Theoretical study on the solvent influence on 1,2,3-triazole tautomeric equilibrium. A comparison of incremental microsolvation and continuum solvation model approaches

The effect of solvent on the 1,2,3-triazole tautomeric equilibrium was studied by calculating Gibbs free energies of tautomerization in the gas-phase and in water solution at MP2/aug-cc-pVDZ, B3LYP/6-311++G(d,p) and wB97XD/6-311++G(d,p) levels of theory. Three models of solvation were investigated: (1) gas-phase incremental microsolvation with explicit water molecules leading to (triazole)(H2O)n clusters where n=1,2,3,4,5; (2) continuum solvation via PCM, SMD and CPCM methodologies applied to an isolated triazole molecule; (3) combination of models (1) and (2) where the system was treated by both: incremental microsolvation and continuum (PCM) approach. Model (3) gives very good agreement with experimental results when the number of water molecules in the (triazole)(H2O)n cluster is equal to 5. However in the case of 3 and 4 water molecules, the results are much less reliable. The reasons behind this behaviour are analyzed and the importance of taking into account the proper number of water molecules in a microsolvation study is discussed.