Molecular Energetics of Cytosine Revisited:  A Joint Computational and Experimental Study.

A static bomb calorimeter has been used to measure the standard molar energy of combustion, in oxygen, at T 298.15 K, of a commercial sample of cytosine. From this energy, the standard (p° 0.1 MPa) molar enthalpy of formation in the crystalline state was derived as −(221.9 ± 1.7) kJ·mol-1. This value confirms one experimental value already published in the literature but differs from another literature value by 13.5 kJ·mol-1. Using the present standard molar enthalpy of formation in the condensed phase and the enthalpy of sublimation due to Burkinshaw and Mortimer J. Chem. Soc., Dalton Trans. 1984, 75, (155.0 ± 3.0) kJ·mol-1, results in a value for the gas-phase standard molar enthalpy of formation for cytosine of −66.9 kJ·mol-1. A similar value, −65.1 kJ·mol-1, has been estimated after G3MP2B3 calculations combined with the reaction of atomization on three different tautomers of cytosine. In agreement with experimental evidence, the hydroxy-amino tautomer is the most stable form of cytosine in the gas phase. The enthalpies of formation of the other two tautomers were also estimated as −60.7 kJ·mol-1and −57.2 kJ·mol-1for the oxo-amino and oxo-imino tautomers, respectively. The same composite approach was also used to compute other thermochemical data, which is difficult to be measured experimentally, such as C−H, N−H, and O−H bond dissociation enthalpies, gas-phase acidities, and ionization enthalpies. [ABSTRACT FROM AUTHOR]