Thermochemistry of Uracils. Experimental and Computational Enthalpies of Formation of 5,6-Dimethyl‑, 1,3,5-Trimethyl‑, and 1,3,5,6-Tetramethyluracils

We describe in the current paper an experimental and computational study of three methylated uracils, in particular, the 5,6-dimethyl-, 1,3,5-trimethyl-, and 1,3,5,6-tetramethyl derivatives. The values of the standard (<i>p</i>⁰ = 0.1 MPa) molar enthalpies of formation in the gas phase at <i>T</i> = 298.15 K have been determined. The energies of combustion were measured by static bomb combustion calorimetry, and from the results obtained, the standard molar enthalpies of formation in the crystalline state at <i>T</i> = 298.15 K were calculated. The enthalpies of sublimation were determined using the transpiration method in a saturated N₂ stream. Values of −(376.2 ± 2.6), −(355.9 ± 3.0), and −(381.7 ± 2.8) kJ·mol^–1 for the gas-phase enthalpies of formation at <i>T</i> = 298.15 K of 5,6-dimethyluracil, 1,3,5-trimethyluracil, and 1,3,5,6-tetramethyluracil, respectively, were obtained from the experimental thermochemical study. An extended theoretical study with the G3 and the G4 quantum-chemical methods has been carried out for all the possible methylated uracils. There is a very good agreement between experimental and calculated enthalpies of formation for the three derivatives studied. A Free–Wilson analysis on G4-calculated enthalpies of formation has been carried out, and the contribution of methylation in the different positions of the uracil ring has been estimated