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

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 DeltaH degrees>(*) over bar * ( g) of an isodesmic reaction and can be calculated separately in the neutral molecules of acids (bases) and in anions (cations). Eight different reacti ons were investigated in which tradition assumes a purely inductive effect. Quantitative estimation was based on the known gas-phase enthalpies of ion ization, Delta H-ion degrees>(*) over bar * (g), and gas-phase enthalpies o f formation, Delta H-f degrees>(*) over bar * (g); some lacking values of t he latter were substituted by calculations at the MP2/6-31+G** or RHF/6-31G** level. Substituent effects in neutral molecules are not negligible, par ticularly in smaller molecules, but are qualitatively different from the co mmon 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 jus t for the energy difference between an ion and the respective neutral molec ule. It can be modeled as the energy of interaction between the given subst ituent and a positive or negative charge but it is quantitatively predicted in terms of electrostatics (Kirkwood-Westheimer theory) only with some con siderable disagreements. In water, this effect is strongly attenuated, diff erently in different classes of compounds; this attenuation can be in a qua litative accord with the electrostatic theory. Copyright (C) 2000 John Wile y & Sons, Ltd.