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Approximate solutions of Heisenberg Hamiltonians (1987)

Sanchez-Marin, J. ; Malrieu, J. P. ; Maynau, D.

New York, NY : Wiley-Blackwell

International Journal of Quantum Chemistry 31 (1987), S. 903-925

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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: Even when each atom of a 2n-center cluster or molecule only brings one active electron in one atomic orbital, the size of the Heisenberg Hamiltonian matrix increases as C2nn. Simple truncations of this matrix would result in size consistence defects, as evident from the isomorphism between Heisenberg and configuration interaction (CI) matrices. Geometry-dependent Heisenberg Hamiltonians derived from accurate ab initio calculations on the two-center systems have proved to be very efficient for conjugated hydrocarbons and for alkali metals; in order to apply this approach to intermediate size systems (10-20 centers), a rational procedure is proposed consisting of the selection of a truncated set of determinants (of low energy) and a dressing of the truncated Hamiltonian matrix under the perturbation of the other determinants. The second order dressing is analogous to a so-called “shift Bk procedure” or Generalized Degenerate Perturbation theory and is weakly dependent on an E0 parameter. Tests performed on various 8- and 10-atom systems show the accuracy of the procedure. An iterative selection of the truncated basis set and proper choices of the E0 values allow one to obtain the whole lower part of the spectrum. The calculated geometries are satisfactory. Some preliminary applications are reported concerning the C12H14 dodecahexene linear chain, perfectly fitting with previous extrapolations.

Zusätzliches Material: 6 Ill.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1002/qua.560310606

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Digitale Medien

Indirect 1H—19F—1H and 1H—19F—13C nuclear overhauser enhancements: Anomalous intensity effects and applications in conformational analysis (1987)

Sánchez-Ferrando, Francisco ; Sanders, Jeremy K. M.

Chichester : Wiley-Blackwell

Organic Magnetic Resonance 25 (1987), S. 539-543

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ISSN: 0749-1581

Schlagwort(e): Indirect heteronuclear NOEs ; 1H-19F-1H and 1H-19F-13C systems ; Three-spin effects ; Anomalous intensity effects ; Conformational analysis ; Chemistry ; Analytical Chemistry and Spectroscopy

Quelle: Wiley InterScience Backfile Collection 1832-2000

Thema: Chemie und Pharmazie

Notizen: Indirect nuclear Overhauser enhancements through 19F have been determined for the three-spin systems 1H-19F-1H and 1H-19F-13C by steady-state presaturation of the appropriate proton, and applied to the conformational analysis of 5-bromo-2-fluoro-4-nitroacetanilide. The 1H NOE difference spectra at 400 MHz showed antiphase doublets for protons undergoing simultaneous dipolar relaxation with, and scalar coupling to, 19F. Similarly, in the 13C selective NOE difference spectrum at 100 MHz, the two lines of the fluorine-bearing carbon showed negative enhancement factors of different magnitude on presaturation of the appropriate proton. The use of these anomalous intensity effects in conformational analysis is illustrated.

Zusätzliches Material: 4 Ill.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1002/mrc.1260250617

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