| Abstract
| - Bond dissociation enthalpy differences, Z−X ΔBDE = BDE(4-YC6H4Z-X) − BDE(C6H5Z-X), forZ = CH2 and O are largely independent of X and are determined mainly by the stabilization/destabilizationeffect of Y on the 4-YC6H4Z• radicals. The effects of Y are small (≤2 kcal/mol for all Y) for Z = CH2, butthey are large for Z = O, where good correlations with σp+(Y) yield ρ+ = 6.5 kcal/mol. For Z = NH, two setsof 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 onN−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 wereunsuccessful; therefore, we turned to density functional theory. Calculations on 15 4-YC6H4NH2 gave N−HΔBDEs correlating with σp+ (ρ+ = 4.6 kcal/mol) and indicated that EW and ED Y's had comparablestrengthening and weakening effects, respectively, on the N−H bonds. To validate theory by connectingit to experiment, the N−H ΔBDEs of four 4,4‘-disubstituted diphenylamines and five 3,7-disubstitutedphenothiazines were both calculated and measured by the radical equilibration EPR technique. For allcompounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N−H ΔBDEs in4-substituted anilines and O−H ΔBDEs in 4-substituted phenols into interaction enthalpies between Y andNH2/OH (molecule stabilization/destabilization enthalpy, MSE) and NH•/O• (radical stabilization/destabilizationenthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilizethe radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituentson the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effectsare of comparable magnitudes. These differences arise from the stronger ED character of NH2 vs OH andthe weaker EW character of NH• vs O•. The relatively large contributions to N−H BDEs in anilines arisingfrom interactions in the molecules suggested that N−X ΔBDEs in 4-YC6H4NH-X would depend on X, incontrast to the lack of effect of X on O−X and CH2-X ΔBDEs in 4-YC6H4O-X and 4-YC6H4CH2-X. Thissuggestion was confirmed for X = CH3, H, OH, and F, for which the calculated NH−X ΔBDEs yielded ρ+= 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively.
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