Effects of double substitution on the thermodynamic stabilities of nitrogen radicals

Abstract

The gas-phase homolytic bond dissociation energies (BDE) of the N—H bonds in forty-eight nitrogencontaining compounds (G-NH-G’) were investigated based on a cycle developed earlier utilizing acidity and electrochemical data in solution. From the relative BDE data thus derived, combined with the relevant BDE’s available in the literature, substituent effects on the thermodynamic stabilities of doubly-substituted nitrogen radicals were systematically studied. The experimental results show that the pattern of substituent effect on this type of nitrogen radicals is quite different from that on similarly substituted carbon radicals. It is generally observed that effects of the second substituent on radical stability are usually attenuated to a certain extent compared with that of the first one due to the socalled “saturation effect”. If, however, the second group is of the type that bears a pair of unshared electron (e.g. NH2, OH, etc.) and thus is capable of forming a two-center-three-electron bond with a neighboring nitrogen radical, or, it is such an electron-withdrawing group that can facilitate formation of an “electron-delocalizing channel” (e.g. Ph3P+, see the text), the two substituents (i. e. G and G’) may then act concertedly to result in a “synergistic effect” on the stability of nitrogen radicals.