Basicity of N-H- and N-methyl-1,2,3-triazoles in the gas phase, in solution, and in the solid state - An experimental and theoretical study

The gas-phase and aqueous basicities of six 1,2,3-triazoles have been deter mined, the former by FT-ICR and the latter by spectrophotometry and LH NMR. The gas-phase experiments agree very well with the Gibbs free energies cal culated at the B3LYP/6-31G* level. In contrast, only semiquantitative ascer tainments are possible when basicities in the gas phase and in solution are compared. It is possible, with the aid of calculations, to obtain a comple te picture of the complex equilibria involved in C-substituted N-H-1,2,3-tr iazoles. The crystal structures of 4(5)-phenyl-1,2,3-triazole (4) and 4(5)- nitro-1,2,3-triazole (15) have been determined. In the gas phase, 2H tautom ers b always predominate, while in aqueous solution, both 1H and 2H tautome rs - a and b - are present. Finally, in the solid state, 1 exists as a 1:1 mixture of 1a and 1b, while 4 is in the 4b tautomeric form and 15 is a 1H t automer 15a. These conclusions - a in the gas phase, a + b in solution, and equal probabilities of finding either a or b in the crystal - are probably general for all 1,2,3-triazoles.