Re‐evaluation of the inductive effect in isolated molecules and in solution

Abstract

The classical concept of inductive effect based originally on dissociation constants in water was revisited on isolated molecules in the gas phase. It then has a strict thermodynamic meaning of ΔHo(g) of an isodesmic reaction and can be calculated separately in the neutral molecules of acids (bases) and in anions (cations). Eight different reactions were investigated in which tradition assumes a purely inductive effect. Quantitative estimation was based on the known gas-phase enthalpies of ionization, ΔionHo(g), and gas-phase enthalpies of formation, ΔfHo(g); some lacking values of the latter were substituted by calculations at the MP2/6–31+G** or RHF/6–31+G** level. Substituent effects in neutral molecules are not negligible, particularly in smaller molecules, but are qualitatively different from the common scale of inductive effect and cannot at present be described in terms of any simple theory. The concept of inductive effect is to be reserved just for the energy difference between an ion and the respective neutral molecule. It can be modeled as the energy of interaction between the given substituent and a positive or negative charge but it is quantitatively predicted in terms of electrostatics (Kirkwood–Westheimer theory) only with some considerable disagreements. In water, this effect is strongly attenuated, differently in different classes of compounds; this attenuation can be in a qualitative accord with the electrostatic theory. Copyright © 2000 John Wiley & Sons, Ltd.