ISSN:
0570-0833
Keywords:
Nonalternant hydrocarbons
;
Alternant hydrocarbons
;
Hydrocarbons
;
Theoretical chemistry
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The present paper is concerned mainly with studies on the electronic structure of nonalternant hydrocarbons by quantum-chemical methods. The references to original publications are by no means exhaustive, but an attempt has been made to cover the entire field. (Analogues and derivatives of non-alternant hydrocarbons are only touched.) The intention is to outline the present situation with regard to quantum-mechanical studies on non-alternant hydrocarbons, and to indicate to the chemist the use of the theoretical characteristics obtained by various approximations of the MO—LCAO method.The simplest modification of the MO-LCAO method is the Hückel approximation (HMO), in which the molecular orbitals ψi are expressed as linear combinations of the atomic orbitals φj; in the π-electron approximation only the 2pz atomic orbitals are considered. For the interaction of n orbitals we have: (a) \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$$\psi_{\rm i} = \sum_{{\rm j}\,=\,1}^{\rm n} {\rm c}_{\rm ij}\varphi_{\rm j}$$\end{document} The π electron densities and the bond ordes can be calculated from the coefficients cij of the molecular orbitals. The energies Ei of the molecular orbitals are found by solving the determinantal equation' the n permitted levels are given by: (b) \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$${\rm E}_{\rm i} = \alpha + {\rm k}_{\rm i}\beta,$$\end{document} where α is the Columb integral of the 2Pz orbital of carbon and β is the resonance intergral of the C—C π-bond (and also the unit of energy in the HMO theory). The occupied molecular orbital with the highest energy is denoted by k1 (normal state), and the unoccupied level with the lowest energy by k-1 (valence state). The transfer of an electron from k1 to k-1 is called N → V1 excitation.The main difference between the HMO method and the more accurate MO—LCAO methods is that electronic interactions are explicitly taken into account in the latter; these methods are Pople's SCF (self-consistent field) method and Pariser and Parr's LCI (limited configuration interaction) method.
Additional Material:
11 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/anie.196510391
Permalink
