Tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution

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High-level ab initio molecular orbital calculations, using basis sets up to 6-311+G(2d,2p) with electron correlation incorporated at the quadratic configuration interaction [QCISD(T)] level, have been used to study the tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution. The solvent effects were investigated by self-consistent reaction field (SCRF) theory. Consistent with experimental observations, the 1H-tetrazole (1)/2H-tetrazole (2) tautomeric equilibrium is calculated to be strongly influenced by the surrounding medium. 2H-Tetrazole is the energetically preferred tautomer in the gas phase. In a nonpolar solution, both the 1H and 2H forms are predicted to exist in comparable amounts. However, in a medium of high dielectric constant the more polar 1H tautomer is the dominant species. The calculated free energy changes for tautomerization of IH-tetrazole in the gas phase and in nonpolar (epsilon = 2) and polar (epsilon = 40) media are -7, 1, and 12 kJ mol-1, respectively. The molecular geometry, charge distribution, and vibrational frequencies of the polar 1H tautomer are found to be altered significantly in the presence of a solvent reaction field. Isomerization of 1 to 2, via a [1,2] hydrogen shift, requires an energy barrier of 207 kJ mol-1 in the gas phase. 5H-Tetrazole (3) is predicted to lie 82 kJ mol-1 above 1, due to its nonaromatic character. However, rearrangement of 3 to 1, via a [1,5] hydrogen shift, is inhibited by an activation barrier of 150 kJ mol-1. Conversely, the energy barrier for the rearrangement of 1 to 3 is 232 kJ mol-1, slightly larger than that required for the isomerization of 1 to 2. These results suggest that 3 is a good candidate for experimental observation. Inclusion of electron correlation leads to a drastic change in the molecular geometry of 3. At the MP2 level, an acyclic structure is predicted, while at the MP3 and QCISD levels the expected cyclic structure is found, The calculated molecular geometry of 1H-tetrazole at the MP2 level is found to differ significantly from the available solid-state structural data.