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

Thermodynamics of sublimation of N-acetyl amides of some amino acids (glyci ne, L-alanine, L-valine, D- and L-leucine, and L-isoleucine) were studied b y calorimetry. The enthalpies of sublimation at temperatures between 373 K and 400 K were determined by means of a Tian-Calvet calorimeter. Standard e nthalpies of sublimation at T = 298.15 K and p(O) = 0.1 MPa were calculated by using the heating enthalpies obtained from calorimetric measurements, o r by integration of published experimental standard molar heat capacities o f the crystalline solids C-p,m(O)(cr). The standard molar heat capacities f or the ideal gases C-p,m(O)(g) were obtained, in the range of temperatures of interest, by the additivity method. Standard entropies of sublimation an d Gibbs energies were also calculated and the results discussed on the basi s of crystal structure data and packing parameters reported in the recent l iterature. A reconsideration of the enthalpy of sublimation values previous ly reported is also discussed. (C) 1999 Academic Press.