A valence bond for several perturbations

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Virginia Polytechnic Institute and State University


A valence bond perturbation method is developed for the purpose of treating the pi electron spectra of molecules related to a parent conjugated hydrocarbon molecule. A spectral perturbation reflects the change in pi electron transition energy of the parent hydrocarbon as a result of heteroatom replacement of one or more carbon atoms or introducing substituents. Such spectral perturbations may be correlated with inductive and mesomeric heteroatom or substituent effects.

Preliminary studies involve a comparison of semi-empirical valence bond and molecular orbital methods using similar approximations and integral values in each.

Zero order wave functions and energy eigenvalues required for the perturbation analysis are obtained from the semi-empirical valence bond calculation for butadiene. First order perturbation theory is applied to acrolein, gyloxal, acrylonitrile and methyl butadienes to study inductive effects. These molecules are iso-pi-electronic with butadiene. First and second order perturbation theory is applied to chloroprene, bromoprene, iodoprene and 1,4-dichlorobutadiene. Inductive effects as well as mesomeric effects which arise as a result of additional pi electrons contributing to the conjugated system by the halogen substituents are considered. Inductive and mesomeric effects of the halogen on spectral perturbations are separated and identified.

Values for empirical parameters required for agreement between calculated and observed spectral perturbations are correlated with relative substituent or heteroatom inductive and mesomeric effects.