Modulating nitric oxide release by S-nitrosothiol photocleavage: mechanism and substituent effects.

The photochemistry and photophysics of a series of S-nitrosothiols (RSNOs) have been studied computationally. The photocleavage mechanism of the model compound CH(3)SNO to release CH(3)S· and ·NO was studied at the CASPT2 level resulting in a barrierless process when irradiating in the visible region (S(1)), in the near UV region (S(2)) and for photosensitized (T(1)) reaction. The absorption energy required to initiate photocleavage was calculated at the CASPT2 and B3P86 levels showing the possibility of the modulation of NO release by RSNO photoactivation as a function of the substituent R. Good correlations between the wavelengths of the lowest energy (1)(n,π*) and (1)(π,π*) transitions of aryl S-nitrosothiols and the corresponding Hammett constants of the substituents have been obtained.