title: Thermochemistry of uracils. Experimental and computational enthalpies of formation of 5,6-dimethyl-, 1,3,5-trimethyl-, and 1,3,5,6-tetramethyluracils.
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.
authors.affiliation: Instituto de Química Física Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain. rnotario@iqfr.csic.es
authors.forename: Rafael
authors.initials: R
authors.lastname: Notario
authors.forename: Vladimir N
authors.initials: VN
authors.lastname: Emel'yanenko
authors.forename: María Victoria
authors.initials: MV
authors.lastname: Roux
authors.forename: Francisco
authors.initials: F
authors.lastname: Ros
authors.forename: Sergey P
authors.initials: SP
authors.lastname: Verevkin
authors.forename: James S
authors.initials: JS
authors.lastname: Chickos
authors.forename: Joel F
authors.initials: JF
authors.lastname: Liebman
mesh_terms: D002152:Calorimetry, Differential Scanning; D008745:Methylation; D008958:Models, Molecular; D015394:Molecular Structure; D044367:Phase Transition; D013816:Thermodynamics; D014498:Uracil / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry*; D055548:Vapor Pressure
chemical_list: D014498:Uracil