Thermodynamic properties of ε-caprolactam and ε-caprothiolactam

Abstract

This paper is concerned with experimental and computational thermochemical studies of ε-caprolactam and ε-caprothiolactam, molecules whose flexible ring structure provides several conformational forms.The gas-phase standard enthalpy of formation at the reference temperature of 298.15 K of the two title compounds have been determined from the enthalpy of formation in the crystalline phase, derived from bomb combustion calorimetry, and from the enthalpy of sublimation, derived from Calvet microcalorimetry and Knudsen effusion techniques.The equilibrium geometries and the thermodynamic properties of three minimum energy conformers obtained with the composite G3(MP2)//B3LYP approach were used to determine the conformational composition of the two lactam derivatives, by means of Boltzmann’s distribution. The computed gas-phase standard molar enthalpies of formation of ε-caprolactam and ε-caprothiolactam have been determined using hypothetical gas-phase reactions, taking into account the conformational compositions of each one of the species. This parameter together with the corresponding experimental one are compared.