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  • 1
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The computed molecular surface electrostatic potentials of a group of anticonvulsants of various chemical types were investigated with the objective of identifying common features that may be related to their activities. The calculations were carried out with the density functional B3P86/6-31G* procedure, using HF/STO-3G*-optimized geometries. Analysis of several statistically based properties of the surface potentials indicates that the negative regions are of primary importance and that an optimum intermediate level of local polarity, or internal charge separation, is required.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1137-1143, 1998
    Additional Material: 5 Ill.
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  • 2
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A general procedure is presented for computing the gas phase heats of formation of a wide variety of organic compounds. ΔE for the formation of the molecule from its elements at 0 K is obtained from density functional calculations (Gaussian 92/DFT) for optimized geometries. This result is converted to ΔH at 298 K by assuming ideal behavior and adding the translational, rotational, and vibrational energies. Additive correction terms corresponding to the various coordination states of the carbons, nitrogens, and oxygens were developed using a database of 54 compounds. The experimental ΔHƒo values of these compounds are then reproduced with an average absolute error of 3 kcal/mol and a standard deviation of 4 kcal/mol. For a group of 10 test cases that were not part of the database, the average absolute error is 3.5 kcal/mol and the standard deviation is 4.1 kcal/mol. © 1995 by John Wiley & Sons, Inc.
    Additional Material: 3 Tab.
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  • 3
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The electrostatic potential V(r) that the nuclei and electrons of a molecule create in the surrounding space is well established as a guide in the study of molecular reactivity, and particularly, of biological recognition processes. Its rigorous computation is, however, very demanding of computer time for large molecules, such as those of interest in recognition interactions. We have accordingly investigated the use of an approximate finite multicenter multipole expansion technique to determine its applicability for producing reliable electrostatic potentials of dibenzo-p-dioxins and related molecules, with significantly reduced amounts of computer time, at distances of interest in recognition studies. A comparative analysis of the potentials of three dibenzo-p-dioxins and a substituted naphthalene molecule computed using both the multipole expansion technique and Gaussian 82 at the STO-5G level has been carried out. Overall we find that regions of negative and positive V(r) at 1.75 Å above the molecular plane are very well reproduced by the multipole expansion technique, with up to a 20-fold improvement in computer time.
    Additional Material: 4 Ill.
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  • 4
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The structures and relative stabilities of furoxan and some of its isomers, e.g., the 1,2-dinitrosoethylenes, have been determined by means of ab initio Hartee-Fock and Møller-Plesset calculations. Geometries were optimized at the HF/3-21G, HF/6-31G* and MP2/6-31G* levels, and subsequently used for computing MP2/6-31G*, MP3/6-31G*, and MP4/6-31G* energies. The results are markedly affected by the inclusion of electronic correlation, which renders three of the isomers unstable. It also emphasizes the importance of a zwitterionic contribution to the structure of furoxan, which promotes ring-opening through a cis 1,2-dinitrosoethylene intermediate/transition state that has an MP4/6-31G*//MP2/6-31G* energy that is 31.6 kcal/mol above furoxan.
    Additional Material: 1 Ill.
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  • 5
    ISSN: 1432-2234
    Keywords: Nitramines ; Vibrational analysis ; Thermal decomposition ; Activation energies ; Dissociation energies
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary We have investigated some possible initial steps in the unimolecular thermal decompositions of 1,3-diazacyclobutane and its 1,3-dinitramine derivative, the latter being selected as the simplest example of a symmetric cyclic nitramine. Vibrational analyses were used to identify normal modes that, in the extreme limits, would correspond to bond rupture and molecular decomposition. The energy requirements for ring fragmentation and N-N bond-breaking were computed at the MP4/6-31G level, using SCF 3-21G optimized structures. It was concluded that ring-fragmentation is a probable initiating step in the decomposition of the unsubstituted molecule, and that it is roughly competitive with N-N bond scission for the dinitramine. The nitronitrite rearrangement is predicted, on the basis of SCF calculations, to be less likely than either of the other two processes. It is proposed that N-N bond-breaking may be of primary importance for nitramine stability, but that energetic performance may be determined more by decomposition pathways having energy barriers.
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  • 6
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A density functional computational study shows that 4-nitro-1,2,3-triazole, which is highly impact sensitive, can decompose through ring opening and subsequent N2 evolution, with the net release of 12 kcal/mol. An input of 52 kcal/mol is required to initiate the process. © 1997 John Wiley & Sons, Inc.
    Additional Material: 2 Tab.
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  • 7
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two density-functional methods, B3/LYP/6-31+G(d) and B3/PW91/6-31G(d, p) have been used to investigate some possible decomposition pathways for methyl nitrate. Two likely ones were found to be (a) loss of NO2, followed by eventual rearrangement to H2C(SINGLE BOND)OH, and (b) formation of H2C(DOUBLE BOND)O and HONO. The initial energy requirement for each process is about 40 kcal/mol, and the second is exothermic, ΔH=-16 kcal/mol of H3C(SINGLE BOND)ONO2. The B3/LYP procedure is found to underestimate the H3CO(SINGLE BOND)NO2 and H3C(SINGLE BOND)ONO2 dissociation energies by about 6 kcal/mol.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 205-210, 1997
    Additional Material: 1 Ill.
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  • 8
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We investigated whether an average local ionization energy defined within the framework of Hartree-Fock theory is physically meaningful in Kohn-Sham density functional theory, in view of the uncertainty in relating the latter orbital energies to electronic ionization energies. In a preliminary study of N2O using several different exchange/correlation functional combinations, it was found that the orbital energies in each case correlate roughly with experimental binding energies. Average local ionization energies are computed on the molecular surfaces of 11 monosubstituted benzene derivatives using both Hartree-Fock and density functional procedures. Both sets of results correctly predict the ortho/para- or meta-directing tendencies of the substituents, as well as their ring-activating or -deactivating effects. Excellent correlations were obtained with the Hammett substituent constants. This initial study suggests that Kohn-Sham average local ionization energies should be physically meaningful, on a relative basis.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 607-613, 1998
    Additional Material: 5 Ill.
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