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 (p
0 = 0.1 MPa) molar enthalpies
of formation in the gas phase at T = 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 T = 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 T =
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.