EFFECTS OF DOUBLE SUBSTITUTION ON THE THERMODYNAMIC STABILITIES OF NITROGEN RADICALS

The gas-phase homolytic bond dissociation energies (BDE) of the N-H bo nds in forty-eight nitrogen-containing compounds( G-NH-G') were invest igated based on a cycle developed earlier utilizing acidity and electr ochemical data in solution. From the relative BDE data thus derived, c ombined with the relevant BDE's available in the literature, substitue nt effects on the thermodynamic stabilities of doubly-substituted nitr ogen radicals were systematically studied. The experimental results sh ow that the pattern of substituent effect on this type of nitrogen rad icals is guile different from that on similarly substituted carbon rad icals. It is generally observed that effects of the second substituent on radical stability are usually attenuated to a certain extent compa red with that of the first one due to the so-called ''saturation effec t''. 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 formin g a two-center-three-electron bond with a neighboring nitrogen radical , or, it is such an electron-withdrawing group that can facilitate for mation of an ''electron-delocalizing channel'' (e. g. Ph3P+, see the t ext), the two substituents (i.e. G and G') may then act concertedly to result in a ''synergistic effect'' on the stability of nitrogen radic als.