title: Conformational behavior and tautomer selective photochemistry in low temperature matrices: the case of 5-(1H-tetrazol-1-yl)-1,2,4-triazole.
abstract: The conformational properties and the photolysis behavior of one of the simplest N-C bonded bicyclic azoles, 5-(1H-tetrazol-1-yl)-1,2,4-triazole (T), were studied in argon and xenon matrices by infrared spectroscopy. Analysis of the experimental results was supported by extensive theoretical calculations carried out at the B3LYP/6-311++G(2d,2p) level of approximation. Out of the eight T minima located on the potential energy surface, the three most stable species were detected in low temperature matrices, namely, 5-(1H-tetrazol-1-yl)-1H-1,2,4-triazole (T1) and two conformers of 5-(1H-tetrazol-1-yl)-2H-1,2,4-triazole (T2a and T2b). With increase of the substrate temperature either during deposition of the matrices or during annealing the T2b → T2a conversion took place, in agreement with the predicted low energy barrier for this transformation (5.38 kJ mol(-1)). Both broad band and narrow band laser UV irradiations of T isolated in Xe and Ar matrices induce unimolecular decomposition involving cleavage of the tetrazole ring of T1 and T2a (T2b) that leads to the production of 1H-1,2,4-triazol-5-yl carbodiimide (P1) and 1H-1,2,4-triazol-3-yl carbodiimide (P2), respectively. When the laser is used, in addition to the main P1 and P2 photoproducts, several minor products could be successfully identified in the matrices: N-cyanocarbodiimide HNCNCN (detected for the first time) associated with nitrilimine HNNCH and HCN. An interesting phenomenon of tautomer-selective photochemistry was observed for the matrix-isolated compound. It could be explained by the different LUMO-HOMO energy gaps estimated for T1, T2a, and T2b, connected with different threshold energies necessary to start the photolysis of T1 and T2a (T2b).
authors.affiliation: Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland.
authors.forename: M
authors.initials: M
authors.lastname: Pagacz-Kostrzewa
authors.forename: I D
authors.initials: ID
authors.lastname: Reva
authors.forename: R
authors.initials: R
authors.lastname: Bronisz
authors.forename: B M
authors.initials: BM
authors.lastname: Giuliano
authors.forename: R
authors.initials: R
authors.lastname: Fausto
authors.forename: M
authors.initials: M
authors.lastname: Wierzejewska
mesh_terms: D008968:Molecular Conformation; D010777:Photochemistry; D011789:Quantum Theory; D013237:Stereoisomerism; D013696:Temperature; D013777:Tetrazoles / Q000138:chemical synthesis / Q000737:chemistry*; D014230:Triazoles / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: C560952:5-(1H-tetrazol-1-yl)-1,2,4-triazole; D013777:Tetrazoles; D014230:Triazoles