A theoretical study on the homolytic dissociation energies of H-N+ bonds

Various levels of theoretical calculations were performed to study the N+-H bond dissociation energies (BDEs) of protonated amines in order to check the experimental results and to investigate the substituent effects. It was found that the reported experimental N+-H BDEs in the gas phase are possibly not accurate. Our best predictions on the basis of CBS-Q and G3 calculations for the N+-H BDEs of NH4+, CH3NH3+, (CH3)(2)NH2+, (CH3)(3)NH+, PhNH3+, and pyridinium are 125 +/- 1, 110 +/- 1, 107 +/- 1, 95 +/- 1, 75 +/- 2, and 124 +/- 1 kcal mol(-1), respectively. In agreement with a previous study, it was also found that the solvent effects on the N+-H homolysis in acetonitrile are large, which significantly increases the N+-H BDEs compared to the gas phase. Further studies on the N+-H BDEs of protonated para-substituted anilines indicated that the substituent effects should have a slope of about 8.7 kcal mol(-1) against the substituent sigma(p)(+) constants. This value is larger than that for the O-H BDEs of phenols (6.7-6.9 kcal mol(-1)) and N-H BDEs of neutral anilines (3.0 kcal mol(-1)). The pattern of substituent effects is also completely different from that for the C-H BDEs of toluenes, as the C-H BDEs of toluenes are reduced by both the electron-withdrawing and -donating groups. Thus, we concluded that it is the electron demand of the system that dictates the substituent effects on BDEs. For the protonated aniline case, the origin of the substituent effects was found to be that an electron-withdrawing group destabilizes X-C6H4-NH2+. more than X-C6H4-NH3+, whereas an electron-donating group stabilizes X-C6H4-NH2+. more than X-C6H4-NH3+.