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

MP2-6-31G** calculations on 1H- and 2H-indazole annular tautomers show that the 1H tautomer 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 Delta E(MP2-6-31G(**)) led to the following thermodynamic values: Delta H-298(1H --> 2H) = 3.9 kcal mol(-1) and Delta G(0)(298)(1H --> 2H) = 4.1 kcal mol(-1). Electronic spectra recorded at 80 degrees C provided experimental evidence for the clear predominance of 1H-indazole in the gas phase.