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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 104 (2000), S. 335-335 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-2234
    Keywords: INDO method for transition metals ; Copper chloride ; Iron chloride
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A complete Intermediate Neglect of Differential Overlap model suitable for the examination of transition metal complexes is described. The model is characterized by the inclusion of all the one-center exchange terms necessary for rotational invariance and accurate spectroscopic predictions, as well as an accurate description of integrals involving 3d atomic orbitals. The model is within the unrestricted Hartree-Fock formalism, and a method for spin purification is described. Problems with convergence of the self-consistent field are discussed, and a method that has been found successful in aiding the convergence is outlined. The model has been applied to many transition metal systems. In this article the results of calculations on the chlorides of Fe, Co and Cu are described. The results of these calculations are compared with experiment, and with the results of calculations by other methods.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-2234
    Keywords: Perturbation theory ; Fourth order perturbation theory ; Electron correlation ; Constant denominator perturbation theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A “constant denominator” perturbation theory is developed based on a zeroth order Hamiltonian characterized by degenerate subsets of orbitals. Such a formulation allows for a decoupling of the numerators of the perturbation sequence, allowing for much more rapid evaluation of the resultant sums. For example, the full fourth order theory can be evaluated as an N 6 step rather than N 7, where N is proportional to the basis set. Although the theory is general, a constant denominator is chosen for this study as the difference between the average occupied and average virtual orbital energies scaled so that the first order wavefunction yields the lowest possible variational bound. The third order correction then appears naturally as a scaled Langhoff-Davidson correction. The full fourth order with this partitioning is developed. Results are presented within the localized bond model utilizing both the Pariser-Parr-Pople and CNDO/2 model Hamiltonians. The second order theory presents a useful bound, usually containing a good deal of the basis set correlation. In all cases examined the fourth order theory shows remarkable stability, even in those cases in which the Nesbet-Epstein partitioning seems poorly convergent, and the Moller-Plesset theory uncertain.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-2234
    Keywords: INDO method for lanthanides ; Lanthanide halides ; Ce(NO3)6 2−
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An intermediate neglect of differential overlap method of use for examining the electronic structure of lanthanide complexes is developed. It is characterized by a basis set obtained from relativistic Dirac-Fock atomic calculations, the inclusion of all one-center two-electron integrals, and a parameter set based on molecular geometry. Lanthanide halides MX2, MX3 and MX4 are studied here, as well as initial results for the twelve coordinate Ce(NO3)6 −2 ion. Geometries obtained are in excellent agreement with experimental values when available. Many MX3 complexes are found to be pyramidal, and EuCl2 and YbCl2 are calculated to be bent even at the SCF level. Models invoking London type forces are therefore not required. Ionization potentials are calculated for the trihalides (δSCF) and are in reasonable agreement with experiment. Contrary to conclusion of others, f-orbital participation, although small, is required — at least in this model — to obtain the spread of metal to halide bond distance observed in these complexes. However f-orbital participation does not seem to be significant even in the twelve coordinate Ce(NO3)6 −2 complex: rather the large coordination number seems to be a consequence of the relatively large size of the lanthanide ion.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 103 (2000), S. 217-218 
    ISSN: 1432-2234
    Keywords: Key words: Roothaan equation – Roothaan–Hall – Hartree–Fock – Roothaan self-consistent field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. This paper reviews the title article by Clemens Roothaan and the huge impact that his paper has had in modern chemistry. In his paper Roothaan converts the molecular Schödinger equation into a matrix equation by systematically introducing the linear combination of atomic orbitals–molecular orbital approximation and by invoking the variational principle.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1432-2234
    Keywords: Inner-shell eigenvalues ; X-ray photoelectron spectra ; ESCA
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Methods are examined for calculating Inner-shell Eigenvalues from molecular orbital models which do not explicitly include core basis functions. If the “valence-orbital only” calculation is a good one, a rather straightforward method can be used to obtain core eigenvalues with a rms error of ±0.01 a.u. compared with ab-initio values. Even simpler methods can be used to reproduce trends among core eigenvalues. The AAMOM valence technique, and to a lesser extent, the INDO model, can be used to yield core eigenvalues for orbitals centered on carbon and nitrogen: for oxygen the agreement is poorer. Extended Hückel or Iterative Extended Hückel methods cannot be used for this purpose with any degree of confidence. Eigenvalues from ab-initio studies or from AAMOM and INDO can be used in assigning ionization processes (XPS) from orbitals localized on carbon and nitrogen: ionization processes from oxygen orbitals are not well treated. An attempt is made to explain this behaviour.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1432-2234
    Keywords: Triplet states, INDO calculations of ∼ ; Benzene ; Pyridine ; Diazines
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The intermediate neglect of differential overlap technique is modified and applied to the calculation of excited triplet states. The resulting method generally reproduces the transition energies of the better-classified observations within a rms error of 1000 cm−1. Trends are well reproduced, and the calculated orders ofn-π* and π-π* triplet states are in good accord with the experimental information to date. The method is applied to benzene and the azines. The lowest four triplet states of benzene are calculated in good accord with experiment. Pyridine is calculated to have an-π* triplet nearly degenerate with the lowest lying π-π* triplet, corroborating suggestions of Japar and Ramsay based on experimental information. A detailed analysis is made of the diazines, and assignments are suggested for the higher lying triplet states not yet classified or not yet observed.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1432-2234
    Keywords: Absorption spectra ; Emission spectra ; p-N,N-dimethylaminobenzonitrile ; Self-consistent reaction field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A self-consistent reaction field (SCRF) method that accounts for full electronic relaxation of the initial and final state in an electron transition is derived. The absorption and emission spectra of p-N,N-dimethylaminobenzonitrile (DMABN) and 6-cyanobenzquinuclidine (CBQ) are calculated in different solvents as a test of the method. The results from the fully relaxed SCRF method compare very well with results from a first-order relaxation SCRF model as well as with the experimental absorption and emission spectra of the two molecules considered in detail in this work.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1432-2234
    Keywords: Key words: Nitrogenase ; Nitrogen fixation ; FeMo enzymes ; Oxidation reduction of FeMo cofactor ; INDO
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. Reduced and oxidized forms of the FeMo- cofactor of Azotobacter vinelandii nitrogenase are examined theoretically within the intermediate neglect of differential overlap model. The results obtained favor one of the experimentally suggested modes of contraction of the metal system which results in an expansion of the central cavity of the cofactor. The bond index analysis indicates marked changes in the Mo coordination upon electron addition which may contribute to an opening of the Mo atom as a possible binding site at the advanced stages of the reduction process. In this work we also compare the 39- and 41-electron [MoFe7] core as possible native resting states, both compatible with known spin and Mössbauer spectroscopies.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0040-5744
    Keywords: Key words: Absorption spectra ; Emission spectra ; p-N ; N-dimethylaminobenzonitrile ; Self-consistent reaction field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  A self-consistent reaction field (SCRF) method that accounts for full electronic relaxation of the initial and final state in an electron transition is derived. The absorption and emission spectra of p-N,N-dimethylaminobenzonitrile (DMABN) and 6-cyanobenzquinuclidine (CBQ) are calculated in different solvents as a test of the method. The results from the fully relaxed SCRF method compare very well with results from a first-order relaxation SCRF model as well as with the experimental absorption and emission spectra of the two molecules considered in detail in this work.
    Type of Medium: Electronic Resource
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