Radical Stabilization Energies of Substituted XNH• Radicals

Abstract

The performances of a number of theoretical methods in the calculation of N−H bond dissociation energies and radical stabilization energies associated with the X−NH• radicals were examined. It was found that the UHF, UMP2, and UMP4 methods were not reliable for the nitrogen radicals because of the spin contamination suffered by them. Surprisingly, the ROHF, ROMP2, and ROB3LYP methods were not found to be reliable for the nitrogen radicals either, because they could lead to unrealistic spin-localization of the radical. This unrealistic spin-localization was even seen with the UCCSD(T) method. The only credible modest-level method for the nitrogen radical was found to be UB3LYP, which could provide at least qualitatively correct radical stabilization energies for the nitrogen radicals. The basis set effect on the UB3LYP calculation was also found to be very small. Nevertheless, G3 and CBS-Q methods were found to be able to provide fairly accurate bond dissociation energies and radical stabilization energies for the substituted nitrogen radicals. According to G3 and CBS-Q results, CH3, NH2, OH, F, Cl, and CN were assigned to be stabilizing substituents for the nitrogen radical, as these groups could effectively delocalize the odd electron on the nitrogen radical. By contrast, COCH3, CONH2, COOH, and CHO were assigned to be destabilizing substituents for the nitrogen radical, although these groups could also delocalize the odd electron on the nitrogen radical. The origin of the destabilization effect was found to be the loss of the conjugation between the NH2 lone-pair electrons and the substituent from the neutral X−NH2 molecule to the X−NH• radical.