-functional computations were carried out for Pd clusters up to Pd22 (1012 electrons). The interaction energy per Pd—Pd bond appears to be approaching a limiting value for Pd22. © 1993 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560480828
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  • 5
    Electronic Resource
    Electronic Resource
    Reaction energetics of tetrahedrane and other hydrocarbons: Ab initio and density functional treatments (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1351-1360 
    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: Reaction energetics of the highly strained tetrahedrane molecule, as well as some smaller hydrocarbons, were computed at the ab initio MP4, QCISD (T), G1, and G2 levels and also by several density functional (DF) approaches: VWN(LDA), PW86, BLYP, and B3LYP. A variety of basis sets were used in the DF calculations. For atomization processes, nonlocal DF procedures are superior to MP4 and QCI. For the other reactions, B3LYP is the most reliable DF approach and is overall competitive with MP4 and QCISD (T), although the MP4/6-311G(2df, p) results are generally the closest to the experimental and/or the G2 values. There is no consistent correlation between the accuracy of the DF results and the size of the basis set. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Gaussian-2 and density functional studies of H2N—NO2 dissociation, inversion, and isomerization (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 497-504 
    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 computational approaches, including Gaussian-2 (G2) and nonlocal density functional theory (DFT-GGA ), have been used to calculate the energy requirements for (a) H2N—NO2 dissociation (through N—N bond scission), (b) inversion of the amine group, and (c) isomerization through the nitro-nitrite rearrangement. Taking zero-point energies into account, the G2 predictions are 53.6 kcal/mole for the dissociation energy and 1.5 kcal/mole for the inversion barrier. The corresponding DFT-QGA values are 48.4 and 0.9 kcal/mole, and an activation energy of 48.7 kcal/mole for the nitro-nitrite rearrangement. The DFF-OGA results indicate that dissociation and rearrangement should be competitive for H2N-NO2. The same conclusion was reached earlier by Saxon and Yoshimine on the basis of MRCISD/6-3lG* calculations, although their computed energy requirements differ from the present ones by approximately 8 kcal/mole. © 1992 John Wiley & Sons, Inc.
    Additional Material: 7 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560440845
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  • 7
    Electronic Resource
    Electronic Resource
    Molecular surface electrostatic potentials and local ionization energies of Group V-VII hydrides and their anions: Relationships for aqueous and gas-phase acidities (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 73-88 
    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: We have computed ab initio HF/6-31+G* electrostatic potentials and average local ionization energies on the molecular surfaces of the Group V-VII hydrides and corresponding anions of the first three rows of the periodic table. The surfaces were defined to be specified contours (0.002 or 0.001 au) of the molecular electronic density. The most negative potentials, VS,min, and lowest ionization energies ĪS,min, were located and determined. Their magnitudes separately satisfy limited correlations with gas-phase protonation enthalpies and aqueous pKa values. Our results indicate that VS,min, and ĪS,min are complementary, the former reflecting electrostatic factors and the latter being related to charge transfer/polarization. More general relationships for protonation enthalpies are obtained when both VS,min and ĪS,min are explicitly included. Solution-phase and gas-phase acidities are shown to correlate very well if electrostatic effects are explicitly taken into account. © 1993 John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560480202
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  • 8
    Electronic Resource
    Electronic Resource
    X-NO2 rotational energy barriers: Local density functional calculations (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 15-20 
    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 X-NO2 rotational energy barriers of nitromethane, nitroethylene, nitrobenzene, and a group of nitramines have been computed using a local density functional (LDF) procedure, using ab initio Hartree-Fock (HF)-optimized structures of the ground and rotational transition states. The results have been discussed in relation to HF and some correlated ab initio values and the available experimental data. Our LDF barriers are overall quite reasonable, in generally satisfactory agreement with the experimental and correlated ab initio results. © 1993 John Wiley & Sons, Inc.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560450104
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  • 9
    Electronic Resource
    Electronic Resource
    Calculation of molecular geometries and energies by a local density functional approach (1991)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 40 (1991), S. 249-259 
    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: We have used density functional theory in the local density approximation, as implemented in the DMol-double numerical plus polarization (DMol/DNP) procedure, to calculate optimized geometries for a group of molecules containing only first-row atoms. The results are in generally satisfactory agreement with experimentally determined structures. The DMol/DNP atomic and molecular energies are rather poor, when compared to high-level ab initio GAUSSIAN 1 values (which are taken as the standard), but the errors do show excellent linear correlations with the total energies. These relationships allow the DMol/DNP results to be corrected to a high degree of accuracy, so that they can then be used to obtain atomization and dissociation energies that compare well with measured values. It is shown that the errors in the DMol/DNP energies tend to increase as the number of electrons becomes larger and the number of nuclei smaller.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560400825
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  • 10
    Electronic Resource
    Electronic Resource
    C—H Bond dissociation of acetylene: Local density functional calculations (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 42 (1992), S. 267-272 
    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 C—H bond dissociation energy of acetylene was computed by both ab initio approaches and density functional theory in a local density approximation (DFT-LDA). Structures and energies for acetylene and its dissociation products (the ethynyl and hydrogen radicals) are presented and compared. Using directly computed HCCH and HCC· energies and the exact H· value, the DFT-LDA calculations are found to yield C—H dissociation energies ranging from 129 to 131 kcal/mol, in good agreement with recent experimental and the highest level theoretical results. The DFT-LDA results show little dependence upon the computational procedure used to obtain geometries.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560420203
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