UV photochemistry of pyridine-water and pyridine-ammonia complexes trapped in cryogenic matrices

In the quest to reach the “Photochemical Graal”, i.e. the photochemical homolytic water dissociation, we have carried out UV irradiation on the pyridine-water and pyridine-ammonia complexes trapped in cryogenic matrices (Ar, Ne and N2). The UV irradiation effects have been followed by FTIR spectroscopy in order to check the solid solvation effect. The experimental results have been compared systematically with theoretical ones, carried out at the UB3LYP and B3LYP/6-311 + G(d) levels of theory. The most salient result is that whatever the matrix and the pyridine partner, a UV irradiation, inducing uniquely pyridine electronic excitation, leads unambiguously to pyridine isomerisation toward the Dewar form while no water/ammonia dissociation and, concomitantly, no transfer toward pyridine of a H species has been observed unlike what we observed in molecular beam [Jouvet et al., PCCP, 2016, 18, 25637-25644]. From native pyridine or its Dewar form, no further photodissociation toward cyclobutadiene and hydrogen cyanide, has been observed.