Sublimation enthalpies of 5-haloderivatives of 1,3-dimethyluracil

Vapor pressure of solid 1,3-dimethyluracil, 5-fluoro-1,3-dimethyluracil, 5-bromo-1,3-dimethyluracil, and 5-iodo-1,3-dimethyluracil were determined by Knudsen effusion method. The sublimation enthalpies and entropies at the average temperatures of the experimental ranges were calculated from the dependence of vapor pressure by the temperature. Utilizing estimated gas-solid differences of molar heat capacity, the standard (p° = 0.1 MPa) molar enthalpy, and entropy of sublimation corrected at 298 K were obtained. The differences in sublimation enthalpies of the haloderivatives of 1,3-dimethyluracil, which follow the sequence in polarizability of the halogen substituents in 5 position, can be used to reveal the weak bonding between the electron-rich atoms and the highly polarizable halogen atoms in these compounds. Our results suggest the possibility that 5-iodo-1,3-dimethyluracil and 5-bromo-1,3-dimethyluracil, but not 5-fluoro-1,3-dimethyluracil, can act as halogen bond donors in the crystal. (Chemical Equation Presented).