A theoretical approach to substituent effects. A comparison of the isoelectronic BH,CH3, and NH 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 is a powerful σ acceptor, with essentially no π interaction, BH 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 and NH with a series of substituents are an order of magnitude larger than corresponding values for CH₃. Interaction energies for BH are of opposite sign to those for NH. The results may be understood qualitatively using perturbation molecular orbital (PMO ) theory.