Importance of aromaticity on the relative stabilities of indazole annular tautomers: an ab initio study

Abstract

MP2-6-31G** calculations on 1H- and 2H-indazole annular tautomers show that the 1Htautomer is more stable than the 2H 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 ΔE(MP2-6-31G**) led to the following thermodynamic values: ΔH²⁹⁸(1H→ 2H)= 3.9 kcal mol^–1 and ΔG₀²⁹⁸(1H→ 2H)= 4.1 kcal mol^–1. Electronic spectra recorded at 80 °C provided experimental evidence for the clear predominance of 1H-indazole in the gas phase.