Heat Capacities of Uracil, Thymine, and Its Alkylated, Cyclooligomethylenated, and Halogenated Derivatives by Differential Calorimetry

Abstract

The molar heat capacity (C p ) of solid uracil, its alkylated and halogenated derivatives, and cyclooligomethylenouracils in the temperature range of (298.15 to 343.15) K by a differential scanning calorimeter (SETARAM TG-DSC 111) were determined. It was demonstrated that the C p value increases by increasing the number of methylene groups attached to the diketopyrimidine ring. The correlations C p = f(T) are given. The contributions of C−CH3, N−CH3, and C−NO2 groups as well as F, Cl, Br, and I atoms in the value of C p for the temperature of 298.15 K are presented. The Chickos method for calculation of C p values is discussed.

Critical Evaluation of the Enthalpies of Formation for Fluorinated Compounds Using Experimental Data and High-Level Ab Initio Calculations

“…Values selected for the generation of the training data set are given in bold; the values not used for averaging are italicized. Reported uncertainties are the expanded uncertainties for the 0.95 level of confidence. b Sources of the condensed-state enthalpies of formation: A, Porto; B, Bartesville; C, Moscow; D, Teddington; E, Windsor; F, Freiburg; G, Gaithersburg; c References and . d Modeled as a mixture of tautomers. e Evaluated with the NIST ThermoData Engine based on multiple sources. f Based on CO 2 analysis g Sublimation pressure data include the solid-to-solid phase-transition region and are not used. h Calorimetric data by Paukov et al are inconsistent with the publications from the Teddington laboratory. The latter are used to maintain consistency among different properties. i References and . j Assuming that the reported combustion energies refer to HF·20H 2 O. k Δ l g C ° p ,m estimated by the NIST ThermoData Engine …”

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

“…Due to the absence of the C p ,m ° [CONH-(C B ) 2 , cr] parameter in the Domalski and Hearing Group Additivity Scheme the value of 46.30 J·K –1 ·mol –1 was evaluated in this work by fitting existing group values to the experimental data reported in table S2 of the Supporting Information and using the Solver software from Microsoft Excel included in the Microsoft Office 2007 software package. Table S2 contains the experimental values for the standard molar heat capacities of a series of cyclic amides, as well as the calculated heat capacities and the difference between the experimental − and calculated values. The differences between experimental and calculated heat capacity values for the compounds presented in Table S2, except for 5,6-dimethyluracil, do not exceed 8 J·K –1 ·mol –1 , which is the uncertainty suggested by Domalski and Hearing for their scheme.…”

Thermochemistry of Uracil, Thymine, Cytosine, and Adenine

“…Instead of the empirical formula of Chickos, the differences of molar heat capacities, Δ cr g H 298 – Δ cr g H T = [ C p,m ° (g) − C p,m ° (cr)]·(298.15 K – T ), have been used, where Δ cr g H T is the sublimation enthalpy at temperature T , and C p,m ° (g) stands for the gas-phase heat capacity obtained from quantum chemical calculations, while C p,m ° (cr) is the solid-phase heat capacity taken from the literature. Furthermore, where the individual p – T data points were provided, the sublimation enthalpies were recalculated according to the following equations where a and b are the fitting parameters, Δ cr g C p,m is the difference of molar heat capacities, and T 0 is an arbitrary reference temperature, taken to be 298.15 K. The following C p,m ° (cr) data were used in this work: 131.8, 163.0, 130.9, and 145.5 J/(mol·K) for uracil, thymine, cytosine, and adenine, respectively. The corresponding C p,m ° (g) values, 108.4, 133.1, 114.1, and 129.2 J/(mol·K), were derived from our calculations using statistical thermodynamics.…”