B—Z bond dissociation energies of para-substituted phenylboranes: a problem for Nau's definition of polar ground-state effects

Abstract

UB3LYP/6-311++g(2df,p) and RMP2/6-311++g(d,p) methods were used to calculate B—Z bond dissociation energies (BDE) of para-substituted phenylboranes (X—C6H4—BH—Z). It was found that the slopes (ρ) of regressions between B—Z BDEs and substituent σp constants increase in the order B—Cl (between –0.24 and 0.21 kcal/mol) < B—H (0.06-0.33 kcal/mol) < B—F (0.48-0.78 kcal/mol) < B—CH3 (0.83–1.28 kcal/mol) < B—Li (2.62-3.73 kcal/mol). Since all the homolysis reactions give the same boron radical, the large variation of ρ indicates that the substituent effects on energies of the ground-state molecules are important for the substituent effects on BDEs. However, the ground-state stabilization energies, calculated using either Nau's method or our method, do not show any correlation with the polarization of the B—Z bond as defined by the electronegativity difference between the group X—C6H4—BH and Z. Therefore, the theory that the remote substituent effects on Y—Z BDEs are dependent on the Y—Z polarity should be discarded.