Estimation of inductive effect PMO-parameter of t-butyl group from charge transfer bands of some π-molecular complexes of furan and thiophene derivatives

Abstract

The systematic red shift of the charge transfer (CT) absorption maxima of π-molecular complexes of tetracyanoethylene with furan and thiophene by <em>t</em>-butyl substitution at 2- and 3-positions of the donors has been utilised to get the inductive effect PMO parameter of <em>t</em>-butyl group. The HOMOs of furan and thiophene have been calculated by HMO method and those of the 2- and 3-<em>t</em>-butyl substituted derivatives have been obtained as a function of the PMO parameter (h_t) of the <em>t</em>-butyl group by applying Coulson-Longuet-Higgins perturbation method. The value of h_t is found to be -0.516, a value which is somewhat higher, as expected, than the PMO parameter for methyl group reported earlier for π -molecular complexes of methylated electron donors. Thiophene MOs have been calculated by two models and one of them, proposed by Longuet-Higgins, has been shown to be more reliable.