Thermodynamics of sublimation of N-acetyl amides of some amino acids.

Thermodynamics of sublimation ofN-acetylamides of some aminoacids (glycine, L-alanine, L-valine , D- and L-leucine, and L-isoleucine) were studied by calorimetry. The enthalpies of sublimation at temperatures between 373 K and 400 K were determined by means of a Tian-Calvet calorimeter. Standard enthalpies of sublimation atT = 298.15 K andpo = 0.1 MPa were calculated by using the heating enthalpies obtained from calorimetric measurements, or by integration of published experimental standard molar heat capacities of the crystalline solidsCp,mo(cr). The standard molar heat capacities for the ideal gasesCp,mo(g) were obtained, in the range of temperatures of interest, by the additivity method. Standard entropies of sublimation and Gibbs energies were also calculated and the results discussed on the basis of crystal structure data and packing parameters reported in the recent literature. A reconsideration of the enthalpy of sublimation values previously reported is also discussed.