Nearest-Neighbor and Non-Nearest-Neighbor Interactions between Substituents in the Benzene Ring. Experimental and Theoretical Study of Functionally Substituted Benzamides

Standard molar enthalpies of formation of 2- and 4-hydroxy­benz­amides were measured by combustion calorimetry. Vapor pressures of benz­amide and 2-hydroxy­benz­amide were derived by the transpiration method. Standard molar enthalpies of sublimation or vaporization of these compounds at 298 K were obtained from vapor pressure temperature dependence. Thermochemical data on benz­amides with hydroxyl, methyl, methoxy, amino, and amide substituents were collected, evaluated, and tested for internal consistency. The high-level G4 quantum-chemical method was used for mutual validation of the experimental and theoretical gas-phase enthalpies of formation. Sets of nearest-neighbor and non-nearest-neighbor interactions between substituents in the benzene ring have been evaluated. A simple incremental procedure has been suggested for a quick appraisal of the vaporization and gas-phase formation enthalpies of the substituted benz­amides

Thermochemical Properties of Xanthine and Hypoxanthine Revisited

The standard molar enthalpies of formation of xanthine and hypoxanthine were measured by using high-precision combustion calorimetry. The standard molar enthalpies of sublimation of these compounds at 298.15 K were derived by the quartz-crystal microbalance technique. Limited thermodynamic data available in the literature are compared with our new experimental data. In addition, we use the G4 method to calculate the molar enthalpies of formation of xanthine and hypoxanthine in the gas phase. There is good agreement between the evaluated experimental data and the quantum-chemical calculations

Thermodynamic Properties of 1,4-Benzoquinones in Gaseous and Condensed Phases: Experimental and Theoretical Studies

A complete study of thermodynamic properties of 1,4-benzoquinones in the condensed and gaseous phases was carried out using experimental techniques and theoretical approaches. Enthalpies of combustion and formation of 2-methyl-1,4-benzoquinone were evaluated using combustion calorimetry. The transpiration method was utilized to determine the temperature dependence of the vapor pressures of 1,4-benzoquinone and 2-methyl-1,4-benzoquinone for the sublimation and vaporization enthalpies calculation. The group additivity scheme was used independently for verification of sublimation enthalpy of 2-methyl-1,4-benzoquinone. For this procedure the enthalpy of solution of 2-methyl-1,4-benzoquinone in benzene was measured at 298.15 K. The experimental values obtained were combined with published data and organized to obtain a reliable set of the experimental enthalpies of formation and enthalpies of phase transition of compounds. The methods of quantum chemistry and statistical physics based on the “rigid rotator–anharmonic oscillator” model were used to calculate thermodynamic functions of 1,4-benzoquinones in the ideal gas state in the temperature range 273.15–1500 K. The strain enthalpy and the enthalpy of π-conjugation were also estimated

Benchmark Thermodynamic Properties of Methyl- and Methoxybenzamides: Comprehensive Experimental and Theoretical Study

The enthalpies of formation of 2-, 3-, and 4-CH₃-benzamide, as well as for 2-CH₃O-benzamide, were measured by using combustion calorimetry. Vapor pressures of the isomeric CH₃- and CH₃O-benzamides were measured by using the transpiration method. The enthalpies of sublimation/vaporization of these compounds at 298 K were obtained from temperature dependencies of vapor pressures. The enthalpies of solution of the isomeric CH₃- and CH₃O-benzamides were measured with solution calorimetry. The enthalpies of sublimation of m- and p-substituted benzamides were independently derived with help of a solution calorimetry-based procedure. The enthalpies of fusion of the CH₃-benzamides were derived from differential scanning calorimetry measurements. Thermochemical data on CH₃- and CH₃O-benzamides were collected, evaluated, and tested for internal consistency. A simple incremental procedure was suggested for a quick appraisal of vaporization enthalpies of substituted benzamides. The high-level G4 quantum-chemical method was used for mutual validation of the experimental and theoretical gas-phase enthalpies of formation. A remarkable ability of the G4-based atomization procedure to calculate reliable enthalpies of formation was established for the set of aliphatic and aromatic amides. An outlook for the proper validation of the G4-AT procedure was discussed