Conformational Analysis, Spectroscopy and Photochemistry of Matrix-Isolated 1H- and 2H- Tautomeric Forms of Ethyl Tetrazole-5- Carboxylate

Abstract The preferred conformations, infrared spectra and photochemistry of matrix-isolated 1H- and 2H- tautomers of ethyl tetrazole-5-carboxylate (ET5C) were investigated. Experimentally, isolated monomeric forms of the two tautomers were trapped in cryogenic argon matrices (15 K), and studied by infrared spectroscopy. Identification of the conformational species trapped in the matrices was undertaken by combining the experimental spectroscopic data with results of quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of theory. For each tautomer, two conformers were observed in the matrices, all forms exhibiting the ester group in the anti configuration (C–O–C–C dihedral equal to 180o). In turn, the conformers having a gauche ester group that are predicted by the calculations to be significantly populated in the gas phase equilibrium (room temperature), were found to convert into the anti forms during matrix deposition (conformational cooling). Very interestingly, and contrarily to what is in general found for tetrazoles, the 1H- tautomer is the most stable tautomer of ET5C, and strongly predominates in the studied matrices. Narrowband UV irradiation (λ =250 nm) was found to lead to tetrazole fragmentation, with release of molecular nitrogen and production of four isomeric photoproducts (diazirine, cyanamide, carbodiimide and nitrile imine), all of them being successfully identified through comparison of the experimental spectroscopic spectra of the irradiated matrices with the DFT(B3LYP)/6-311++G(d,p) predicted infrared spectra of these species. This study also stresses the structural relevance of the N–H//O = C intramolecular interaction between the ring hydrogen atom and the carbonyl group of the substituent in the 1H-tautomer of ET5C.