Disulfidicity: A scale to characterize the disulfide bond strength via the hydrogenation thermodynamics

Is Uracil Aromatic? The Enthalpies of Hydrogenation in the Gaseous and Crystalline Phases, and in Aqueous Solution, as Tools to Obtain an Answer

“…The enthalpy of formation, in the gaseous phase, of 5,6-dihydrouracil was determined experimentally using results from static bomb combustion calorimetry, Knudsen effusion, and Calvet microcalorimetry, and the enthalpy of hydrogenation of uracil was calculated and compared with other reference molecules. Enthalpies of hydrogenation have played a fundamental role as an experimental probe for solving chemical problems dealing with aromaticity, conjugation, strain energies, and the relative stabilities of molecules. − Besides the quintessential example of the resonance energy of benzene, there are recent examples in which the enthalpy of hydrogenation has given further insight into questions such as the aromaticity of a model cyclohexatriene, acenaphthylene and pyracylene, N , N -dihydrodiazaacenes, , and 1,2-azaborines; the conjugation of polyynes − and disulfide bonds; the strain energies of alkenes, alkynes, and allenes; and the relative stabilities of carbenes. , The hydrogenation of uracil, in particular, also plays a fundamental role in the catabolism of pyrimidines − and the derivatives of 5,6-dihydrouracil are important compounds in cancer research. − Moreover, in recent years, the energetic properties of pyrimidine − and uracil − derivatives, as well as other related compounds such as cytosine and barbituric acid derivatives, − have gained particular interest within the thermochemical community. For this reason, the experimental enthalpy of formation of 5,6-dihydrouracil, in the gaseous phase, obtained in this work and the derived enthalpy of hydrogenation of uracil, will give further insight into the energetic properties of uracil and can also be used as a benchmark for computational studies in other research areas.…”

“…Enthalpies of hydrogenation have played a fundamental role as an experimental probe for solving chemical problems dealing with aromaticity, conjugation, strain energies, and the relative stabilities of molecules. 18−20 Besides the quintessential example of the resonance energy of benzene, there are recent examples in which the enthalpy of hydrogenation has given further insight into questions such as the aromaticity of a model cyclohexatriene, 21 acenaphthylene and pyracylene, 22 N,N-dihydrodiazaacenes, 23,24 and 1,2-azaborines; 25 the conjugation of polyynes 26−30 bonds; 31 the strain energies of alkenes, alkynes, and allenes; 32 and the relative stabilities of carbenes. 33,34 The hydrogenation of uracil, in particular, also plays a fundamental role in the catabolism of pyrimidines 35−37 and the derivatives of 5,6dihydrouracil are important compounds in cancer research.…”