Tautomerism and Aromaticity in 1,2,3-Triazoles: The Case of Benzotriazole

This paper provides an explanation for the extraordinary difference in stability between 1.2.3-triazole and benzotriazole tautomers. In the gas phase, the 2H tautomer of 1,2,3-triazole represents more than 99.9% of the equilibrium mixture, whereas in benzotriazole the reverse is true (more than 99.99% of 1H tautomer at equilibrium). To understand the origin of this different behavior, an ab initio study at the 6-31G level was carried out on both tautomers of benzotriazole, on benzotriazolate anion, and on both tautomers of benzotriazolium cation (the 1,2- and the 1,3-H H+ ions). Theoretical results (the proton affinity of 1H-benzotriazole is 10.2 kcal mol-1 larger than that of 2H-benzotriazole) was checked against ICR measurements with excellent agreement (1-methylbenzotriazole is 10.4 kcal mol-1 more basic than 2-methylbenzotriazole). Thermodynamic measurements (enthalpies of solution, vaporization, sublimation, and solvation) in three solvents (water, methanol, and dimethyl sulfoxide) confirm the predominance of the 1H tautomer in solution. Taking into account lone pair/lone pair repulsions and aromaticity, it is possible to explain the different behavior of 1,2,3-triazole and benzotriazole in the case of neutral molecules and their similarity in the case of protonated species. © 1989, American Chemical Society. All rights reserved.