A theoretical approach to substituent effects. A comparison of the isoelectronic BH 3−, CH₃, and NH 3+ groups and their interaction with substituents in disubstituted benzenes

Ab initio molecular orbital theory with the STO‐3G basis set is used to examine both charge and energy interactions in a series of meta‐ and para‐substituted phenylborate anions and toluenes. Comparison of the results is made with data for substituted anilinium cations. It is concluded that whereas NH 3+ is a powerful σ acceptor, with essentially no π interaction, BH 3− is primarily a π donor, and, to a slight extent only, a π donor. CH₃ is indicated to be both a weak σ and π donor. Energies of interaction of BH 3− and NH 3+ with a series of substituents are an order of magnitude larger than corresponding values for CH₃. Interaction energies for BH 3− are of opposite sign to those for NH 3+. The results may be understood qualitatively using perturbation molecular orbital (PMO) theory.