Ab initio calculations on 4-substituted styrenes: a theoretical model for the separation and evaluation of field and resonance substituent parameters

Abstract

Ab initio (STO-3G minimal basis set) molecular orbital calculations on 4-substituted styrenes and CH3X–C2H4 pairs indicate that ΔqH (the charge density difference between β-vinyl protons) provides a direct theoretical measure of the field effect of a polar substituent. Σqcπ, the total substituent-induced π electron density change in styrene provides a theoretical measure of the resonance effect of the substituent. ΔqH and Σqcπ provide the basis of a theoretical dual substituent parameter scale, TF and TR0. This scale compares very closely with experimental DSP scales, indicating the latter scales are fundamentally valid. The calculations also indicate that field effects and σ inductive effects do not show parallel trends. It is concluded that field effects are more important than σ inductive effects except at very close range. Individual carbon π electron densities reflect both resonances effects and polar π inductive effects.