Photochemical Fragmentation of Unsubstituted Tetrazole, 1,2,3-Triazole, and 1,2,4-Triazole: First Matrix-Spectroscopic Identification of Nitrilimine HCNNH

Equilibria between the tautomers of heterocyclic azoles like 1, 2, and 4 have been studied many times. We here show that the application of density functional methods in combination with matrix IR spectroscopy is a useful tool for determining which protomer is preferred in rare gas matrices and thus also can be assumed to dominate in the gas phase. Photolysis of tetrazole (4) in cryogenic matrices allows the IR-spectroscopic identification of a new CH2N2 isomer, the long soughtafter nitrilimine HCNNH (6). Flash pyrolysis of 4 also yielded nitrilimine. Upon irradiation this species is converted to a second, previously unknown “isomer”, an HCN/NH complex 13. Nitrilimine (6) can also be generated by photolyzing 1,2,3- (1) and 1,2,4-triazole (2) in Ar matrices. The complex photochemistry of all three heterocyclic precursor compounds was unveiled. Our findings were supported by isotopic substitution experiments and by high-level ab initio calculations. Moreover, the IR bands of iminocyanide HNCN were tentatively assigned. This radical has up to now not been observed in a matrix.