An SCMO calculation of the spin densities in the methyl substituted benzene negative ions

Abstract

Most of the many attempts to explain ‘hyperconjugation’ invoke an explicit delocalization of the electrons of the methyl group. This work is part of an attempt to calculate these effects by a method which requires no such delocalization. A calculation of the spin densities in the methyl substituted benzene negative ions is reported here. In this work the methyl substituent is considered to exert a purely inductive effect which manifests itself in a change in the scale of the orbital exponent of the 2pπ atomic orbital at the carbon to which it is attached. (This is in contrast to previous ‘inductive’ treatments in the HMO method which alter only the α of the substituted carbon.) This change is evaluated by considering the change in the ionization potential of the methyl radical on successive methyl substitution. (There is no reference to any EPR data for the calibration of the parameters.) A comparison with the EPR results for the toluene, m-xylene and p-xylene negative ions reveals excellent agreement for these compounds. An open-shell SCMO procedure based on that of Roothaan is used in the calculations.