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  • 1
    Electronic Resource
    Electronic Resource
    Average local ionization energies computed on the surfaces of some strained molecules (1990)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 645-653 
    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 ab initio self-consistent-field molecular orbital (SCF-MO) procedure has been used to compute the average local ionization energies [Ī(r)] of some strained molecules on three-dimensional surfaces defined by the contour of electronic density equal to 0.002 electrons/bohr3. Ī(r) is rigorously defined within the framework of SCF-MO theory and can be interpreted as the average energy needed to ionize an electron at any point in the space of a molecule. Thus, the positions of the smallest Ī(r) values (ĪSmin) on the surfaces of molecules are the sites expected to be the most reactive to electrophiles. We find ĪSmin, near the C—C bond midpoints of saturated three-membered, but not foru-membered, hydrocarbon rings. These ĪS,min are interpreted as reflecting the “σ-aromatic” character of the former. Our Ī(r) data effectively provide a “fingerprint” characterizing saturated three-membered rings, which is useful, for example, in analyzing molecules such as [1.1.1] propellane. Our results for the latter are consistent with the interpretation of it having a biradical character, as has been suggested earlier.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560382462
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  • 2
    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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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    An analysis of molecular electrostatic potentials obtained by a local density functional approach (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 113-122 
    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: Local density functional theory (DFT-LDA) has been explored as a tool for obtaining the molecular electrostatic potential V(r), using the code DMol. We have presented and discussed DFT-LDA electrostatic potentials for a representative series of molecules: ethylene, benzene, formamide, cytosine, and 2,3,7,8-tetrachlorodibenzo-p-dioxin. V(r) results obtained with a double numerical plus polarization (DNP) basis set show the key features that are characteristic of the ab initio potentials of these compounds and suggest that this is a useful approach, especially for large molecules that are difficult to study by ab initio methods.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560440204
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  • 5
    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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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    Calculation of intramolecular force fields from second-derivative tensors (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1271-1277 
    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 practical procedure (FUERZA) to obtain internal force constants from Cartesian second derivatives (Hessians) is presented and discussed. It allows a systematic analysis of pair atomic interactions in a molecular system, and it is fully invariant to the choice of internal coordinates of the molecule. Force constants for bonds or for any pair of atoms in general are defined by means of the eigenanalysis of their pair interaction matrix. Force constants for the angles are obtained from their corresponding two-pair interaction matrices of the two bonds or distances forming the angle, and the dihedral force constants are similarly obtained using their corresponding three-pair interaction matrices. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    A study of small systems containing H and O atoms using nonlocal functionals: comparisons with ab initio and experiment (1994)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 52 (1994), S. 655-666 
    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: Structures and energies have been calculated for HO, H2O, O2, HO2, H2O2, and O3 molecules using the nonlocal functionals PW, PW91, B-P86, and B-LYP with the goal of obtaining their atomization energies. Results were compared with those from highly correlated methods and experiment. It was found that all nonlocal functionals perform similarly to or better than correlated methods MP4 and QCI (using relatively equivalent basis sets). All nonlocal energies were self-consistently calculated using the optimized geometries for each functional. None of the results contain any empirical correction except those inherent to some of the functionals. Increasing the size of the basis set when using the nonlocal functionals does not lead to any significant improvement of the energies and surprisingly it worsens the results for one of the functionals. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560520858
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