Substituent effects on the bond dissociation enthalpies of aromatic amines

Bond dissociation enthalpy differences, Z-X ΔBDE = BDE(4-YC6H4Z-X) - BDE(C6H5Z-X), for Z = CH2 and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC6H4Z• radicals. The effects of Y are small (≤2 kcal/mol for all Y) for Z = CH2, but they are large for Z = O, where good correlations with σp+(Y) yield ρ+ = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H ΔBDEs correlate with σp+(Y), yielding ρ+ = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H ΔBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory, Calculations on 15 4-YC6H4NH2 gave N-H ΔBDEs correlating with σp+ (ρ+ = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H ΔBDEs of four 4,4′-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H ΔBDEs in 4-substituted anilines and O-H ΔBDEs in 4-substituted phenols into interaction enthalpies between Y and NH2/OH (molecule stabilization/destabilization enthalpy, MSE) and NH•/O• (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH2 vs OH and the weaker EW character of NH• vs O•. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X ΔBDEs in 4-YC6H4NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH2-X ΔBDEs in 4-YC6H4O-X and 4-YC6H4CH2-X. This suggestion was confirmed for X = CH3, H, OH, and F, for which the calculated NH-X ΔBDEs yielded p+ = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively.