Importance of aromaticity on the relative stabilities of indazole annular tautomers: an ab initio study.
MP2-6-31G** calculations on both indazole annular tautomers, the 1H and the 2H, show that the first one is more stable than the second one by 3.6 kcal mol-1. In the case of 1H-indazole, the excellence of MP2-6-31G** results is confirmed by the fact that its microwave rotational constants are reproduced with great accuracy. The addition of the thermal energy correction and entropy effects to DE(MP2-6-31G**) led to the following thermodynamic values: DH298(1H->2H) = 3.9 kcal mol-1 and DG0298(1H->2H) = 4.1 kcal mol-1. Electronic spectra recorded at 80ºC provided experimental evidence for the clear predominance of 1H-indazole in the gas phase.