A theoretical approach to substituent effects. Interaction between directly bonded groups in the isoelectronic series XNH₃⁺, XCH₃, and XBH₃^-

Ab initio molecular orbital theory including full geometry optimization at the 4-31G level is used to examine the interactions between substitutents X(X = Li, BeH, BH₂, CH₃, NH₂, OH and F) and substrates Y(Y = NH₃⁺, CH₃, BH₃^-) in the isoelectronic series XNH₃⁺, XCH₃ and XBH₃^-. The results indicate that the interaction energies are dominated by σ-effects. NH₃⁺ is found to interact favorably with the σ-donors (e.g. Li, BeH and BH₂) and unfavorably with the σ-acceptors (e.g. F, OH, NH₂). The reverse pattern a observed for XBH₃^-. The range of interaction energies for XCH₃ is considerably smaller than for XNH₃⁺ and XBH₃^-.