Ab initio calculations on the hydration of dimethylpyrazole and indazole. Solvent effects on tautomeric energies.

Abstract

The effect of hydration upon the energies of the tautomeric pairs of indazoles and dimethylpyrazoles was investigated (i) by attaching one water molecule to each nitrogen, and (ii) by the continuum model. In the calculation (i) the STO-3G basis set was used. The geometry optimization resulted in strongly hydrogen bonded cyclic structures with notable co-operative effect. This was explored in more detail with 3,4-dimethylpyrazole hydrate using the 3-21G basis set and correction for the basis set superposition error. In the calculation (ii) on the indazole pair the 6-31G basis set was used together with the continuum based SCF algorithm of Tomasi and co-workers [1]. The results of both approaches are in good agreement. The tautomeric energy of the indazole tautomers calculated by the extended basis set (10 kcal mol −1) is 2.9 kcal smaller than that obtained from STO-3G calculations. The continuum solvent effects reduces it to 5 kcal mol −1.