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  • 1
    Electronic Resource
    Electronic Resource
    Ziegler-Natta Ethylene Insertion Reaction for a Five-Coordinate Titanium Chloride Complex Bridged to an Aluminum Hydride Cocatalyst (1995)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 117 (1995), S. 4109-4117 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja00119a027
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  • 2
    Electronic Resource
    Electronic Resource
    Titanium-Ethylene Complexes Proposed To Be Intermediates in Ziegler-Natta Catalysis. Can They Be Detected through Vibrational Spectroscopy? (1995)
    s.l. : American Chemical Society
    Organometallics 14 (1995), S. 4349-4358 
    Details
    ISSN: 1520-6041
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/om00009a044
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  • 3
    Electronic Resource
    Electronic Resource
    Evaluation of PM3(tm) as a Geometry Generator in Theoretical Studies of Transition-Metal-Based Catalysts for Polymerizing Olefins (1997)
    Springer
    Journal of molecular modeling 3 (1997), S. 193-202 
    Details
    ISSN: 0948-5023
    Keywords: Keywords: PM3(tm) ; polymerization ; ethylene ; geometry optimization ; insertion barrier
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The PM3(tm) method has been applied to several systems of relevance to catalytic polymerization of olefins, for catalysts containing Ti, Zr or Cr. With some exceptions, PM3(tm) calculations reproduce experimental geometries of stable, closed-shell, precursors well. For stationary points along the path of monomer insertion into a metal-alkyl bond, the comparison is made to structures obtained by optimization using various first-principle methods. Large errors are uncovered for the transient structures, in particular pertaining to metal-ethylene coordination and agostic interactions. The energy profiles for four insertion reactions are computed by gradient-corrected density functional (DFTG) methods, using molecular structures taken from PM3(tm) and first-principle geometry optimizations, respectively. The chromium case is promising, giving values for the barrier to monomer insertion of 11 and 9 kcal/mol based on PM3(tm) and DFTG geometries, respectively. The Ti- and Zr-based systems are predicted to proceed downhill based on PM3(tm) structures, whereas small barriers are found when using first-principle structures. A hybrid PM3(tm)-DFTG procedure is suggested for geometry optimization, which facilitates an accurate estimate of the barrier when applied to one of the zirconium systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s008940050031
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  • 4
    Electronic Resource
    Electronic Resource
    Accurate and approximate calculations of Franck–Condon intensities in the carbon 1s photoelectron spectrum of methane (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7979-7985 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The level of theory needed in order to accurately compute the bond contraction following core ionization in methane is examined with an emphasis on one-electron bases and treatment of electron correlation. At our highest level of theory, including core–valence electron correlation, a value of 4.82 pm is computed for the bond contraction in core-ionized methane, in good agreement with experimental findings. The associated potential energy curve in the symmetric stretching coordinate is used to form relative intensities of the peaks in the C 1s photoelectron spectrum of methane. For use in more approximate studies, it is proposed that the ionized core may be conveniently represented by effective core potentials during geometry optimization, and the prospect of this approach is explored in some depth. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481398
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  • 5
    Electronic Resource
    Electronic Resource
    Evidence of Fermi resonance in core-ionized methane (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7986-7991 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A full quartic potential energy surface is determined for core-ionized methane and used to investigate coupling between vibrational modes. A strong Fermi resonance is found between the first excited state of the symmetric stretching mode ν1′ and a doubly excited bending mode, whereas the corresponding interaction is less pronounced for v1′=2. In terms of the carbon 1s photoelectron spectrum of methane, the net effect of the mode coupling is to reduce the apparent contribution from anharmonicity to peak positions. The contribution from anharmonicity to the intensity of each peak is dominated by cubic and quartic terms in the symmetric stretching coordinate, and remains significant. This resolves a paradox pointed out in a recent experimental work [Carroll et al., Phys. Rev. A 59, 3386 (1999)]. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481399
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  • 6
    Electronic Resource
    Electronic Resource
    Molecular-field splitting and vibrational structure in the phosphorus 2p photoelectron spectrum of PF3 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 4472-4477 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The experimental phosphorus 2p spectrum of PF3 shows vibrational structure corresponding to the symmetric phosphorus-fluorine stretching mode, with a fundamental frequency of 121.7 meV. In addition, the spectrum shows that the 2p3/2 peak is a doublet split by 146 meV. This molecular-field splitting arises because the anisotropic molecular environment lifts the threefold degeneracy of the 2p orbitals. The observed vibrational structure is compared with that predicted by theory, either treating the core hole explicitly or using the equivalent-cores approximation to simulate the effects of the core hole. Ab initio calculations of the molecular-field splitting that include core-valence electron correlation predict a value in good agreement with the experimental value. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479739
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  • 7
    Electronic Resource
    Electronic Resource
    Molecular-field splitting of the 2p3/2 peak in x-ray photoelectron spectra of second-row atoms: A theoretical study of phosphine and phosphorus trifluoride (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 4478-4486 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory of molecular-field-induced splitting of the 2p3/2 peak in x-ray photoelectron spectra of second-row atoms is reexamined, using PH3 and PF3 as examples. Existing theory, which includes crystal field and pseudoatomic valence effects, is found to describe correctly the electrostatic field in the 2p region, but, nevertheless, to severely underestimate the magnitude of the resulting molecular-field splitting. Numerical simulations show that core-valence correlation in the core ionized, final states is responsible for amplifying the molecular-field splitting to the extent that it is observable with the present resolution. A detailed analysis based on second-order Møller–Plesset theory is presented of these correlation effects. A qualitative model is developed which predicts that the contribution from core-valence correlation scales with the asymmetry of the valence shell at the atom of interest. This represents an extension of the pseudoatomic valence theory, to take into account not only electrostatic and exchange effects, but also core-valence correlation. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479211
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  • 8
    Electronic Resource
    Electronic Resource
    Methane dissociation on a nonplanar MgO(001) surface. Theoretical modeling of surface defects (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4626-4631 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate ab initio quantum mechanical calculations are presented for the abstraction of hydrogen from methane over three- and four-coordinated oxygen sites at an imperfect MgO surface. A local portion of the lattice close to the reaction site is described using a cluster model, whereas the surrounding crystal is represented by its Madelung potential. The four-coordinated oxygen has a closed-shell electronic structure and shows no activity towards hydrogen abstraction. The three-coordinated oxygen is a singly charged radical with high hydrogen affinity and the abstraction takes place here with a barrier of 15 kcal/mol. Subsequent adsorption of the methyl radical is found to be feasible and the adsorption geometry is partly optimized. An extension of the point-charge model in terms of projection operators is found useful when describing Pauli repulsion between the adsorbed methyl entity and neighboring Mg2+ cores.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461731
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  • 9
    Electronic Resource
    Electronic Resource
    An investigation of the quantum chemical description of the ethylenic double bond in reactions. I. The electrophilic addition of hydrochloric acid to ethylene (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6910-6920 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The bimolecular reaction mechanism for adding hydrochloric acid to ethylene is studied within a number of contemporary quantum chemical models. The transition state structure and energy is examined in detail, and high-level calculations support a picture of an intimate association of a chloride anion and a bridged ethyl cation, with some covalent bonding retained between chlorine and hydrogen. A tunneling correction of 1 kcal/mol in the reaction barrier is obtained by the Bell equation. The methods employed include configuration interaction (CI) and gradient-corrected density functional theory (DFTG), with a range of basis sets. High-end CI and DFT methods perform equally well with respect to enthalpies of reaction and activation, when used in connection with large bases. However, when bases of double-ζ plus polarization quality are used, an unfortunate accumulation of errors makes DFTG inferior. This is in contrast to the excellent values for a number of ground state properties of relevance to the reaction barrier which are obtained at this level of theory. A recently proposed extrapolation procedure for CI energies (PCI-80) is shown to fail for the electron affinity of chlorine, leading to an exaggerated estimate of the barrier for the title reaction. The single-particle description of the π bond in ethylene converges very slowly with the number of basis functions, and this affects the reaction enthalpy as computed by DFT and CI methods to a similar degree. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471984
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  • 10
    Electronic Resource
    Electronic Resource
    High resolution photoelectron spectroscopy of sulfur 2p electrons in H2S, SO2, CS2, and OCS (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 9035-9039 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-resolution photoelectron spectra for the 2p electrons in H2S, SO2, CS2, and OCS show the effects of vibrational excitation in the core-excited species as well as the splitting of the 2p3/2 hole state by the molecular field. Theoretical calculations at the Hartree–Fock level account reasonably well for the vibrational structure. The molecular-field splitting is calculated with a configuration interaction-based method using large basis sets. This produces values for the 2p3/2 splitting of 108, 96, 129, and 144 meV for the title molecules, to be compared with experimental values of 110, 105, 140, and 150 meV. Thus all observed features in the spectra are quantitatively accounted for by theoretical modeling. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472761
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