Thermochemical study of 1-, 3- and 4-piperidinecarboxamide derivatives

The standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, of three piperidinecarboxamide derivatives, namely 1-, 3- and 4-piperidinecarboxamide, were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by Calvet microcalorimenty, respectively.The final results are analysed and discussed in terms of molecular structure.     

Thermochemical properties of 4-N,N-dialkylamino-7-nitrobenzofurazan derivatives (alkyl = methyl,ethyl)

The standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, for two nitrobenzofurazan derivatives, 4-N,N-dimethylamino-7-nitrobenzofurazan (DMANBF) and 4-N,N-diethylamino-7-nitrobenzofurazan (DEANBF), were derived from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry and by the Knudsen effusion technique, respectively. The results are compared with the corresponding data calculated by the G3(MP2)//B3LYP approach. Computationally, the molecular structures of both compounds were established and the geometrical parameters were determined at the B3LYP/6-31G(d) level of theory.     

Experimental and computational study on the molecular energetics of benzyloxyphenol isomers

This paper reports a combined experimental and computational thermochemical study of 4-benzyloxyphenol. Static bomb combustion calorimetry and Knudsen mass-loss effusion technique were used to determine the standard (p^° = 0.1 MPa) molar enthalpy of combustion, , and of sublimation, , respectively, from which the standard (p^° = 0.1 MPa) molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, were derived.For comparison purposes, the gas-phase enthalpy of formation of this compound was estimated by G3(MP2)//B3LYP calculations, using a set of gas-phase working reactions; the results are in excellent agreement with experimental data. G3(MP2)//B3LYP computations were also extended to the calculation of the gas-phase enthalpies of formation of the 2- and 3-benzyloxyphenol isomers. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies and, finally, O–H bond dissociation enthalpies.     

Thermochemical study of two anhydrous polymorphs of caffeine

The mean values of the standard massic energy of combustion of caffeine in phase I (or alpha) and in phase II (or beta) measured by static-bomb combustion calorimetry in oxygen, at T = 298.15 K, are Δ_cu^° (C₈H₁₀O₂N₄, I) = −(21823.27 ± 0.68) J · g⁻¹ and Δ_cu^° (C₈H₁₀O₂N₄, II) = −(21799.96 ± 1.08) J · g⁻¹, respectively.The standard (p^° = 0.1 MPa) molar enthalpy of formation in condensed phase for each form was derived from the corresponding standard molar enthalpies of combustion as, and .The difference between the standard enthalpy of formation of the two polymorphs in condensed phase was also evaluated by using reaction-solution calorimetry. The obtained result, 2.04 ± 0.25 kJ · mol⁻¹, is in agreement, within the uncertainty, with the difference between the molar enthalpies of formation obtained from combustion experiments (4.5 ± 3.2) kJ · mol⁻¹, which can be considered as an internal test for consistency of the results.A value for the standard enthalpy of formation of caffeine in the gaseous state was proposed: , estimated from the values of the standard enthalpies of formation of both crystalline forms obtained in this work, and the data on standard enthalpies of sublimation collected from the literature.     

Thermochemical study of 5-methyluracil, 6-methyluracil, and 5-nitrouracil

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, for 5-methyluracil, 6-methyluracil, and 5-nitrouracil were derived from the values of the standard massic energies of combustion measured by static bomb combustion calorimetry. The results obtained together with literature values of the enthalpies of sublimation yielded the standard molar enthalpies of formation, in gaseous phase, at T = 298.15 K. These values are discussed in the terms of structural enthalpic increments.     

Standard molar enthalpies of formation of 2-, 3- and 4-cyanobenzoic acids

The standard (p = 0.1 MPa) molar enthalpies of formation of 2-, 3- and 4-cyanobenzoic acids were derived from their standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The Calvet high temperature vacuum sublimation technique was used to measure the enthalpies of sublimation of 2- and 3-cyanobenzoic acids. The standard molar enthalpies of formation of the three compounds, in the gaseous phase, at T = 298.15 K, have been derived from the corresponding standard molar enthalpies of formation in the condensed phase and standard molar enthalpies for phase transition. The results obtained are −(150.7 ± 2.0) kJ · mol⁻¹, −(153.6 ± 1.7) kJ · mol⁻¹ and −(157.1 ± 1.4) kJ · mol⁻¹ for 2-cyano, 3-cyano and 4-cyanobenzoic acids, respectively. Standard molar enthalpies of formation were also estimated by employing two different methodologies: one based on the Cox scheme and the other one based on several different computational approaches. The calculated values show a good agreement with the experimental values obtained in this work.     

Thermochemical study of aliphatic and phenyl-substituted geminal diacetates

The standard enthalpies of combustion _c H ^o of aliphatic diacetates1 and aromatic diacetates2 were measured calorimetrically. The enthalpies of vaporization _vap H ^o or sublimation _sub H ^o of1 and2 were obtained from the temperature function of the vapor pressure measured in a flow system. From _f H ^o(g) of1 and2 new values of group increments for the estimation of standard enthalpies of formation of these classes of compounds were derived. The geminal interaction energy between the geminal acyloxy groups shows no anomeric stabilization.Geminal Substituent Effects, Part 12, for part 11 see Ref. 7.     

Thermochemical Study of 1-Methylhydantoin

Using static bomb combustion calorimetry, the combustion energy of 1-methylhydantoin was obtained, from which the standard molar enthalpy of formation of the crystalline phase at T = 298.15 K of the compound studied was calculated. Through thermogravimetry, mass loss rates were measured as a function of temperature, from which the enthalpy of vaporization was calculated. Additionally, some properties of fusion were determined by differential scanning calorimetry, such as enthalpy and temperature. Adding the enthalpy of fusion to the enthalpy of vaporization, the enthalpy of sublimation of the compound was obtained at T = 298.15 K. By combining the enthalpy of formation of the compound in crystalline phase with its enthalpy of sublimation, the respective standard molar enthalpy of formation in the gas phase was calculated. On the other hand, the results obtained in the present work were compared with those of other derivatives of hydantoin, with which the effect of the change of some substituents in the base heterocyclic ring was evaluated.     

Thermochemical Study of the Cyanophenol Isomers

The standard (p° = 0.1MPa) molar enthalpies of formation for 2-, 3- and 4-cyanophenol in the gaseous phase were derived from the standard molar enthalpies of combustion in oxygen at T = 298.15 K, measured by static bomb combustion calorimetry, and the standard molar enthalpies of sublimation at 298.15 K, measured by Calvet microcalorimetry: 2-cyanophenol, (32.8 ± 2.1) kJ-mol^–1; 3-cyanophenol, (37.8 ± 2.2) kJ-mol^–1; 4-cyanophenol, (35.1 ± 2.5)-kJ-mol^–1. Ab initio geometry optimizations of the three cyanophenols and respective phenoxyl radicals and phenoxide anions were performed using the 6-31G* basis sets. Single-point MP2 and DFT energy calculations allowed the estimation of the enthalpies of formation in the gaseous phase, the O—H bond dissociation energies, and the gas-phase acidities of the three cyanophenols. The theoretical results are generally in good agreement with the experimental findings.     

Structural studies of cyclic ureas: 2. Enthalpy of formation of parabanic acid

Thermophysical and thermochemical studies have been carried out for crystalline parabanic acid. The thermophysical study was made by differential scanning calorimetry, DSC, over the temperature interval between T = (263 and 473) K. Two phase transitions were found: at T = (392.3 ± 1.6) K with the enthalpy of transition of (2.1 ± 0.4) kJ · mol⁻¹ and at T = (509.8 ± 1.5) K, when the compound was scanned to its fusion temperature. The standard (p = 0.1 MPa) molar enthalpy of formation, at T = 298.15 K, for crystalline parabanic acid was determined using static-bomb combustion calorimetry as −(590.2 ± 1.0) kJ · mol⁻¹. The standard molar enthalpy of sublimation, at T = 298.15 K, was derived from the variation of their vapour pressures, measured by the Knudsen-effusion method, with the temperature. These two thermochemical parameters yielded the standard molar enthalpy of formation in the gaseous phase, at T = 298.15 K, as −(470.8 ± 1.2) kJ · mol⁻¹.     

Thermochemical study of 2- and 3-alkyl substituted thiophenes

The standard (p ⁰=0.1 MPa) molar enthalpies of formation, in the condensed phase, of nine linear-alkyl substituted thiophenes, six in position 2- and three in position 3-, at T=298.15 K, were derived from the standard massic energies of combustion, in oxygen, to yield CO₂(g) and H₂SO₄·115H₂O(aq), measured by rotating-bomb combustion calorimetry. The standard molar enthalpies of vaporization of these compounds were measured by high temperature Calvet Microcalorimetry, so their standard molar enthalpies of formation, in the gaseous phase, were derived. The results are discussed in terms of structural contributions to the energetics of the alkyl-substituted thiophenes, and empirical correlations are suggested for the estimation of the standard molar enthalpies of formation, at T=298.15 K, for 2- and 3-alkyl-substituted thiophenes, both in the condensed and in the gaseous phases.     

Thermochemical and theoretical study of some methyldiazines

The standard (p ⁰ = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, atT=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G^* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.     

Thermochemistry of aromatic ketones. Experimental enthalpies of formation and structural effects

The standard enthalpies of formation Δ_fH^o (liq. or cr.) at the temperature T = 298.15 K were measured using combustion calorimetry for benzophenone (A), 1-indanone (B), -tetralone (C), 9-fluorenone (D), anthrone (E) and dibenzosuberone (F). The standard enthalpies of vaporization Δ_vH^o or sublimation Δ_sH^o of A-F and 5,7-dihydro-6H-dibenzo[a,c]cyclohepten-6-one (G) were obtained from the temperature function of the vapor pressure measured in a flow system. Enthalpies of fusion Δ_mH of solid compounds were measured by DSC. From the enthalpies of formation of the gaseous compounds of A-G the values of their strain enthalpies were derived and structural effects discussed.

     

Structural studies of cyclic ureas: 3. Enthalpy of formation of barbital

A thermochemical and thermophysical study has been carried out for crystalline barbital [5,5′-diethylbarbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard () molar enthalpy of formation of the crystalline barbital, at T = 298.15 K, was derived as −(753.0 ± 1.8) kJ · mol⁻¹. The thermophysical study was made by differential scanning calorimetry over the temperature interval (265 to 470) K. A solid–solid phase transition was found at T = 413.3 K. The vapour pressures of the crystalline barbital were measured at several temperatures between T = (355 and 377) K, by the Knudsen mass-loss effusion technique, from which the standard molar enthalpy of sublimation, at T = 298.15 K was derived as (117.3 ± 0.6) kJ · mol⁻¹. The combination of the experimental results yielded the standard molar enthalpy of formation of barbital in the gaseous phase, at T = 298.15 K, as −(635.8 ± 1.9) kJ · mol⁻¹. This value is compared and discussed with our theoretical calculations by several methods (Gaussian-n theories G2 and G3, complete basis set CBS-QB3, density functional B3P86 and B3LYP) by means of atomization and isodesmic reaction schemes.     

Thermodynamic properties of caffeine: Reconciliation of available experimental data

Molar enthalpies of sublimation of two crystal forms of caffeine were obtained from the temperature dependence of the vapour pressure measured by the transpiration method. A large number of primary experimental results on the temperature dependences of vapour pressure and phase transitions have been collected from the literature and have been treated in a uniform manner in order to derive sublimation enthalpies of caffeine at T = 298.15 K. This collection together with the new experimental results reported here has helped to resolve contradictions in the available sublimation enthalpies data and to recommend a consistent and reliable set of sublimation and formation enthalpies for both crystal forms under study. Ab initio calculations of the gaseous molar enthalpy of formation of caffeine have been performed using the G3MP2 method and the results are in excellent agreement with the selected experimental data.     

Thermodynamic properties of xanthone: Heat capacities,phase-transition properties,and thermodynamic-consistency analyses using computational results

Heat capacities and phase-transition properties for xanthone (IUPAC name 9H-xanthen-9-one and Chemical Abstracts registry number [90-47-1]) are reported for the temperature range 5 < T/K < 524. Statistical calculations were performed and thermodynamic properties for the ideal gas were derived based on molecular geometry optimization and vibrational frequencies calculated at the B3LYP/6-31+G(d,p) level of theory. These results are combined with sublimation pressures from the literature to allow critical evaluation of inconsistent enthalpies of sublimation for xanthone, also reported in the literature. Literature values for the enthalpy of combustion of xanthone are re-assessed, a revision is recommended for one result, and a new value for the enthalpy of formation of the ideal gas is derived. Comparisons with thermophysical properties reported in the literature are made for all other reported and derived properties, where possible.     

Thermochemical properties of polycyclic hydrocarbons including fragments of norbornane and small cycles

Combustion enthalpies of three polycyclic hydrocarbons were measured by the precision bomb calorimetry method and their enthalpies of formation in the liquid state were calculated: for pentacyclo[6.3.1.1^3.6.0^2.7.0^9.11]tridecene, –7713.9±3.8 and 25.8±3.8 kJ mol^–1; for 10-met hylpentacyclo[6.3.1.1^3.6.O^2,.7.0^9.11 It ridecene, -8348.8±3.9 and -18.7+-3.9 k] mol^–1; and for 11-methylpentacyclo[6.4.1.1^3.6.0^2.7.0^9.12]tetradecene-10-spirocyclopropane,clopropane, –10157.9±3.4 and 38.1±3.8 kJ mol^–1. The thermochemical I parameters obtained agree with calculated values as well as with experimental and calculated enthalpies of formation of some hydrocarbons that contain the same fragments as the compounds studied.The authors thank Academician O. M. Nefedov and Yu. V. Tomilov for submitting the samples and for discussion of the results obtained.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2676–2678, November, 1996.