A Reconsideration of the Validity of Correlating Substituent-induced Proton Chemical Shifts in Aromatic Derivatives with Reactivity Parameters

Abstract

Comparisons of 1H chemical shifts and charge densities (determined by CNDO/2 MO calculations) for 4-substituted derivatives of styrene, toluene, benzylchloride, and N,N,N-trimethylphenylammonium ion indicate that the chemical shifts primarily reflect intramolecular electronic effects. These effects are reflected by correlations of 1H chemical shifts with the F and R reactivity parameters of Swain and Lupton. It is concluded that it is valid to correlate substituent-induced chemical shifts for aromatic derivatives with divided reactivity parameters provided that (1) chemical shifts are measured at infinite dilution in a non-polar medium and (2) at least ten derivatives of each family are investigated.Calculations for 4-substituted toluenes indicate that there will be a small conformational dependence for substituent-induced benzylic proton chemical shifts.