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

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• 作者：Rafael Notario ; Vladimir N. Emel'yanenko ; Mar铆a Victoria Roux ; Francisco Ros ; Sergey P. Verevkin ; James S. Chickos ; Joel F. Liebman
• 刊名：The Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：January 10, 2013
• 年：2013
• 卷：117
• 期：1
• 页码：244-251
• 全文大小：303K
• 年卷期：v.117,no.1(January 10, 2013)
• ISSN：1520-5215

文摘

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 (p⁰ = 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 鈭?376.2 卤 2.6), 鈭?355.9 卤 3.0), and 鈭?381.7 卤 2.8) kJ路mol^鈥? 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鈥揥ilson 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.