Abstract: The heterolytic and homolytic N—NO bond dissociation energies of seven N-nitrosoindole derivatives were evaluated by using titration calorimetry and relative thermodynamic cycles. The energetic scales of the heterolytic and homolytic N—NO bond dissociation energies of N-nitrosoindoles cover the ranges from 206.1 to 246.2 kJ/mol and from 119.1 to 124.6 kJ/mol, respectively, which indicates that N-nitrosoindoles are much easier to release a NO radical(NO^·) rather than a NO cation(NO⁺). The estimation of the heterolytic and homolytic(N—NO)^-· bond dissociation energies of the N-nitrosoindoles radical anions gives the energetic ranges from 25.5 to 33.4 kJ/mol and from 5.0 to 40.5 kJ/mol for the(N—NO)^-· bond homolysis and heterolysis, respectively, which means that N-nitrosoindole radical anions are unstable at room temperature.

Key words: N-Nitrosoindoles, Radical anions of N-nitrosoindoles, N—NO bond energy, Titration calorimetry