TAUTOMERISM OF 1,2,3-TRIAZOLE AND 1,2,4-TRIAZOLE IN THE GAS-PHASE AND IN AQUEOUS-SOLUTION - A COMBINED ABINITIO QUANTUM-MECHANICS AND FREE-ENERGY PERTURBATION STUDY

The energy differences between the tautomers of 1,2,3- and 1,2,4-triazole have been estimated at the 6-31G**(MP2)//3-21G level including zero-point effects. The results confirm that only a single tautomer of each isomer is expected to be observed in the gas phase. In aqueous solution the free energy separation of each pair of tautomers is considerably reduced and has been estimated by use of the free energy perturbation method. These calculations agree with the experimental findings that for 1,2,3-triazole both tautomers are observed in a polar solvent, whereas for 1,2,4-triazole the concentration of the rare tautomer is still extremely low. © 1990 American Chemical Society.