Determination of inductive effect parameter of methyl group from gas-phase charge transfer bands of molecular complexes of N-phenyl-tetrachlorophthalimide with methyl substituted benzenes

Abstract

The charge transfer absorption bands of electron donor-acceptor (EDA) complexes of N-phenyltetrachlorophthalimide as acceptor with a series of methyl substituted benzenes as donors have been studied in dioxan, chloroform, dichloromethane and dimethylformamide as solvents. The absorption maxima (v_CT) of the complexes in these solvents deviate greatly from Mulliken's equation when plotted against donor ionisation potentials but the gas-phase values of v_CT, determined from the CT transition energies in the first three weakly polar solvents, give a reasonable fit to Mulliken type plot with the donor ionisation potentials. The energies of the HOMOs of methyl substituted benzenes, obtained as functions of the inductive effect PMO parameter (h_Me) of the methyl group by Coulson-Longuet- Higgins perturbation technique, give a linear correlation with the gas-phase hv_CT values of the complexes. From the slope of this plot, h_Me is found to be -0.1843, a value in fair agreement with that (-0.21) obtained by Lepley from studies of CT complexes of TCNE with a series of methyl substituted benzenes and naphthalenes.