title: Molecular energetics of cytosine revisited: a joint computational and experimental study.
abstract: 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 degrees = 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(-1) and -57.2 kJ.mol(-1) for 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.
authors.affiliation: CIQUP/Centro de Investigação em Química da Universidade do Porto, Department of Chemistry, Faculty of Science, University of Porto, Rua do Campo Alegre 687, P-4169-007 Porto, Portugal.
authors.forename: José R B
authors.initials: JR
authors.lastname: Gomes
authors.forename: Maria D M C
authors.initials: MD
authors.lastname: Ribeiro da Silva
authors.forename: Vera L S
authors.initials: VL
authors.lastname: Freitas
authors.forename: Manuel A V
authors.initials: MA
authors.lastname: Ribeiro da Silva
mesh_terms: D000465:Algorithms; D000588:Amines / Q000737:chemistry; D002151:Calorimetry; D003596:Cytosine / Q000737:chemistry*; D005740:Gases; D006863:Hydrogen-Ion Concentration; D007097:Imines / Q000737:chemistry; D007553:Isotope Labeling; D010100:Oxygen / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D000588:Amines; D005740:Gases; D007097:Imines; D003596:Cytosine; D010100:Oxygen