Low-Temperature Heat Capacity of L- and DL-Phenylglycines
Journal of Thermal Analysis and Calorimetry
, E-ISSN: 1572-8943
||Adiabatic calorimetry L-phenylglycine DL-phenylglycine Heat capacity Molecular crystals Chiral/racemic counterparts
Kovalevskaya Yulia A.
Boldyreva Elena V.
Nikolaev Institute of Inorganic Chemistry SB RAS, Pr. Ac. Lavrentieva 3, Novosibirsk 630090, Russia
Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
Institute of Solid State Chemistry and Mechanochemistry SB RAS, ul. Kutateladze 3, Novosibirsk 630128, Russia
Heat capacity of crystalline L- and DL-phenylglycines was measured in the temperature range from 6 to 305 K. For L-phenylglycine, no anomalies in the C p (T) dependence were observed. For DL-phenylglycine, however, an anomaly in the temperature range 50–75 K with a maximum at about 60 K was registered. The enthalpy and the entropy changes corresponding to this anomaly were estimated as 20 J mol−1 and 0.33 J K−1 mol−1, respectively. In the temperature range 205–225 K, an unusually large dispersion of the experimental points and a small change in the slope of the C p (T) curve were noticed. Thermodynamic functions for L- and DL-phenylglycines in the temperature range 0–305 K were calculated. At 298.15 K, the values of heat capacity, entropy, and enthalpy are equal to 179.1, 195.3 J K−1 mol−1, and 28590 J mol−1 for L-phenylglycine and 177.7, 196.3 J K−1 mol−1 and 28570 J mol−1 for DL-phenylglycine. For both L- and DL-phenylglycine, the C p (T) at very low temperatures does not follow the Debye law C – T 3 . The heat capacity C p (T) is slightly higher for L-phenylglycine, than for the racemic DL-crystal, with the exception of the phase transition region. The difference is smaller than was observed previously for the L-/DL-cysteines, and considerably smaller, than that for L-/DL- serines.