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The enthalpies of combustion heat capacities, enthalpies of sublimation and enthalpies of formation of 2-tert-butylbenzimidazole and 2-phenylimidazole are reported. An attempt is made to relate the crystal structure to the enthalpies of sublimation.

The gas-phase hemolytic N-H bond dissociation enthalpy (BDE) was investigated for a large series of molecules containing at least one N-H bond by means of accurate density-functional theory calculations. The molecules studied belong to different classes of compounds namely, amines, amides, and anilines, amino acids, phenoxazines, indolamines, and other compounds of general interest, such as anti-inflammatory drugs.

High-level density functional theory calculations are carried out for a benchmark set of benzene derivatives. The best performance is obtained when different atom equivalent schemes are used.

An experimental and computational thermochemical study on the relative stabilities of 2- and 3-thiopheneacetic acid methyl esters was carried out. The enthalpies of combustion and vaporization were measured by a rotating bomb combustion calorimeter, Calvet microcalorimetry and correlation gas chromatography and the gas phase enthalpies of formation at T=298.15 K were determined.

The energetics of the C F, C-Cl, C-Br, and C-I bonds in 2-haloethanols is examined by employing a combination of experimental and theoretical methods. It is concluded that the major qualitative trends exhibited by the C-X bond dissociation enthalpies for the series of compounds can be predicted by Pauling's electrostatic-covalent model.

Research is presented describing the use of static bomb combustion calorimetry to measure the standard molar enthalpies of formation for gas phase 2-, 3-, and 4-phenylpyridine, obtained from the standard molar enthalpies of combustion in oxygen at 298.15 K.

The kinetics of the aggregation process of DiesterB, a homologous coproduct generated in Verteporfin synthesis is investigated by visible spectrophotometry. The model proposed states that at high temperatures the equilibrium is shifted toward monomers, reducing the catalyst nucleus formation and resulting in an overall reaction velocity decrease.

A computational study of the structural and thermochemical properties of N-phenyl (open) and N-alkyl (cyclic) ureas, through the use of M05-2X and B3LYP density functional theory calculations has been carried out. The consistency of the literature experimental results has been confirmed, and using mainly isodesmic reactions, the unknown ΔfH0(g) of the other urea derivatives were estimated. The experimental results together with the theoretical information have permitted the study of the effect of phenyl, p-and m-chlorophenyl, alkyl, and carbonyl substitutions on the thermodynamical stability of urea and its cyclic derivatives. The peculiar behavior of the N-tert-butyl substituent in cyclic ureas has been related to geometric deformations. [ABSTRACT FROM AUTHOR]

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]

An enthalpic value for the N-methyllactam/O-methyllactim isomerization,in the gaseous phase, is reportedin this work for the conversion between 2,4-dimethoxypyrimidine and1,3-dimethyluracil. For this purpose, the enthalpy of formation of2,4-dimethoxypyrimidine, in the gaseous phase, was obtained experimentallycombining results from combustion calorimetry and Calvet microcalorimetry,and the enthalpy of formation of 1,3-dimethyluracil, in the gaseousphase, reported previously in the literature, is also discussed. Theenthalpy of hydrogenation of 1,3-dimethyluracil is compared with theenthalpy of hydrogenation of uracil and interpreted in terms of aromaticity,considering the influence of the hyperconjugation and the hindranceof the solvation of the ring by the methyl groups. The enthalpy ofsublimation of 2,4-dimethoxypyrimidine was obtained combining Calvetmicrocalorimetry and differential scanning calorimetry results. Thisenthalpy is compared with the enthalpy of sublimation of 1,3-dimethyluracilpreviously reported in the literature and analyzed herein. From theinterplay between the experimental results and the theoretical simulationof dimers of these molecules, the influence of stereochemical hindranceon the in-plane intermolecular contacts and aromaticity on the π···πinteractions is analyzed. [ABSTRACT FROM AUTHOR]

The relative stabilities of 2,2′- and 3,3′-bithiophenes were evaluated by experimental thermochemistry and the results compared with data obtained from state of the art calculations, which were also extended to 2,3′-bithiophene. The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline 2,2′-bithiophene and 3,3′-bithiophene were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g) and H2SO4·115H2O, measured by rotating-bomb combustion calorimetry at T= 298.15 K. The vapor pressures of these two compounds were measured as a function of temperature by Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation, at T= 298.15 K, were derived from the Clausius−Clapeyron equation. The experimental values were used to calculate the standard (p° = 0.1 MPa) enthalpies of formation of the title compounds in the gaseous phase; the results were analyzed and interpreted in terms of enthalpic increments and molecular structure. Standard ab initio molecular orbital calculations at the G3(MP2)//B3LYP level were performed. Enthalpies of formation, using homodesmotic reactions, were calculated and compared with experimental data. The computational study was also extended to the isomeric compound 2,3′-bithiophene. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out. Finally, bond dissociation enthalpies (BDE) and enthalpies of formation of thienyl radicals were also computed. [ABSTRACT FROM AUTHOR]

Static bomb calorimetry, Calvet microcalorimetry and the Knudsen effusion technique were used to determine the standard molar enthalpy of formation in the gas phase, at T= 298.15 K, of the indole and indoline heterocyclic compounds. The values obtained were 164.3 ± 1.3 kJ·mol−1and 120.0 ± 2.9 kJ·mol−1, respectively. Several different computational approaches and different working reactions were used to estimate the gas-phase enthalpies of formation for indole and indoline. The computational approaches support the experimental results reported. The calculations were further extended to the determination of other properties such as bond dissociation enthalpies, gas-phase acidities, proton and electron affinities and ionization energies. The agreement between theoretical and experimental data for indole is very good supporting the data calculated for indoline. [ABSTRACT FROM AUTHOR]

The standard (po 0.1 MPa) molar enthalpies of formation of 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dichloroanilines were derived from the standard molar energies of combustion, in oxygen, to yield CO2(g), N2(g) and HCl·600H2O(l), at T 298.15 K, measured by rotating bomb combustion calorimetry. The Calvet high-temperature vacuum sublimation technique was used to measure the enthalpies of sublimation of the six isomers. These two thermodynamic parameters yielded the standard molar enthalpies of formation of the six isomers of dichloroaniline, in the gaseous phase, at T 298.15 K. The gas-phase enthalpies of formation were also estimated by G3MP2B3 calculations, which were further extended to the computation of gas-phase acidities, proton affinities, and ionization enthalpies. [ABSTRACT FROM AUTHOR]