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Ab initio study of the methyl internal rotation and aldehyde hydrogen wagging of thioacetaldehyde in the X1A′ and a3A″ states (1988)

Smeyers, Yves G. ; Niño, A. ; Bellido, M. N.

Springer

Theoretical chemistry accounts 74 (1988), S. 259-267

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ISSN: 1432-2234

Keywords: Internal rotation ; Aldehyde hydrogen wagging ; Potential energy functions ; Conformation in triplet excited state

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology

Notes: Abstract Ab initio SCF calculations have been performed for the methyl internal rotation and for aldehyde hydrogen wagging of thioacetaldehyde in the first triplet excited state, a3A″, as well as in the singlet ground state, X1A′. The preferred conformations for these states are the anti-eclipsed and the eclipsed ones, respectively. The calculated barrier heights to methyl rotation (118.3 and 455.6 cm−1 for a3A″ and X1A′, respectively) are in good agreement with the available experimental data. The singlet ground state and the triplet excited state exhibit a planar and pyramidal configuration, respectively. The inversion barrier of the pyramidal configuration is found to be very low 67.4 cm−1. Finally, the change of conformation and structure with the transition is explained by a change of hydridization of the aldehyde carbon atom due to an n → π * excitation.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00530223

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Atomic charge models for polypeptides derived from ab initio calculations (1989)

Bellido, M. N. ; Rullmann, J. A. C.

New York, NY [u.a.] : Wiley-Blackwell

Journal of Computational Chemistry 10 (1989), S. 479-487

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ISSN: 0192-8651

Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry

Source: Wiley InterScience Backfile Collection 1832-2000

Topics: Chemistry and Pharmacology , Computer Science

Notes: Ab Initio charge distributions for amino acid dipeptides are derived utilizing two medium-sized basis sets. Peptide charges differ in two ways from those of existing force fields: the magnitude of the peptide dipole and the dependency on the residue type. The merging of charge distributions of side chain and backbone fragments within a semiclassical model including polarization is investigated. Polarization plays a small, but distinct role in improving the correspondence with ab initio data derived for the complete dipeptide. A description in terms of partly overlapping, interacting fragments correlates well with the ab initio data. The method can be used to derive the electrostatic properties of biological macromolecules by combining accurate descriptions of short range interactions (using good quality basis sets on not too small fragments) with good classical models of long range interactions (using multicenter multipole expansions and atomic polarizability tensors). Factors limiting the accuracy of the present representations are discussed.

Additional Material: 2 Ill.