Sublimation Thermodynamics of Chiral and Racemic Forms of Valine

The thermodynamics of sublimation of l-, d-, and dl-valine was investigated by Knudsen effusion mass spectrometry. These amino acids have been shown to sublimate without decomposition. The dependence of the saturated vapor pressure in the studied temperature range is described by a linear equation ln(p/Pa) = −(17313 ± 312)/T + (41.691 ± 0.363) for l-valine (350–410 K), ln(p/Pa) = −(16981 ± 301)/T + (40.788 ± 0.451) for d-valine (348–408 K), and ln(p/Pa) = −(16970 ± 204)/T + (41.360 ± 0.405) for dl-valine (355–410 K). The pressures of l- and d-valine are close to each other. dl-valine is ∼1.7 times more volatile than the chiral forms. The enthalpy of sublimation was determined by the methods of the second and third laws of thermodynamics; the recommended values were selected as 142.3 ± 4.3 kJ·mol–1 for l-valine, 140.9 ± 2.3 kJ·mol–1 for d-valine, and 141.8 ± 1.1 kJ·mol–1 for dl-valine. The enthalpy of formation of the gaseous forms of valine was found to be 472.1 ± 4.5 kJ·mol–1 for l-valine, 472.1 ± 2.8 kJ·mol–1 for d-valine, and 475.4 ± 1.5 kJ·mol–1 for dl-valine.