Determination of N-NO bond dissociation energies of N-methyl-N-nitrosobenzenesulfonamides in acetonitrile and application in the mechanism analyses on NO transfer

Abstract

The heterolytic and homolytic N-NO bond dissociation energies of seven substituted N-methyl-N-nitrosobenzenesulfonamides (abbreviated as G-MNBS, G = p-OCH(3), p-CH(3), p-H, p-Cl, p-Br, 2,5-2Cl, m-NO(2)) in acetonitrile solution were evaluated for the first time by using titration calorimetry and relative thermodynamic cycles according to Hess' law. The results show that the energetic scales of the heterolytic and homolytic N-NO bond dissociation energies of G-MNBS in acetonitrile solution cover the ranges from 44.3 to 49.5 and from 33.0 to 34.9 kcal/mol for the neutral G-MNBS, respectively, which indicates that N-methyl-N-nitrosobenzenesulfonamides are much easier to release a NO radical (NO(*)) than to release a NO cation (NO(+)). The estimation of the heterolytic and homolytic (N-NO)(-)(*) bond dissociation energies of the seven G-MNBS radical anions in acetonitrile solution gives the energetic ranges of -15.8 to -12.9 and -3.1 to 1.8 kcal/mol for the (N-NO)(-)(*) bond homolysis and heterolysis, respectively, which means that G-MNBS radical anions are very unstable at room temperature and able to spontaneously or easily release a NO radical or NO anion (NO(-)), but releasing a NO radical is easier than releasing NO anion. These determined N-NO bond dissociation energies of G-MNBS and their radical anions have been successfully used in the mechanism analyses of NO transfer from G-MNBS to 3,6-dibromocarbazole and the reactions of NO with the substituted N-methyl-benzenesulfonamide nitranions (G-MBSN(-)) in acetonitrile solution.