Tautomer selective photochemistry in 1-(tetrazol-5-yl)ethanol.

A combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(d,p) study of the molecular structure and photochemistry of 1-(tetrazol-5-yl)ethanol [1-TE] was performed. The potential energy surface landscapes of the 1H and 2H tautomers of the compound were investigated and the theoretical results were used to help characterize the conformational mixture existing in equilibrium in the gas phase prior to deposition of the matrices, as well as the conformers trapped in the latter. In the gas phase, at room temperature, the compound exists as a mixture of 12 conformers (five of the 1H tautomer and seven of the 2H tautomer). Upon deposition of the compound in an argon matrix at 10 K, only three main forms survive, because the low barriers for conformational isomerization allow extensive conformational cooling during deposition. Deposition of the matrix at 30 K led to further simplification of the conformational mixture with only one conformer of each tautomer of 1-TE surviving. These conformers correspond to the most stable forms of each tautomer, which bear different types of intramolecular H-bonds: 1H-I has an NH···O hydrogen bond, whereas 2H-I has an OH···N hydrogen bond. Upon irradiating with UV light (λ > 200 nm), a matrix containing both 1H-I and 2H-I forms, an unprecedented tautomer selective photochemistry was observed, with the 2H tautomeric form undergoing unimolecular decomposition to azide + hydroxypropanenitrile and the 1H-tautomer being photostable.