Thermochemical study of the dicyanoimidazole isomers

The standard (p° = 0.1 MPa) molar enthalpy of formation at T = 298.15 K for 4,5-dicyanoimidazole, in the crystalline phase, was derived from the standard molar energy of combustion measured by static bomb combustion calorimetry. This value and the literature value of the standard molar enthalpy of sublimation of the compound allow the calculation of the corresponding gas-phase standard molar enthalpy of formation, at T = 298.15 K. Additionally, theoretical calculations for 4,5-dicyanoimidazole were performed by density functional theory with the hybrid functional B3LYP and the 6-31G(d) basis set, extending the study to the 2,4- and 2,5-dicyanoimidazole isomers. Single-point energy calculations for both molecules were determined at the B3LYP/6-311+G(2df,2p) level of theory. With the objective of assessing the quality of the results, standard ab initio molecular orbital calculations at the G3 level were also performed. Enthalpies of formation, obtained using appropriate working reactions, were calculated and compared with the experimental data.     

Thermochemical study of the ethylpyridine and ethylpyrazine isomers

The standard (p(o) = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of four liquids: 2-ethylpyridine, 4-ethylpyridine, ethylpyrazine and 2,3-diethylpyrazine were measured by static bomb calorimetry in an oxygen atmosphere. The values of the standard molar enthalpies of vaporization, at T = 298.15 K, were obtained by Calvet microcalorimetry, allowing the calculation of the standard molar enthalpies of formation of the compounds, in the gas phase, at T= 298.15 K: 2-ethylpyridine (79.4 +/- 2.6) kJ mol(-1); 4-ethylpyridine (81.0 +/- 3.4) kJ mol(-1); ethylpyrazine (146.9 +/- 2.8) kJ mol(-1); and 2,3-diethylpyrazine (80.2 +/- 2.9) kJ mol(-1). The most stable geometries of all ethylpyridine and ethylpyrazine isomers were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G* and 6-311G**. These calculations were then used to obtain estimates of the enthalpies of formation of all isomers, including those not experimentally studied, through the use of isodesmic reactions. A discussion of the relationship between structure and energetics of the isomers is also presented.     

Thermochemical study of three dimethylpyrazine derivatives

The standard (p ⁰=0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T-298.15 K, for 2,5-dimethylpyrazine (2,5-DMePz) and for the two dimethylpyrazine-N,N′-dioxide derivatives, 2,3-dimethylpyrazine-1,4-dioxide (2,3-DMePzDO) and 2,5-dimethylpyrazine-1,4-dioxide (2,5-DMePzDO), were derived from the measurements of standard massic energies of combustion, using a static bomb calorimeter, and from the standard molar enthalpies of vaporization or sublimation, measured by Calvet microcalorimetry. The mean values for the molar dissociation enthalpy of the nitrogen-oxygen bonds, 〈DH _m⁰〉(N-O), were derived for both N,N′-dioxide compounds. These values are discussed in terms of the molecular structure of the two N,N′-dioxide derivatives and compared with 〈DH _m⁰〉(N-O) values previously obtained for other N-oxide derivatives.     

Thermochemical study of the solvation of alkanes in mixtures of methanol with butyl alcohol isomers

A calorimetric method was applied at 25 °C to measure the enthalpies of dissolution of cyclohexane, heptane, and decane in the methanol-n-butanol mixed solvent and hexadecane in mixtures of methanol withn-, iso-, andtert-butyl alcohols. The standard enthalpies of dissolution of alkanes were determined. It was shown that the equation proposed in the literature for calculation of the enthalpies of dissolution of alkanes in mixtures with nonspecific intermolecular solvent-solvent interactions describes satisfactorily the enthalpies of dissolution of alkanes in mixtures of methanol withn- andiso-butyl alcohols. It was suggested that there is no preferential solvation of alkanes by one of the mixed solvent components in the MeOH−BuⁿOH and MeOH−BuⁱOH mixtures; in the MeOH−Bu^tOH system, the composition of alkane solvation shell differs slightly from the solvent composition in the bulk. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 271–274, February, 1999.     

Thermochemical properties of three piperidine derivatives

The standard (p ^o=0.1 MPa) molar energies of combustion for the crystalline 1-benzyl-4-piperidinol and 4-piperidine-piperidine, and for the liquid 4-benzylpiperidine, were measured by static bomb calorimetry, in oxygen, at T=298.15 K. The standard molar enthalpies of sublimation or vaporization, at T=298.15 K, of these three compounds were determined by Calvet microcalorimetry. Those values were used to derive the standard molar enthalpies of formation, at T=298.15 K, in their condensed and gaseous phase, respectively.     

A quantum chemical study of three isomers of N20

Ab initio molecular orbital (MP2) and density functional theory (B3LYP) calculations using different basis sets have been employed to study the structures, energetics and vibrational frequencies of the large homonuclear polynitrogen compound, N₂₀. In the present study, three distinct forms were found to represent local minima on the potential energy surface. They are the fullerene-type cage form of I_h symmetry, a corannulene-like bowl form of C_5v symmetry, and a ring isomer with D₅ symmetry of which the cage form turns out to be the highest energy form. Both the bowl and ring forms are calculated to be more stable than the cage form by about 200 kcal/mol. The molecular properties calculated for these isomers may serve as valuable predictions for future experimental searches for new high energy density materials (HEDM).     

DFT study of three C22H14 isomers of tripentaprismane

Structures, energies, and vibrational frequencies have been calculated for the three C₂₂H₁₄ isomers of tripentaprismane at the B3LYP/6‐31G** level of theory. Thus, the three C₂₂H₁₄ isomers of tripentaprismane have the form of coplanar tripentaprismane‐cage molecules. Symmetries of isomer 1, 2, and 3 are C_2v, C_s, and C_2v, respectively. Heats of formation of the three C₂₂H₁₄ isomers are estimated in the present work. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Thermochemical study of some methoxytetralones

The standard (p^° = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of 5-, 6- and 7-methoxy-α-tetralone were measured by static bomb calorimetry. The values of the standard molar enthalpies of sublimation were obtained by Calvet microcalorimetry and corrected to T = 298.15 K. Combining these results, the standard molar enthalpies of formation of the compounds, in the gas phase, at T = 298.15 K, have been calculated, 5-methoxy-α-tetralone -(244.8 ± 1.9) kJ · mol^?1, 6-methoxy-α-tetralone ?(243.0 ± 2.8) kJ · mol^?1 and 7-methoxy-α-tetralone ?(242.3 ± 2.6) kJ · mol^?1.Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange–correlation energy functional with extended basis sets and more accurate correlated computational techniques of the MCCM/3 suite have been performed for the compounds. The agreement between experiment and theory gives confidence to estimate the enthalpy of formation of 8-methoxy-α-tetralone. Similar calculations were done for the 5-, 6-, 7- and 8-methoxy-β-tetralone, for which experimental work was not done.     

Thermochemical study of some cubane derivatives

The thermochemical study of cubane-1,4-dicarboxylic acid (1), diethyl cubane-1,4-dicarboxylate (2), diisopropyl cubane-1,4-dicarboxylate (3), and bis(2-fluoro-2,2-dinitro)ethyl cubane-1,4-dicarboxylate (4) was performed. The standard enthalpies of combustion (_c H°) and formation (_f H°) of these compounds were estimated using the method of combustion in a calorimetric bomb in an oxygen atmosphere. Using the additive group method, calculated values for _f H° of these substances which agreed satisfactorily with the experimental ones were obtained. The strain energies (E _s) of the cubic structure of derivatives1–4 were calculated. It was concluded thatE _s did not change on substitution of hydrogen atoms in cubane for various functional groups and was equal toE _s of the structure of cubane itself. The reliability of the single published value of _f H° in the cubane crystal state, 541.8 kJ mol^–1 (129.5 kcal mol^–1), was confirmed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2471–2473, October, 1996.     

Thermodynamic study of the three isomers of bromo and iodobenzoic acids. Part II

Using our technique of combustion of small amount of a substance, we determined by calorimetry the standard molar enthalpy of formation in the condensed state and atT=298.15 K of the three isomers of bromo and iodobenzoic acids. Associating to these values their standard molar enthalpies of sublimation previously measured, it was possible to determine their standard molar enthalpies of formation in the gaseous state and atT=298.15 K. The experimental values of the thermodynamic properties _f H _m ^o (cr, 298.15 K), _f H _m ^o (cr, 298.15 K), _sub H _m ^o (298.15 K), and _f H _m ^o (g, 298.15 K) are given for the two series. From the experimental value of the standard molar enthalpy of atomization, it was possible to determine an enthalpy value for the C_b-Br and C_b-I bonds. The experimental and theoretical values of the resonance energy of bromo and iodobenzoic acids are compatible. The relative stability of some monosubstituted derivatives of benzoic acid studied in our laboratory is also discussed.Part I is concerned with Ref. 22 (for bromobenzoic acids) and with Ref. 23 (for iodobenzoic acids).     

Photo-induced isomerization of three nitrotoluene isomers: A matrix-isolation infrared spectroscopic and quantum-chemical study

The photo-induced isomerization reactions of ortho-, meta- and para-nitrotoluene molecules were investigated by matrix isolation infrared spectroscopy and quantum chemical calculations. Under UV irradiation of ortho-nitrotoluene in solid argon, the hydrogen atom transfer isomer was formed, as reported previously. It was found that the hydrogen atom transfer isomer is unstable and rearranged to its nitro isomer upon annealing. In addition, the nitrite isomer as well as its dissociation product tolyloxy radical was also formed. Only the nitrite isomers and the tolyloxy radicals were formed upon UV excitation of the meta- and para-nitrotoluene molecules. Infrared spectra and vibrational frequency assignments of the newly observed nitrite isomers and tolyloxy radicals are reported, which are supported by quantum chemical calculations.