Low-Temperature Thermodynamic Properties of Pyrimidine Bases: Thymine and Uracil

This study reports precise heat capacity data for crystalline thymine (C5H6N2O2; CAS Number: 65-71-4; fraction purity: 0.999) and uracil (C4H4N2O2; CAS Number: 66-22-8; fraction purity: 0.999) over the temperature range of 6 to 330 K. Adiabatic calorimetry was used to perform the measurements. The calorimeter was validated by measuring the heat capacity of reference standards (copper and benzoic acid). The experimental heat capacity data were fitted using Einstein-Planck functions and extrapolated to 0 K following the Debye law. The data enabled the calculation of thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) for thymine and uracil between 0 and 330 K.