Experimental and computational study on the energetics of the cyclic anhydrides of glycine and alanine

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, of the cyclic anhydrides of glycine and DL-alanine, at T = 298.15 K, were derived from the standard molar energies of combustion in oxygen, measured by static bomb combustion calorimetry. For these compounds, the standard molar enthalpies of sublimation, at T = 298.15 K, were determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method. Through the values for the heat capacity differences between the gas and the crystalline phases of the studied compounds, the standard (po = 0.1 MPa) molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were derived. From the experimental values, the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of glycine anhydride and DL-alanine anhydride were calculated as –(321.0 ± 1.9) kJ·mol−1 and –(382.7 ± 2.0) kJ·mol−1, respectively. A theoretical study at the G3 and G4 levels has been carried out, and the calculated enthalpies of formation have been compared with the experimental values.

► Study on the energetics of the cyclic anhydrides of glycine and alanine. ► Experimental and computational thermochemistry of glycine and alanine anhydrides. ► Ab initio calculations for two cyclopeptides obtained by G3(MP2)//B3LYP method.