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 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.     

An efficient approach for theoretical study on the low-energy isomers of large fullerenes C90-C140

An approach that consists of a molecular mechanics method based on the second generation reactive empirical bond order (REBO) potential and the more accurate semiempirical method PM3 (Parametric Method No. 3) was proposed to predict the energetically favored isomers of the fullerenes from C90 to C140 at the semiempirical level. All the 578,701 isolated-pentagon-rule isomers of fullerenes from C90 to C140 were enumerated from topological structures and systematically searched using an energy minimization method to select the best 100 low-energy isomers based on the REBO potential for each fullerene. Then these candidate isomers were further optimized by PM3 and ranked again to determine the top low-energy isomers. This approach was applied to calculate the energetically favored isomers of C90-C140. The results of C90-C120 are in good agreement with the published results by quantum-chemical methods. Furthermore, the top five low-energy isomers of C90-C120, as well as C122-C140 which have scarcely been systematically studied before, are also predicted with the approach. The analysis of the structures showed that the hexagon-neighbor rule is an important factor to the stability of C90-C140. The time cost for the systematical search based on the REBO potential was also discussed. It indicates that the approach proposed is efficient for predicting the energetically favored isomers of large fullerenes at the semiempirical level.     

Thermochemical study of 2,5-dimethyl-3-furancarboxylic acid, 4,5-dimethyl-2-furaldehyde,and 3-acetyl-2,5-dimethylfuran

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous state, at T = 298.15 K, for 2,5-dimethyl-3-furancarboxylic acid, 3-acetyl-2,5-dimethylfuran, and 4,5-dimethyl-2-furaldehyde were derived from the values of the standard molar enthalpies of formation, in the condensed phase, and the standard molar enthalpies of phase transition from the condensed to the gaseous state. The values of the standard molar enthalpies of formation of the compounds in the condensed phases were calculated from the measurements of the standard massic energies of combustion obtained by static bomb combustion calorimetry. The enthalpies of vaporization/sublimation were measured by Calvet high temperature microcalorimetry. For 2,5-dimethyl-3-furancarboxylic acid the standard enthalpy of sublimation was also calculated, by the application of the Clausius–Clapeyron equation, to the temperature dependence of the vapor pressures measured by the Knudsen effusion technique.     

10.

Thermochemical properties and thermokinetic behavior of energetic triazole ionic salts     

ZHAO FengQi*  XUE Liang  XING XiaoLing  HU RongZu  ZHOU ZhiMing  GAO HongXu  YI JianHua  XU SiYu & PEI Qing National Key Lab of Science  Technology on Combustion  Explosion  Xi’an Modern Chemistry Research Institute  Xi’an  China School of Chemistry  Materials Science  Shaanxi Normal University  Xi’an  China School of Chemical Engineering  Environment  Beijing Institute of Technology  Beijing  China 《中国科学B辑(英文版)》2011,(3)

The properties of dissolution in different solvents,the specific heat capacity and thermal decomposition process under the non-isothermal conditions for energetic triazole ionic salts 1,2,4-triazolium nitrate(1a),1,2,3-triazolium nitrate(1b),3,4,5triamino-1,2,4-triazolium nitrate(2a),3,4,5-triamino-1,2,4-triazolium dinitramide(2b)were precisely measured using a Calvet Microcalorimeter.The thermochemical equation,differential enthalpies of dissolution(△difH m ),standard molar enthalpies of dissolution(△difH ...     

11.

Structure of the vinyldiazomethyl anion and energetic comparison to the cyclic isomers     

Ichino T  Gianola AJ  Kato S  Bierbaum VM  Lineberger WC 《The journal of physical chemistry. A》2007,111(34):8374-8383

The 351.1 nm photoelectron spectrum of the vinyldiazomethyl anion has been measured. The ion is generated through the reaction of the allyl anion with N(2)O in helium buffer gas in a flowing afterglow source. The spectrum exhibits the vibronic structure of the vinyldiazomethyl radical in its electronic ground state as well as in the first excited state. Electronic structure calculations have been performed for these molecules at the B3LYP/6-311++G(d,p) level of theory. A Franck-Condon simulation of the X (2)A' state portion of the spectrum has been carried out using the geometries and normal modes of the anion and radical obtained from these calculations. The simulation unambiguously shows that the ions predominantly have an E conformation. The electron affinity (EA) of the radical has been determined to be 1.864 +/- 0.007 eV. Vibrational frequencies of 185 +/- 10 and 415 +/- 20 cm(-1) observed in the spectrum have been identified as in-plane CCN bending and CCC bending modes, respectively, for the X (2)A' state. The spectrum for the A (2)A' state is broad and structureless, reflecting large geometry differences between the anion and the radical, particularly in the CCN angle, as well as vibronic coupling with the X (2)A' state. The DFT calculations have also been used to better understand the mechanism of the allyl anion reaction with N(2)O. Collision-induced dissociation of the structural isomer of the vinyldiazomethyl anion, the 1-pyrazolide ion, has been examined, and energetics of the structural isomers is discussed.     

12.

An isomer of coronopilin     

V. I. Novikov  Yu. N. Forostyan 《Chemistry of Natural Compounds》1969,5(5):312-313

     

13.

An isomer of coronopilin     

V. I. Novikov  Yu. N. Forostyan 《Chemistry of Natural Compounds》1972,5(5):312-313

Conclusions From the racemes ofC. xanthifolia Fresen. have been isolated a sesquiterpene lactone C₁₆H₂₀O₄, the structural (+)-isomer of (–)-coronopilin, which has not been studied previously; structure I has been proposed for it.Khimiya Prirodnykh Soedinenii, Vol. 5, No. 5, pp. 375–377, 1969     

14.

povel isomers of hexasulfur: Prediction of a stable prism isomer and implications for the thermal reactivity of elemental sulfur     

Wong MW  Steudel Y  Steudel R 《The Journal of chemical physics》2004,121(12):5899-5907

High-level ab initio molecular orbital calculations were employed to explore the potential energy hypersurface of hexasulfur, S(6). Twelve isomeric structures of S(6) have been identified: two unbranched rings (chair and boat), one trigonal prism of D(3h) symmetry, two singly branched rings (S(5)double bondS), three triplet chains, one singlet chain, and three doubly branched rings (Sdouble bondS(4)double bondS). The prism structure is essentially a cluster of three S(2) molecules connected via a six-center pi(*)-pi(*)-pi(*) interaction. It is by 51 kJ mol(-1) less stable than the lowest-energy chair form. The reactions to generate the boat, the prism, and the singly branched isomers from the chair form are predicted to have lower barriers than the ring opening reaction of cyclo-S(6), which requires an activation energy of 149 kJ mol(-1). The prism and singly branched isomers are found to be more reactive species than the chair form and they are potential sources of S(2) in chemical reactions involving elemental sulfur.     

15.

An NMR study of optical isomers in solution     

M.I. Kabachnik  T.A. Mastryukova  E.I. Fedin  M.S. Vaisberg  L.L. Morozov  P.V. Petrovsky  A.E. Shipov 《Tetrahedron》1976,32(14):1719-1728

NMR ³¹P-{¹H} spectra of stereoisomeric N - [S - (methylethoxyphosphinyl) - thioglycolyl]valines in solution to reveal association of the molecules, and interaction of the chiral centres. Under fast inter-associate exchange in achiral media, these interactions lead to the following: (i) The chemical shift of the racemic mixture of enantiomers deviates from the shift of the individual species; (ii) the spectra of non-racemic mixtures are doublets; (iii) there are 2ⁿ lines in the spectrum of a mixture containing unequal concentrations of stereoisomers with n asymmetric centres. The integrated intensity ratio is equal to the concentration ratio in all cases. The concept of statistically controlled associate diastereomerism (SCAD) is introduced and the respective formalism is given to describe the spectral effects accompanying variations of temperature and concentration. It is also shown applicable to more complicated cases involving ion exchange of chiral fragments between stereoisomers.     

16.

First-principle computational study on the full conformational space of L-threonine diamide, the energetic stability of cis and trans isomers     

Sahai MA  Fejer SN  Viskolcz B  Pai EF  Csizmadia IG 《The journal of physical chemistry. A》2006,110(40):11527-11536

First-principle computations were carried out on the conformational space of trans and cis peptide bond isomers of HCO-Thr-NH2. Using the concept of multidimensional conformational analysis (MDCA), geometry optimizations were performed at the B3LYP/6-31G(d) level of theory, and single-point energies as well as thermodynamic functions were calculated at the G3MP2B3 level of theory for the corresponding optimized structures. Two backbone Ramachandran-type potential energy surfaces (PESs) were computed, one each for the cis and trans isomers, keeping the side chain at the fully extended orientation (chi1=chi2=anti). Similarly, two side chain PESs for the cis and trans isomers were generated for the (phi=psi=anti) orientation corresponding to approximately the betaL backbone conformation. Besides correlating the relative Gibbs free energy of the various stable conformations with the number of stabilizing hydrogen bonds, the process of trans-->cis isomerization is discussed in terms of intrinsic stabilities as measured by the computed thermodynamic functions.     

17.

A new NMR approach for the assignment of symmetric isomers     

Lunazzi L  Mazzanti A 《Journal of the American Chemical Society》2004,126(38):12155-12157

Irradiation of the (13)C satellite (1% natural abundance) of the NMR (1)H signals yields NOE effects on the spatially close hydrogens of the same isotopomer but not on the hydrogens of the 99% isotopomer having only (12)C atoms. In a DPFGSE-NOE sequence, the latter signals are completely canceled, and it is possible, therefore, to detect NOE effects experienced by isochronous lines that would not be otherwise observable. This allows the structural assignments of symmetric isomers to be unambiguously obtained. Examples are reported for the cases of the cis and trans dimethylstilbene, cis and trans stilbeneoxide, 2,5- and 3,4-dimethylthiophene, 1,8- and 1,5-dimethylnaphthalene, syn and anti 1,2-bis(2-methyl-1-naphthyl)benzene, 1,3-cyclooctadiene, and cycloheptatriene.     

18.

Separation and identification of biliverdin isomers and isomer analysis of phycobilins and bilirubin   总被引：11，自引：0，他引：11  

P O'Carra  E Colleran 《Journal of chromatography. A》1970,50(3):458-468

     

19.

Chemical algebra III: Thermochemical approach to completely G-invariant distances     

Remi Chauvin 《Journal of mathematical chemistry》1994,16(1):269-283

The search for a definition of distances over sets of skeletal analogs (identified to G-Hilbert spaces of vector ligand parameters) is initiated from the algebraic formulation of the constant of stereogenic pairing equilibria (pairing product). A basic definition equation is devised from thermodynamical speculations. The equation is proved to have always a single potential distance solution D_p as soon as the pairing product is discriminating. The equation of D_p is constructed in order to satisfy three consistency requirements: completeG-invariance (arbitrary orientations selected to describe skeletal analogs do not affect the value of D_p); extension properties (D_p coincides with two standard completelyG-invariant distances or with the Euclidean distance in borderline cases); all the distance properties except, perhaps, the triangular inequality. The latter point remains challenging in general, and is computationally verified in some examples.     

20.

Use of gas chromatography to study the thermodynamic parameters of solutions of isomers of 2,5-dimethoxytetrahydrofuran and 2,3-dichlorobutane     

T. M. Shcherbina  V. B. Bondarev  S. V. Vitt 《Russian Chemical Bulletin》1970,19(4):907-908

     

Thermochemical properties and thermokinetic behavior of energetic triazole ionic salts

The properties of dissolution in different solvents,the specific heat capacity and thermal decomposition process under the non-isothermal conditions for energetic triazole ionic salts 1,2,4-triazolium nitrate(1a),1,2,3-triazolium nitrate(1b),3,4,5triamino-1,2,4-triazolium nitrate(2a),3,4,5-triamino-1,2,4-triazolium dinitramide(2b)were precisely measured using a Calvet Microcalorimeter.The thermochemical equation,differential enthalpies of dissolution(△difH m ),standard molar enthalpies of dissolution(△difH ...     

Structure of the vinyldiazomethyl anion and energetic comparison to the cyclic isomers

The 351.1 nm photoelectron spectrum of the vinyldiazomethyl anion has been measured. The ion is generated through the reaction of the allyl anion with N(2)O in helium buffer gas in a flowing afterglow source. The spectrum exhibits the vibronic structure of the vinyldiazomethyl radical in its electronic ground state as well as in the first excited state. Electronic structure calculations have been performed for these molecules at the B3LYP/6-311++G(d,p) level of theory. A Franck-Condon simulation of the X (2)A' state portion of the spectrum has been carried out using the geometries and normal modes of the anion and radical obtained from these calculations. The simulation unambiguously shows that the ions predominantly have an E conformation. The electron affinity (EA) of the radical has been determined to be 1.864 +/- 0.007 eV. Vibrational frequencies of 185 +/- 10 and 415 +/- 20 cm(-1) observed in the spectrum have been identified as in-plane CCN bending and CCC bending modes, respectively, for the X (2)A' state. The spectrum for the A (2)A' state is broad and structureless, reflecting large geometry differences between the anion and the radical, particularly in the CCN angle, as well as vibronic coupling with the X (2)A' state. The DFT calculations have also been used to better understand the mechanism of the allyl anion reaction with N(2)O. Collision-induced dissociation of the structural isomer of the vinyldiazomethyl anion, the 1-pyrazolide ion, has been examined, and energetics of the structural isomers is discussed.     

An isomer of coronopilin

Conclusions From the racemes ofC. xanthifolia Fresen. have been isolated a sesquiterpene lactone C₁₆H₂₀O₄, the structural (+)-isomer of (–)-coronopilin, which has not been studied previously; structure I has been proposed for it.Khimiya Prirodnykh Soedinenii, Vol. 5, No. 5, pp. 375–377, 1969     

povel isomers of hexasulfur: Prediction of a stable prism isomer and implications for the thermal reactivity of elemental sulfur

High-level ab initio molecular orbital calculations were employed to explore the potential energy hypersurface of hexasulfur, S(6). Twelve isomeric structures of S(6) have been identified: two unbranched rings (chair and boat), one trigonal prism of D(3h) symmetry, two singly branched rings (S(5)double bondS), three triplet chains, one singlet chain, and three doubly branched rings (Sdouble bondS(4)double bondS). The prism structure is essentially a cluster of three S(2) molecules connected via a six-center pi(*)-pi(*)-pi(*) interaction. It is by 51 kJ mol(-1) less stable than the lowest-energy chair form. The reactions to generate the boat, the prism, and the singly branched isomers from the chair form are predicted to have lower barriers than the ring opening reaction of cyclo-S(6), which requires an activation energy of 149 kJ mol(-1). The prism and singly branched isomers are found to be more reactive species than the chair form and they are potential sources of S(2) in chemical reactions involving elemental sulfur.     

An NMR study of optical isomers in solution

NMR ³¹P-{¹H} spectra of stereoisomeric N - [S - (methylethoxyphosphinyl) - thioglycolyl]valines in solution to reveal association of the molecules, and interaction of the chiral centres. Under fast inter-associate exchange in achiral media, these interactions lead to the following: (i) The chemical shift of the racemic mixture of enantiomers deviates from the shift of the individual species; (ii) the spectra of non-racemic mixtures are doublets; (iii) there are 2ⁿ lines in the spectrum of a mixture containing unequal concentrations of stereoisomers with n asymmetric centres. The integrated intensity ratio is equal to the concentration ratio in all cases. The concept of statistically controlled associate diastereomerism (SCAD) is introduced and the respective formalism is given to describe the spectral effects accompanying variations of temperature and concentration. It is also shown applicable to more complicated cases involving ion exchange of chiral fragments between stereoisomers.     

First-principle computational study on the full conformational space of L-threonine diamide, the energetic stability of cis and trans isomers

First-principle computations were carried out on the conformational space of trans and cis peptide bond isomers of HCO-Thr-NH2. Using the concept of multidimensional conformational analysis (MDCA), geometry optimizations were performed at the B3LYP/6-31G(d) level of theory, and single-point energies as well as thermodynamic functions were calculated at the G3MP2B3 level of theory for the corresponding optimized structures. Two backbone Ramachandran-type potential energy surfaces (PESs) were computed, one each for the cis and trans isomers, keeping the side chain at the fully extended orientation (chi1=chi2=anti). Similarly, two side chain PESs for the cis and trans isomers were generated for the (phi=psi=anti) orientation corresponding to approximately the betaL backbone conformation. Besides correlating the relative Gibbs free energy of the various stable conformations with the number of stabilizing hydrogen bonds, the process of trans-->cis isomerization is discussed in terms of intrinsic stabilities as measured by the computed thermodynamic functions.     

A new NMR approach for the assignment of symmetric isomers

Irradiation of the (13)C satellite (1% natural abundance) of the NMR (1)H signals yields NOE effects on the spatially close hydrogens of the same isotopomer but not on the hydrogens of the 99% isotopomer having only (12)C atoms. In a DPFGSE-NOE sequence, the latter signals are completely canceled, and it is possible, therefore, to detect NOE effects experienced by isochronous lines that would not be otherwise observable. This allows the structural assignments of symmetric isomers to be unambiguously obtained. Examples are reported for the cases of the cis and trans dimethylstilbene, cis and trans stilbeneoxide, 2,5- and 3,4-dimethylthiophene, 1,8- and 1,5-dimethylnaphthalene, syn and anti 1,2-bis(2-methyl-1-naphthyl)benzene, 1,3-cyclooctadiene, and cycloheptatriene.     

Chemical algebra III: Thermochemical approach to completely G-invariant distances

The search for a definition of distances over sets of skeletal analogs (identified to G-Hilbert spaces of vector ligand parameters) is initiated from the algebraic formulation of the constant of stereogenic pairing equilibria (pairing product). A basic definition equation is devised from thermodynamical speculations. The equation is proved to have always a single potential distance solution D_p as soon as the pairing product is discriminating. The equation of D_p is constructed in order to satisfy three consistency requirements: completeG-invariance (arbitrary orientations selected to describe skeletal analogs do not affect the value of D_p); extension properties (D_p coincides with two standard completelyG-invariant distances or with the Euclidean distance in borderline cases); all the distance properties except, perhaps, the triangular inequality. The latter point remains challenging in general, and is computationally verified in some examples.