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Electronic Structure and Properties of Non‐Alternant Hydrocarbons
Author(s) -
Zahradník R.
Publication year - 1965
Publication title -
angewandte chemie international edition in english
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 0570-0833
DOI - 10.1002/anie.196510391
Subject(s) - linear combination of atomic orbitals , valence bond theory , molecular orbital theory , molecular orbital , molecular orbital diagram , atomic orbital , chemistry , slater type orbital , natural bond orbital , electronic structure , atomic physics , computational chemistry , valence (chemistry) , quantum mechanics , electron , physics , density functional theory , molecule
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 MOLCAO 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 2p z 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 2P z orbital of carbon and β is the resonance intergral of the CC 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 MOLCAO 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.