Article
Thermochemistry of uracils. experimental and computational enthalpies of formation of 5,6-dimethyl-, 1,3,5-trimethyl-, and 1,3,5,6-tetramethyluracils
Journal of Physical Chemistry A
(2013)
Abstract
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 N(2) 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.
Disciplines
Publication Date
January 10, 2013
DOI
10.1021/jp311057h
Citation Information
Rafael Notario, Vladimir N. Emel'yanenko, María Victoria Roux, Francisco Ros, et al.. "Thermochemistry of uracils. experimental and computational enthalpies of formation of 5,6-dimethyl-, 1,3,5-trimethyl-, and 1,3,5,6-tetramethyluracils" Journal of Physical Chemistry A Vol. 117 Iss. 1 (2013) p. 244 - 251 Available at: http://works.bepress.com/james-chickos/30/