Caged Nitric Oxide-Thiyl Radical Pairs.

S-Nitrosothiols (RSNO) are exogenous and endogenous sources of nitric oxide in biological systems due to facile homolytic cleavage of the S-N bonds. By following the photolytic decomposition of prototypical RSNO (R = Me and Et) in Ne, Ar, and N2 matrixes (<10 K), elusive caged radical pairs consisting of nitric oxide (NO•) and thiyl radicals (RS•), bridged by O···S and H···N connections, were identified with IR and UV/vis spectroscopy. Upon red-light irradiation, both caged radical pairs (RS•···•ON) vanish and reform RSNO. According to the calculation at the CASPT2(10,8)/cc-pVDZ level (298.15 K), the dissociation energy of MeS•···•ON amounts to 4.7 kcal mol-1.