Energetic Effect of the Carboxylic Acid Functional Group in Indole Derivatives

The standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, was calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, yielding −(222.2 ± 3.5) kJ·mol–1 and −(234.1 ± 2.1) kJ·mol–1 for indole-3-carboxylic acid and 1-methylindole-3-carboxylic acid, respectively. Computational studies, at the G3(MP2) composite level, were conducted for indole-3-carboxylic acid and 1-methylindole-3-carboxylic acid as a complement of the experimental work, and they were also extended to the remaining isomers, indole-2-carboxylic acid, 1-methylindole-2-carboxylic acid, 3-methylindole-2-carboxylic acid, and 2-methylindole-3-carboxylic acid, to provide reliable estimates of the corresponding thermochemical parameters. The agreement of the estimates of the standard gas-phase enthalpy of formation so obtained, indole-2-carboxylic acid −(223.6 ± 0.8) kJ·mol–1, 1-methylindole-2-carboxylic acid −(223.7 ± 0.8) k...