Vapour pressures and sublimation enthalpies of thymine and cytosine

The vapour pressures of cytosine and thymine were measured using the torsion-effusion technique. The sublimation processes of cytosine and thymine were investigated over the temperature ranges 480–553 K and 420–503 K, respectively. The following pressure—temperature equations were derived by least-squares treatment of the vapour pressure data

The standard sublimation enthalpies were obtained by second-and third-law treatment of the experimental data and the values ΔH⁰₂₉₈ = 167 ± 10 kJ mole^?1 and ΔH⁰₂₉₈ = 138 ± 10 kJ mole^?1 were derived for cytosine and thymine, respectively. IR and Raman spectra were recorded in the gas phase in order to evaluate the thermodynamic functions of gaseous cytosine and thymine.