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  • 1
    Digitale Medien
    Digitale Medien
    Chlorogallanes (GaClH2, GaCl2H, and GaCl3) and their dimer isomers (1991)
    s.l. : American Chemical Society
    Inorganic chemistry 30 (1991), S. 4225-4229 
    Details
    ISSN: 1520-510X
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/ic00022a025
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  • 2
    Digitale Medien
    Digitale Medien
    Direct inversion in the iterative subspace (DIIS) optimization of open-shell, excited-state, and small multiconfiguration SCF wave functions (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 5728-5734 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The direct inversion in the iterative subspace (DIIS) method is applied to several simple SCF wave functions in an effective Fock matrix formulation. The following cases are treated: high-spin-restricted open shell, open-shell singlet, and two-configuration wave functions. Open-shell singlet states are described by a three-determinant 2×2 CAS expansion which is equivalent to Davidson's nonorthogonal SCF method in the case of the first open-shell singlet. Very sharp convergence is usually obtained in less than 20 cycles. The method is applicable to slowly convergent or even inherently divergent cases, and able to enforce convergence to excited states not the lowest of their symmetry. For these simple wave functions, the present first order method is asymptotically more efficient than second-order methods. Examples are presented for H2O, H2O2, C2H4, F2, several states of NO2, C2H5, formaldehyde, and ketene.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.449880
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  • 3
    Digitale Medien
    Digitale Medien
    Ammonia alane (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 5310-5317 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Ab initio studies employing basis sets approaching triple-ζ plus double-polarization plus f function quality are reported for AlH3NH3, AlH3, and NH3. The self-consistent-field method was employed for all basis sets; the single and double excitation configuration interaction and the comparable coupled-cluster methods were utilized with a double-ζ plus polarization functions basis set. The dissociation energy of AlH3NH3 is predicted to be 26 kcal/mol after correcting for zero-point vibrational energy, in qualitative agreement with the rough estimate of 30 kcal/mol by Haaland for Al–N bond strengths. In keeping with the dative nature of the Al–N bond, the AlH3 and NH3 moieties do not undergo large changes upon the formation of AlH3NH3. The resemblance of AlH3NH3 to the related compound BH3NH3 is quite strong. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities, atomic charges, and dipole moment of ammonia alane are reported.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462716
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  • 4
    Digitale Medien
    Digitale Medien
    Classical and nonclassical forms of the vinyl cation: A coupled cluster study (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3653-3658 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The energy difference between the classical and nonclassical forms of the vinyl cation is studied using the single and double excitation configuration interaction (CISD) and coupled cluster methods (CCSD and CCSDT-1). The basis sets employed range from double ζ to those including f functions on carbon and d functions on hydrogen. It is shown that the theoretical energy differences are very sensitive to the inclusion of higher-order polarization functions and the inclusion of triple excitations in the correlated wave functions. The results are compared with other theoretical studies in the literature and with the very recent experimental work of Crofton, Jagod, Rehfuss, and Oka. The most reliable single theoretical treatment involves the CCSDT-1 method with basis set C(10s5p2d1f/4s3p2d1f), H(5s2p/3s2p), and predicts 3.3 kcal/mol for the classical/nonclassical energy difference. Basis set extension is expected to increase this separation to about 4.0 kcal/mol, still less than the experimental value, 〉6 kcal/mol.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.457821
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  • 5
    Digitale Medien
    Digitale Medien
    Silaketene: A product of the reaction between silylene and carbon monoxide? (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1031-1035 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Molecular structures and vibrational frequencies for silaketene, H2SiCO, are predicted using ab initio techniques. A weakly bound nonplanar structure is found to be the equilibrium geometry, whereas the ketene-like planar structure is the transition state for inversion between the two equivalent nonplanar conformations. The barrier to inversion about the silicon atom is 18 kcal/mol. The ab initio heat of reaction of SiH2 and CO to form H2SiCO is 16 kcal/mol. The results are discussed in light of recent matrix isolation experiments attempting to make and detect dimethylsilaketene.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456155
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  • 6
    Digitale Medien
    Digitale Medien
    Structure and energetics of the lowest 1A1 and 1B1 states of dichlorocarbene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 2063-2067 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A recent (1989) partial structural analysis of spectroscopic measurements by Choe, Tanner, and Harmony led to the conclusion that the first excited electronic state (1B1 state) of CCl2 has a longer bond distance than does the 1A1 ground state. This is consistent with typical Franck–Condon pictures of diatomic potential curves and with experimental results for the analogous CF2 system. However, the earlier (1985) theoretical study of Nguyen, Kerins, Hegarty, and Fitzpatrick came to the qualitatively opposite conclusion, predicting the 1B1 bond distance to be shorter by 0.044 A(ring), which agrees with the 1989 analysis of rotationally resolved spectra by Clouthier and Karolczak. In this research high level ab initio quantum mechanical methods are used to clarify this question. The most reliable values predicted here for the CCl2 bond distances are r0=1.713 A(ring) (1A1) and r0=1.657 A(ring) (1B1). Large basis sets and high levels of electron correlation are required for excellent agreement with experiment in both structures and energies. The differences between CF2 and CCl2 in the behavior of the C–X bond length upon excitation is discussed. It is also argued that most closed shell carbenes are not two configuration problems.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459928
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  • 7
    Digitale Medien
    Digitale Medien
    An assessment for the full coupled cluster method including all single, double, and triple excitations: The diatomic molecules LiH, Li2, BH, LiF, C2, BeO, CN+, BF, NO+, and F2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 568-573 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The full coupled cluster singles, doubles, and triples (CCSDT) model for molecular electronic structure theory has been used in conjunction with two less complete coupled cluster methods (CCSD, CCSDT-1) to predict the equilibrium bond distances and harmonic vibrational frequencies of a family of closed-shell ground state diatomic molecules. The results are compared with the configuration interaction method including all single, double, triple, and quadruple excitations (CISDTQ) and experiment. With previous CCSDT results for HF, OH−, N2, and CO, the present work provides a realistic assessment of the CCSDT method. The BeO molecule in particular places great demands on the theory, and the linearized triple excitation model CCSDT-1 fails badly.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.458407
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  • 8
    Digitale Medien
    Digitale Medien
    The reaction of methane with molecular oxygen: A semiquantitative estimate of the activation energy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6391-6394 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The geometry and vibrational frequencies at the transition state for the abstraction of hydrogen atom from CH4 by O2 have been determined by ab initio quantum mechanical techniques. Configuration interaction including all single and double excitations (a total of 164 377 configurations in C1 symmetry) was used in conjunction with a double zeta plus polarization basis set. The molecular structures and vibrational frequencies for the reactants and products are also given so that reaction rate constants may be obtained for the forward and reverse reactions. The ab initio activation energy agrees quite well with previous rough estimates based on kinetic modeling and by analogy with related reactions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.456304
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  • 9
    Digitale Medien
    Digitale Medien
    UHF natural orbitals for defining and starting MC-SCF calculations (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 4926-4933 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A simple way of generating starting orbitals for multiconfigurational SCF calculations (particularly of the CAS type) is to use the natural orbitals of the unrestricted Hartree–Fock wave function. Significant fractional occupancy of a UHF natural orbital indicates that the orbital should be included in the active space; this is illustrated for symmetrically stretched water, for NO, N2O4, and for the transition state on the Me3CO→Me2CO+CH3 reaction surface. Average natural orbitals should be used if there are several UHF solutions for the same state; this restores symmetry broken at the the UHF level. Configuration selection based on fractional occupation appears to be the basis for a reliable automated procedure. The UHF natural orbitals provide good starting orbitals for the two-configuration SCF and for the 4×4 CAS wave function in stretched F2, H2O2, and C2H6, methane, water, in twisted ethylene, in ozone, and for various CAS wave functions in the first asynchronous transition state of the HCNO+H2C2 1,3-cycloaddition. Comparison is made with the extended Hartree–Fock method.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.454704
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  • 10
    Digitale Medien
    Digitale Medien
    Vinylidene: the final chapter? (1990)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 112 (1990), S. 8714-8719 
    Details
    ISSN: 1520-5126
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/ja00180a011
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