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  • 1
    Digitale Medien
    Digitale Medien
    Protonated forms of 2-(2-furyl)pyrroles and their interconversion: proton NMR and quantum-chemical (MNDO) study (1992)
    s.l. : American Chemical Society
    The @journal of organic chemistry 57 (1992), S. 3934-3938 
    Details
    ISSN: 1520-6904
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/jo00040a039
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  • 2
    Digitale Medien
    Digitale Medien
    Theoretical evidence for a bound doubly-excited 1B2(C 1s,n→π*2) state in H2CO below the C 1s ionization threshold (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6716-6723 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The group of three lowest singlet C 1s-excited states of formaldehyde H2CO is studied theoretically. The equilibrium geometries are determined at the restricted open-shell Hartree–Fock (ROHF) level and refined total energies are obtained using the multireference configuration interaction (MRCI) approach. In agreement with an earlier prediction [Chem. Phys. 122, 9 (1988)] the second lowest singlet state, 1B2, is characterized by a doubly excited, "two particle–two hole" (2p–2h), configuration C 1s,n→π*2. Our calculations predict that H2CO in the 1B2(2p–2h) state has a stable pyramidal equilibrium structure with a barrier to inversion of 0.28 eV, the valence angle being close to 107°. The calculated length of the CO bond is 1.390 Å. The 1B2(2p–2h) state is shown to be also bound with respect to all possible dissociation and rearrangement processes. The lowest predicted dissociation energy for the 1B2 state (H2CO*→H2+CO* reaction) is 0.29 eV (6.69 kcal/mol). The rationalization of the great stability of the 1B2(2p–2h) state is the similarity of its electronic structure to that of the first singly-excited state 2A″(n) of nitroxyl radical H2NO⋅. The neighboring states 1B1(C 1s→π*) and 1A1(C 1s→3s) are characterized within the same framework. Spectroscopic implications and possibilities for the experimental identification of the 1B2(2p–2h) state are discussed. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311296
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  • 3
    Digitale Medien
    Digitale Medien
    A consistent third-order propagator method for electronic excitation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9982-9999 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A propagator method referred to as third-order algebraic–diagrammatic construction [ADC(3)] for the direct computation of electronic excitation energies and transition moments is presented. This approach is based on a specific reformulation of the diagrammatic perturbation expansion for the polarization propagator, and extends the existing second-order [ADC(2)] scheme to the next level of perturbation theory. The computational scheme combines diagonalization of a Hermitian secular matrix and perturbation theory for the matrix elements. The characteristic properties of the method are compact configuration spaces, regular perturbation expansions, and size-consistent results. The configuration space is spanned by singly and doubly excited states, while the perturbation expansions in the secular matrix extend through third order in the p-h block, second order in the p-h/2p-2h coupling block, and first order in the 2p-2h block. While the simpler ADC(2) method, representing a counterpart to the MP2 (second-order Møller–Plesset) ground-state method, recommends itself for application to larger molecules, the ADC(3) scheme is aimed at a more accurate description of molecular excitation spectra. The relationship of the ADC(3) scheme with coupled cluster methods is discussed, focusing here in particular on the treatment of transition moments. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.480352
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  • 4
    Digitale Medien
    Digitale Medien
    Vibronic structure of the valence π-photoelectron bands in furan, pyrrole, and thiophene (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 1025-1040 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The 2A2 and 2B1 states formed in the ionization of the outermost π orbitals in furan, pyrrole and thiophene are shown to interact vibronically via nontotally symmetric b2 vibrational modes. The interaction is strongest in pyrrole and thiophene, where the conical intersection between the two adiabatic surfaces occurs near the minimum of the upper (2B1) state. The resulting nonadiabatic effects manifest themselves in the 2B1 bands by a lack of resolved structure in case of pyrrole and thiophene, and by a line broadening in case of furan. The spectra are investigated using a linear vibronic coupling model. All totally symmetric a1 (tuning) modes and nontotally symmetric b2 (coupling) modes describing the ring motion are taken into account. The parameters of the model are obtained with the aid of ab initio calculations. The ground state optimized geometries and vibrational frequencies are computed at the level of the second-order Møller–Plesset perturbation theory, while the dependence of the ionization energies on the nuclear configuration is evaluated using the outer valence Green's function method. Where appropriate, assignments of the observed structure are given. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.476645
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  • 5
    Digitale Medien
    Digitale Medien
    A non-Dyson third-order approximation scheme for the electron propagator (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4734-4744 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An efficient third-order propagator method to compute ionization potentials and electron affinities of atoms and molecules is presented. The development is based on the algebraic diagrammatic construction (ADC) representing a specific reformulation of the diagrammatic perturbation series of the electron propagator G(ω). In contrast with previous approximation schemes, relying on the Dyson equation and approximations for the self-energy part, the ADC procedure here is applied directly to the (N(minus-plus)1)-electron parts G−(ω) and G+(ω), respectively, of the electron propagator. This leads to decoupled secular equations for the ionization energies ((N−1)-electron part) and electron affinities ((N+1)-electron part), respectively. In comparison with the Dyson-type approach, there is a substantial reduction of the secular matrix dimension opposed by a small additional expense in computing some second- and third-order contributions to the secular matrix elements. The relationship of the non-Dyson ADC(3) method to coupled cluster methods is outlined. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.477085
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  • 6
    Digitale Medien
    Digitale Medien
    Different formulations of restricted hartree-fock method for transition-metal atoms (1993)
    Springer
    Journal of structural chemistry 33 (1993), S. 786-794 
    Details
    ISSN: 1573-8779
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00745598
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  • 7
    Digitale Medien
    Digitale Medien
    Protonated forms of 2-(2-furyl)pyrroles and their interconversions (1989)
    Springer
    Chemistry of heterocyclic compounds 25 (1989), S. 1122-1133 
    Details
    ISSN: 1573-8353
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract It was shown by 1H NMR spectroscopy that 2-(5-R1-2-furyl)-3-R2-1-R1-pyrroles (R1= H, Me; R2=H, Me; R3=H, Et, CH=CH2) are protonated by acids (HSO3F, HCO2CF3, HCl, HBr) either at the C(5) atom of the pyrrole ring or at the C(5′) atom of the furan ring, depending on the conditions. The energies of formation (δH), the charges, and the partial electron densities of the boundary orbitals of 2-(2-furyl)pyrrole (I) and its protonated forms (IA and IB) were calculated by the MNDO method. The calculated δH values for the IA and IB forms are in agreement with their experimental ratio. According to the calculated reactivity indexes, the protonation of pyrrole is subject to orbital control and may include the prior formation of less stable protonated forms.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00470690
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  • 8
    Digitale Medien
    Digitale Medien
    Quantum-chemical investigation of the protonated forms of 2-(2-furyl)pyrrole (1991)
    Springer
    Chemistry of heterocyclic compounds 27 (1991), S. 583-590 
    Details
    ISSN: 1573-8353
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The electron and conformational structures, as well as the internal rotation, of 2-(2-furyl)pyrrole and its α-protonated forms were studied by the MNDO method with complete optimization of the geometry. In conformity with the experiments (PMR), the two delocalized cations with a cis orientation of the heteroatoms that are formed as a result of protonation of the pyrrole or furan ring have the greatest and virtually equal stabilities (δH=738.7 and 740.6 kJ/mole).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00472502
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  • 9
    Digitale Medien
    Digitale Medien
    Protonated forms of 2-(2-thienyl)pyrroles. Investigation by means of 1H NMR spectroscopy and MNDO calculations (1993)
    Springer
    Chemistry of heterocyclic compounds 29 (1993), S. 711-719 
    Details
    ISSN: 1573-8353
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract By means of 1H NMR spectroscopy it was demonstrated that 2-(2-thienyl)pyrrole and its 1-vinyl-substituted derivatives are protonated by adds (HSO3F, CF3CO2H, HCl, and HBr) at the C5 atom of the pyrrole ring. The reaction with the superacid system HSO3F/SbF5/SO2 leads to an equilibrium mixture of pyrrolium and thiophenium ions. The MNDO method was used to calculate the heats of formation (δH), charges, and partial electron densities of the highest occupied molecular orbitals (HOMO) of 2-(2-thienyl)pyrrole and its protonated forms. The calculated δH values of the thienylpyrrolium and pyrrolylthiophenium ions are in agreement with the experimentally established relationship.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00531554
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  • 10
    Digitale Medien
    Digitale Medien
    Coupling coefficients for systems with two open electronic shells: Transition metal ions with pM dN configuration (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 45 (1993), S. 363-383 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We derived the necessary conditions to which the vector coupling coefficients (VCC) amn and bmn, describing atomic L, S-multiplets of pMdN and dNs1 configurations (1 ≤ N ≤ 9, 1 ≤ M ≤ 5), should satisfy. It is shown that for two-open-shell systems under consideration the unknown VCC should satisfy not only usual restrictions resulting from the spheric symmetry, but also some additional equation introduced in the present paper in the form of a postulate. VCC obtained were used for the ab initio calculations (by the general SCF coupling operator method) of several transitionmetal atoms and ions with electronic configurations 3d14p1, 3p43d3, and 3p53d3. To check the presented theory, we carried out a detailed comparison between these results and analogous data, obtained by the atomic Roothaan-Hartree-Fock method [2]. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560450404
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