The thermodynamic characteristics of 15-pentadecanolide and 16-hexadecanolide

Combustion calorimetry was used to determine the enthalpies of formation of 15-pentadecanolide (I) and 16-hexadecanolide (II). The temperature dependences of vapor pressure of lactones and the enthalpies of vaporization were determined by the transpiration method. Differential scanning calorimetry measurements were performed to find the temperatures and enthalpies of fusion of lactones. Conformational analysis and quantum-chemical calculations of the structural, vibrational, and energy characteristics of stable conformers of I were performed. The molecular and spectral data were used to calculate the thermodynamic properties of I in the ideal gas state. An explanation was suggested of the special features of changes in the enthalpies of vaporization in the series of unsubstituted lactones as the size of their rings increased.