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 g as matrices and thus also can be assumed to dominate in the gas phase. Photolysis of tetrazole (4) in cryogenic matrices allows the IR-spect roscopic identification of a new CH2N2 isomer, the long sought-after n itrilimine HCNNH (6). Flash pyrolysis of 4 also yielded nitrilimine. U pon irradiation this species is converted to a second, previously unkn own ''isomer'', an HCN/NH complex 13. Nitrilimine (6) can also be gene rated by photolyzing 1,2,3- (1) and 1,2,4-triazole (2) in Ar matrices. The complex photochemistry of all three heterocyclic precursor compou nds was unveiled. Our findings were supported by isotopic substitution experiments and by high-level ab initio calculations. Moreover, the I R bands of iminocyanide HNCN were tentatively assigned. This radical h as up to now not been observed in a matrix.