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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 779-788 
    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 computational study of substituent effects on methane activation and elimination by high-valent zirconium complexes is reported. Substituent (Z) effects (in a structural, electronic, and enthalpic sense) are substantially less important for the imido (LnZr(DOUBLE BOND)NZ) and imidolike TS than the amido (LnM(SINGLE BOND)NHZ). For the microscopic reverse reaction, methane CH activation, it suggests that tailoring imido reactivity through electronic modification of nitrogen substituents will be difficult. Analysis of the earliest part of the reaction coordinate for methane elimination entails structural deformation of the Zr - amido to assume an appropriate geometry for elimination, which, in some cases, is directly reflected in substantially higher elimination barriers. Lower elimination barriers correlate with stronger agostic bonding, providing further support for the crucial importance of agostic bonding in facilitating alpha-elimination processes for high-valent transition-metal complexes. © 1996 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 793-806 
    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 analysis of the reaction of molecular hydrogen with the Zr-imido complex (NH2)2Zr= NH is reported. Several interesting points are noted. The calculated stretching frequency of the Zr=N bond in (NH2)2Zr=NH is 860 cm-1 when properly scaled to account for electron correlation effects. The value supports the assignment of an infrared (IR) band at 865 cm-1 to the Zr=N stretch of a tetrahydrofuran (THF) adduct of the putative reactive intermediate (NHSi′)2Zr=NSi′. Although a weakly bound, H2 complex is found, the interaction is small (1.3 kcal mol-1) compared with experimentally characterized H2 complexes. Variations in bond lengths and intrinsic stretching frequencies demonstrate that π-bonding for amido (NH2) ligands can be substantial in a coordinatively saturated complex. H2 activation by the bis(amido)imido reactive intermediate is calculated to be significantly more favorable by the 1,2-addition of H2 across the Zr=N bond to form the tris(amido)hydride than a sigma-bond metathesis pathway. The transition state (TS) for the addition of H2 across the Zr=N bond of the bis(amido )imido complex is 9.8 kcal mol-1 above the charge transfer (CT) complex, (NH2)2Zr=NH = H2 at the MP2 level; the reverse process, extrusion of H2 from the tris(amido)hydride has a 28.2 kcal mol-1 barrier. The geometry of the four-center TS is of interest, deviating markedly from that of a square and being more “kite” shaped (i.e., one obtuse and three acute angles). Mulliken Bond Overlap Populations suggest that there is some interaction between the Zr and the H atom being transferred (Hl) in the various TSs studied. It is suggested that the interaction plays an important role in the ability of the Zrimido complexes, and indeed other high-valent, multiply bonded complexes, to activate X=H (X=H, C, Si, N, and H) bonds. The design of materials and catalysis precursors which enhance the metalhydrogen interaction in the TS could conceivably lead to lower chemical vapor deposition (CVD) processing temperatures and higher catalytic activities. © 1992 John Wiley & Sons, Inc.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 36 (1989), S. 773-790 
    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 of the pathways and intermediates for the epoxidation of olefins using a Ru(IV)-oxo model complex is presented. Simple bonding concepts, coupled with INDO/1 semiempirical MO calculations, provide a description of the metal-oxo/olefin interaction that agrees well with experimental facts. Concerted [1 + 2] and [2 + 2] pathways are investigated and found to lead to unfavorable orbital interactions as with organic analogues. Nonconcerted process (those in which one bond is formed before the second), which connect the two concerted pathways, are preferred. A nonconcerted [1 + 2] pathway is preferred over a nonconcerted [2 + 2] pathway on the basis of less steric repulsions between the olefin and the metal ligands. Also, all open structures (those with one C—O bond formed) investigated minimized to bound epoxides. In the most favorable pathway, a nonconcerted [1 + 2] pathway, radical cation character on the terminal carbon increases as the nonconcerted process is induced. Preference for the bound epoxide intermediate over the oxometallocycle is simply explained by partial charge considerations.
    Additional Material: 7 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 665-678 
    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 INDO/1 semi-empirical method is used to analyze the hydroxylation of alkanes by models for the putative active species in cytochromes P-450. In the insertion of oxygen into the C—H bond by a M - O complex there are four atoms of particular interest, the metal and oxygen atoms of the oxidant plus the C and H atom of the C—H bond to be activated. With four atoms being studied (3N-6=), 6 degrees of freedom need to be described and a nearly intractable computational problem results. Thus, two reaction coordinates (CHI and OAT) were developed in an attempt to describe alkane hydroxylation in a chemically meaningful way. CHI (C—H insertion) is a measure of the extent to which the C—H bond has been activated and transferred to the O atom. OAT (oxygen atom transfer) is a measure of the extent to which the C—O bond in the alcohol product has formed. The models chosen for the present study are manganyl (MnO) and ferryl (FeO) porphyrin complexes. Experimental evidence for manganese prophyrin complexes shows that as the axial ligand is made a stronger donor the insertion of the O atom into the C—H bond becomes less concerted. The INDO/1 method was used to calculate the points on the PES determined by the changes in OAT and CHI. The results point to the axial ligand modifying the pathway by changing the relative electro-/nucleophilicity of the oxygen atom.
    Additional Material: 2 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 52 (1994), S. 181-194 
    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 computational study, using relativistic effective core potentials, is presented of transition metalmain group multiply bonded complexes, of interest in the context of catalysis and chemical vapor deposition of TM/MG materials. Model d0 transition metal complexes chosen are of the general form ClnME where M = Zr (n = 2), Ta (n = 3), and W (n = 4). Main group elements of interest are the tetrels (E = C, Si, Ge, Sn), pnictogens (E = N, P, As, Sb), and chalcogens (E = O, S, Se, Te). A comparison between calculated metric data and available experimental data for a wide range of TM = MG complexes will help in further assessing efficient computational approaches to TM complexes, particularly of the heavier MG elements, as a function of metal, ligand and level of theory. In the present work restricted Hartree Fock (RHF) and Møller-Plesset second order perturbation theory (MP2) wavefunctions were employed. In most cases there are small differences between RHF and MP2 calculated geometries, with both methods showing good agreement with experimental data, suggesting these approaches will be suitable for the study of larger, more experimentally relevant models. Changes in ZrE bond lengths for E = chalcogen (upon going from RHF to MP2) suggest a fundamentally different description between the Zr-oxo bond and heavier chalcogens, a result supported by recent experimental data for a series of Zr-chalcogenidos. To date no examples have been reported of arsinidene and stibinidene complexes. Computational results show similar behavior among the heavier pnictogen complexes, i.e., LnM = EH (E = P, As, Sb), suggesting that strategies used to synthesize phosphinidenes may be suitable in the search for the first LnM = AsR and LnM = SbR complexes. Additionally, calculations suggest that design of ligand sets which yield linearly coordinated phosphinidenes (and presumably As and Sb analogues) will lead to phosphinidenes with stronger metal-pnictogen bonds and increased thermodynamic stability versus nonlinearly coordinated examples. © 1994 John Wiley & Sons, Inc.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 16 (1995), S. 91-104 
    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 parallel direct self-consistent field (SCF) algorithm for distributed memory computers is described. Key features of the algorithm are its ability to achieve a load balance dynamically, its modest memory requirements per processor, and its ability to utilize the full eightfold index permutation symmetry of the two-electron integrals despite the fact that entire copies of the Fock and density matrices are not present in each processor's local memory. The algorithm is scalable and, accordingly, has the potential to function efficiently on hundreds of processors. With the algorithm described here, a calculation employing several thousand basis functions can be carried out on a distributed memory machine with 100 or more processors each with just 4 MBytes of RAM and no disk. The Fock matrix build portion of the algorithm has been implemented on a 16-node Intel iPSC/2. Results from benchmark calculations are encouraging. The algorithm shows excellent load balance when run on 4, 8, or 16 processors and displays almost ideal speed-up in going from 4 to 16 processors. Preliminary benchmark calculations have also been carried out on an Intel Paragon. © 1995 by John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 9 (1988), S. 771-778 
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Methods for the computation of one- and two-electron spin-orbit integrals over Gaussian-type basis functions are presented. We show that existing nuclear-attraction and electron-repulsion integral codes can be readily adapted for the efficient evaluation of spin-orbit integrals; in particular, one can take advantage of recent advances in the computation of derivative integrals. Recurrence relations for the nuclear attraction integrals are also developed.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 39 (1991), S. 31-45 
    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 intermediate neglect of differential overlap (INDO) model for elements of the second transition series is described. Values of exponents for the radial portion of the Slater basis atomic orbitals, the orbital ionization energies, and the beta (or bonding) parameters for these elements are reported. Use of the model to investigate equilibrium geometries, relative energy ordering of geometric isomers, spectroscopic transition energies, energy ordering of state wave functions of different symmetry, trends in dipole moments, and equilibrium stretching frequencies is reported. The results indicate that the INDO method is capable of yielding quality results for species containing atoms of the second transition series.
    Additional Material: 11 Tab.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 55 (1995), S. 315-328 
    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 effective core potential study of a model chemical vapor deposition (CVD) reaction is reported. Compounds studied are those in which a Group IVA (E = C, Si, Ge, Sn) main group element is directly bonded to a Group IVB (M = Ti, Zr, Hf) transition metal. Model reactants (H3M - EH3) possess Group IVA-Group IVB single bonds while products (H2M = EH2, formed by H2 elimination from reactants) have Group IVA-Group IVB multiple bonds. The main findings of this research are as follows: First, a single-determinant (RHF) description is appropriate for singly bonded Group IVA-Group IVB complexes. Agreement between experimental and calculated M-E single-bond lengths is very good at all levels studied. Second, electron correlation is of much greater importance for describing the ME π bond than the ME σ bond. Third, analysis of calculated double-to-single bond ratios (Rdouble/Rsingle) suggest that π-bonding remains nearly constant over the entire series of compounds studied and is weaker than in the main group-main group analogs, suggesting a reason for the lack of reported examples with a Group IVA-Group IVB double bound. Fourth, barriers to elimination of H2 from H3M - EH3 show two significant trends. We propose that the trends as a function of transition metal (Ti 〈 Zr 〈 Hf) and main group element (C 〉 Si 〉 Ge 〉 Sn) are due to trends in M - H and E - H bond energies. © 1995 John Wiley & Sons, Inc.
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