Inductive effect of methyl group in the light of HMO and DFT methods

Inductive effect parameter hMe of methyl group and the Coulomb integral α of sp2 hybridized carbon have been estimated from the HOMO energies of a series of methylbenzenes calculated by DFT/B3LYP method using the basis sets 6-31++G∗∗ and 6-311+G(2d,p). By correlating the DFT results with those from Coulson-Longuet-Higgins perturbation method in HMO formalism and with charge transfer transition energies of three series of CT complexes of methylbenzenes, it is established that the fundamental concepts used by organic chemists and the simple Hückel method still serve as a useful and easy alternative to the present-day computer-intensive methods in explaining experimental trends at least for π-conjugated systems. The parameter (hMe) value has been shown to be transferable from one series of compounds to another. Further it has been shown that hEt is about 15% higher than hMe, which is supported by the experimental observation that ethyl group exerts greater +I effect than methyl.