Experimental and computational thermochemical study of oxindole

An experimental and computational thermochemical study was performed for oxindole. The standard (p∘=0.1MPa) molar enthalpy of formation of solid oxindole was derived from the standard molar energy of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The respective standard molar enthalpy of sublimation, at T = 298.15 K, was measured by Calvet microcalorimetry. The standard molar enthalpy of formation in the gas phase was derived as −(66.8 ± 3.2) kJ · mol−1.Density functional theory calculations with the B3LYP hybrid functional and the 6-31G∗ and 6-311G∗∗ sets have also been performed in order to obtain the most stable conformation of oxindole. A comparison has been made between the structure of oxindole and that of the related two-ring molecules: indoline and 2-indanone and the one-ring molecules: pyrrolidine and 2,3-dihydropyrrole. The G3(MP2)//B3LYP method and appropriate reactions were used to obtain estimates of the standard molar enthalpy of formation of oxindole in the gas phase, at T = 298.15 K. Computationally obtained estimates of the enthalpy of formation of oxindole are in very good agreement with the experimental gas phase value. The aromaticity of oxindole was evaluated through the analysis of the nucleus independent chemical shifts (NICS) obtained from the B3LYP/6-311G∗∗ wave functions.