Thermodynamic properties of quinoxaline-1,4-dioxide derivatives: a combined experimental and computational study

The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported.     

Thermochemical studies on 3-methyl-quinoxaline-2-carboxamide-1,4-dioxide derivatives: enthalpies of formation and of N-O bond dissociation

The standard molar enthalpies of formation of the 3-methyl-N-R-2-quinoxalinecarboxamide-1,4-dioxides (R = H, phenyl, 2-tolyl) in the gas phase were derived using the values for the enthalpies of combustion of the crystalline compounds, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, at T = 298.15 K. These values have also been used to calibrate a computational procedure that has been employed to estimate the gas-phase enthalpies of formation of the corresponding 3-methyl-N-R-2-quinoxalinecarboxamides and also to compute the first, second, and mean N-O bond dissociation enthalpies in the gas phase. It is found that the size of the substituent almost does not influence the computed N-O bond dissociation enthalpies; the maximum enthalpic difference is approximately 5 kJ.mol-1.     

Energetics of flavone and flavanone

In this work, we have determined the experimental standard (p° = 0.1 MPa) molar enthalpies of formation, in gas phase, of flavone and flavanone.These results were obtained by combining the standard molar enthalpies of formation in the condensed phase with the standard molar enthalpies of sublimation. The former values were derived from combustion experiments in oxygen, at T = 298.15 K, in a static bomb calorimeter. The values of the standard molar enthalpies of sublimation were obtained by Calvet microcalorimetry and corrected to T = 298.15 K.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 obtained results, experimental and computational, for flavone and flavanone were compared with those obtained for chromone and chromanone, respectively.     

Comparative computational and experimental study on the thermochemistry of the chloropyrimidines

The standard (p0 = 0.1 MPa) molar enthalpies of formation, Delta fH(0)(M), for liquid 2,4,6-trichloropyrimidine and for crystalline 2-chloropyrimidine, 2,4- and 4,6-dichloropyrimidine, and 2,4,5,6-tetrachloropyrimidine compounds were determined at T = 298.15 K by rotating-bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation, Delta (g)(cr,l) H(0)(M), of these compounds at T = 298.15 K were determined by Calvet microcalorimetry. The experimental standard molar enthalpies of formation of those compounds, in the gaseous state, at T = 298.15 K, were thus obtained by combining these two sets of results. The latter values have been employed in the calibration of the computational procedure, which has been used to estimate the gas-phase enthalpies of formation for the other chloropyrimidines that were not possible to obtain in a pure form for the experimental study. It is found that the exchange-correlation functional based on the local spin density approximation (LSDA) seems to be a cheap choice for the estimation of enthalpies of formation for heterocycles containing nitrogen atoms; the well-known B3LYP hybrid method yields larger differences, with respect to the experimental values, for 2,4,6-tri- and 2,4,5,6-tetrachloropyrimidines.     

Experimental and computational thermochemical study of barbituric acids: structure-energy relationship in 1,3-dimethylbarbituric acid

This paper reports an experimental and computational thermochemical study on 1,3-dimethylbarbituric acid. The value of the standard (p° = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -639.6 ± 1.9 kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as 92.3 ± 0.6 kJ·mol(-1). From these results a value of -547.3 ± 2.0 kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.     

Experimental and computational thermochemical study of 2-thiobarbituric acid: structure-energy relationship

This paper reports an experimental and computational thermochemical study on 2-thiobarbituric acid (2-thioxodihydropyrimidine-4,6(1H,5H)-dione), [CAS 504-17-6]. The value of the standard (p(0) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by bomb combustion calorimetry, using a rotatory bomb, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(396.8 ± 0.9) kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as (118.3 ± 2.2) kJ·mol(-1). From these results a value of -(278.5 ± 2.4) kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.     

Thermodynamic study of sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid: a combined experimental and theoretical investigation

The standard (p(o)= 0.1 MPa) molar energies of combustion in oxygen, at T= 298.15 K, of four 1,3-benzodioxole derivatives (sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid) were measured by static bomb calorimetry. The values of the standard molar enthalpies of sublimation, at T= 298.15 K, were derived from vapour pressure-temperature measurements using the Knudsen effusion technique. Combining these results the standard molar enthalpies of formation of the compounds, in the gas phase, at T= 298.15 K, have been calculated: sesamol (-325.7 +/- 1.9) kJ mol(-1); piperonyl alcohol (-329.0 +/- 2.0) kJ mol(-1); piperonylic acid (-528.9 +/- 2.6) kJ mol(-1) and homopiperonylic acid (-544.5 +/- 2.9) kJ mol(-1). The most stable geometries of all the compounds were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G** and 6-311G**. The nonplanarity of the molecules was analyzed in terms of the anomeric effect, which is believed to arise from the interaction between a nonbonded oxygen p orbital and the empty orbital sigma*(CO) involving the other oxygen atom. Calculations were performed to obtain estimates of the enthalpies of formation of all the benzodioxoles using appropriate isodesmic reactions. There is a perfect agreement between theoretical and experimental results.     

Calorimetric and computational thermochemical study of 3,3-tetramethyleneglutaric acid, 3,3-tetramethyleneglutaric anhydride, and 3,3-tetramethyleneglutarimide

The standard (p(o) = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of solid 3,3-tetramethyleneglutaric acid and the related 3,3-tetramethyleneglutaric anhydride and 3,3-tetramethyleneglutarimide were measured by static bomb combustion calorimetry. The values of the standard molar enthalpies of sublimation, 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 gaseous state, at T = 298.15 K. The geometries of the experimentally studied compounds were fully optimized using density functional theory with the B3LYP functional and extended basis sets. More accurate energies were also obtained from single-point calculations at the most stable B3LYP/6-311G** geometries, using the cc-pVTZ basis set. From these calculations the standard molar enthalpies of formation of 3,3-tetramethyleneglutaric acid, 3,3-tetramethyleneglutaric anhydride, and 3,3-tetramethyleneglutarimide were estimated using isodesmic reactions involving glutaric acid, glutaric anhydride, and glutarimide, respectively. Experimental and computational results were used in the discussion of the interrelation of energetics and structure in these compounds and compared with other structurally related compounds.     

Cubane, cuneane, and their carboxylates: a calorimetric, crystallographic, calculational, and conceptual coinvestigation

[reaction: see text] This study is a multinational, multidisciplinary contribution to the thermochemistry of dimethyl1,4-cubanedicarboxylate and the corresponding isomeric, cuneane derivative and provides both structural and thermochemical information regarding the rearrangement of dimethyl 1,4-cubanedicarboxylate to dimethyl 2,6-cuneanedicarboxylate. The enthalpies of formation in the condensed phase at T = 298.15 K of dimethyl 1,4-cubanedicarboxylate (dimethyl pentacyclo[4.2.0.0.(2,5)0.(3,8)0(4,7)]octane-1,4-dicarboxylate) and dimethyl 2,6-cuneanedicarboxylate (dimethyl pentacyclo[3.3.0.0.(2,4)0.(3,7)0(6,8)]octane-2,6-dicarboxylate) have been determined by combustion calorimetry, delta(f) H(o)m (cr)/kJ x mol(-1) = -232.62 +/- 5.84 and -413.02 +/- 5.16, respectively. The enthalpies of sublimation have been evaluated by combining vaporization enthalpies evaluated by correlation-gas chromatography and fusion enthalpies measured by differential scanning calorimetry and adjusted to T = 298.15 K, delta(cr) (g)Hm (298.15 K)/kJ x mol(-1) = 117.2 +/- 3.9 and 106.8 +/- 3.0, respectively. Combination of these two enthalpies resulted in delta(f) H(o)m (g., 298.15 K)/kJ x mol(-1) of -115.4 +/- 7.0 for dimethyl 1,4-cubanedicarboxylate and -306.2 +/- 6.0 for dimethyl 2,6-cuneanedicarboxylate. These measurements, accompanied by quantum chemical calculations, resulted in values of delta(f) Hm (g, 298.15 K) = 613.0 +/- 9.5 kJ x mol(-1) for cubane and 436.4 +/- 8.8 kJ x mol(-1) for cuneane. From these enthalpies of formation, strain enthalpies of 681.0 +/- 9.8 and 504.4 +/- 9.1 kJ x mol(-1) were calculated for cubane and cuneane by means of isodesmic reactions, respectively. Crystals of dimethyl 2,6-cuneanedicarboxylate are disordered; the substitution pattern and structure have been confirmed by determination of the X-ray crystal structure of the corresponding diacid.     

Experimental and computational investigation of the energetics of the three isomers of monochloroaniline

The standard (p degrees = 0.1 MPa) molar enthalpies of formation of 2-, 3-, and 4-chloroaniline were derived from the standard molar energies of combustion, in oxygen, 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 vaporization or sublimation of the three isomers. These two thermodynamic parameters yielded the standard molar enthalpies of formation of the three isomers of chloroaniline, in the gaseous phase, at T = 298.15 K, as 53.4 +/- 3.1 kJ.mol(-1) for 2-chloroaniline, 53.0 +/- 2.8 kJ.mol(-1) for 3-chloroaniline, and 59.7 +/- 2.3 kJ.mol(-1) for 4-chloroaniline. These values, which correct previously published data, were used to test the computational methodologies used. Therewith, gas-phase acidities, proton affinities, electron donor capacities, and N-H bond dissociation enthalpies were calculated and found to compare well with available experimental data for these parameters.     

Experimental and computational study of the thermochemistry of the fluoromethylaniline isomers

The standard (po = 0.1 MPa) molar enthalpies of formation in the condensed phase of seven isomers of fluoromethylaniline were derived from the standard molar energies of combustion, in oxygen, to yield CO2(g), N2(g) and HF.10H2O(l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation of these compounds, also at T = 298.15 K, were determined using Calvet microcalorimetry, while the enthalpies of fusion of the solid compounds were determined by differential scanning calorimetry. The standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived from the former two experimental quantities. G3MP2//B3LYP calculations were performed for all possible fluoromethylanilines allowing the estimation of data for the isomers that were not studied experimentally. The Cox scheme was applied with two different approaches for the estimation of the standard molar enthalpies of formation of all the isomers studied, and this led to the conclusion that the literature values for the enthalpies of formation of the meta and para isomers of methylaniline seem to be not reliable. Further G3MP2//B3LYPs calculations on the methylaniline isomers yielded new values for the standard molar enthalpies of formation of the isomers of methylaniline, which have been tested under the Cox scheme, resulting in better estimates.     

Experimental and computational thermochemical study of 2- and 3-thiopheneacetic acid methyl esters

Thiophene-based compounds have widespread use in modern drug design, biodiagnostics, electronic and optoelectronic devices, and conductive polymers. The present study reports an experimental and computational thermochemical study on the relative stabilities of 2- and 3-thiopheneacetic acid methyl esters. 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. Standard ab initio molecular orbital calculations at the G3 level were performed, and a theoretical study of the molecular and electronic structure of the compounds studied was carried out. Calculated enthalpies of formation, using atomization and isodesmic reactions are in very good agreement with the experimental results.     

Experimental and computational study on the molecular energetics of indoline and indole

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 x mol(-1) and 120.0 +/- 2.9 kJ x 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.     

Combined experimental and computational study of the thermochemistry of the fluoroaniline isomers

The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the condensed phase of all the fluoroanilines, with the exception of the 2,3,5-trifluoroaniline compound, were derived from the standard molar energies of combustion in oxygen at T = 298.15 K, measured by rotating bomb combustion calorimetry. Calvet high-temperature vacuum sublimation experiments were performed to measure their enthalpies of vaporization or sublimation. These experiments allowed the determination of the standard molar enthalpies of formation in the gaseous phase and at T = 298.15 K. These values are also compared with estimates based on G3MP2B3 and BP86/6-31+G(d) computations, which have been extended also to the fluoroaniline that was not studied experimentally. The results are in close agreement with a mean deviation of approximately 3 kJ.mol-1. The largest difference between experimental and G3MP2B3 values is found for the pentafluoroaniline (-7.0 kJ.mol-1). For the three monofluoroanilines, the composite approach has been used also to compute gas-phase acidities, electron and proton affinities, ionization enthalpies and N-H bond dissociation enthalpies. The computed values compare well with available experimental results supporting the new computed data.     

Experimental and computational thermochemistry of 1,4-benzodioxan and its 2-R derivatives

The standard molar energies of combustion, at T = 298.15 K, of crystalline 1,4-benzodioxan-2-carboxylic acid and 1,4-benzodioxan-2-hydroxymethyl were measured by static bomb calorimetry in an oxygen atmosphere. The standard molar enthalpies of sublimation, at T = 298.15 K, were obtained by Calvet microcalorimetry. These values were used to derive the standard molar enthalpies of formation of the compounds in the gas phase at T = 298.15 K: 1,4-benzodioxan-2-carboxylic acid ?(547.7 ± 3.0) kJ · mol^?1 and 1,4-benzodioxan-2-hydroxymethyl ?(374.2 ± 2.3) kJ · mol^?1.In addition, density functional theory calculations using the B3LYP hybrid exchange–correlation energy functional with extended basis sets, 6-311G⁷ and cc-pVTZ, have been performed for the compounds studied. We have also tested two more accurate computational procedures involving multiple levels of electron structure theory in order to get reliable estimates of the thermochemical parameters of the compounds studied. The agreement between experiment and theory gives confidence to estimate the enthalpies of formation of other 2-R derivatives of 1,4-benzodioxan (R = –CH₂COOH, –OH, –COCH₃, –CHO, –CH₃, –CN, and –NO₂).     

Calorimetric and computational study of indanones

Condensed phase standard (p degrees = 0.1 MPa) molar enthalpies of formation for 1-indanone, 2-indanone, and 1,3-indandione were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation for 1-indanone and 2-indanone, at T = 298.15 K, were measured both by correlation-gas chromatography and by Calvet microcalorimetry leading to a mean value for each compound. For 1,3-indandione, the standard molar enthalpy of sublimation was derived from the vapor pressure dependence on temperature. The following enthalpies of formation in gas phase, at T = 298.15 K, were then derived: 1-indanone, -64.0 +/- 3.8 kJ mol(-1); 2-indanone, -56.6 +/- 4.8 kJ mol(-1); 1,3-indandione, -165.0 +/- 2.6 kJ mol(-1). The vaporization and fusion enthalpies of the indanones studied are also reported. In addition, theoretical calculations using the density functional theory with the B3LYP and MPW1B95 energy functionals and the 6-311G** and cc-pVTZ basis sets have been performed for these molecules and the corresponding one-ring species to obtain the most stable geometries and to access their energetic stabilities.     

Di-hydroxybenzenes: Catechol, resorcinol, and hydroquinone: Enthalpies of phase transitions revisited

Molar enthalpies of sublimation of 1,2-di-hydroxybenzene, 1,3-di-hydroxybenzene, and 1,4-di-hydroxybenzene were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. The molar enthalpies of fusion of 1,2- and 1,4-isomers were measured by differential scanning calorimetry (DSC). A large number of the primary experimental results on the temperature dependences of vapor pressure and phase transitions have been collected from the literature and have been treated in a uniform manner in order to derive sublimation, vaporization and fusion enthalpies of di-hydroxybenzenes at the reference temperature 298.15 K. The data sets on phase transitions were checked for internal consistency. This collection together with the new experimental results reported here has helped to resolve contradictions in the available thermochemical data and to recommend consistent and reliable sublimation, vaporization and fusion enthalpies for all three isomers under study.     

Structure-energy relationship in barbituric acid: a calorimetric, computational, and crystallographic study

This paper reports the value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K for barbituric acid. The enthalpies of combustion and sublimation were measured by static bomb combustion calorimetry and transference (transpiration) method in a saturated N2 stream and a gas-phase enthalpy of formation value of -(534.3 +/- 1.7) kJ x mol(-1) was determined at T = 298.15 K. G3-calculated enthalpies of formation are in very good agreement with the experimental value. The behavior of the sample as a function of the temperature was studied by differential scanning calorimetry, and a new polymorph of barbituric acid at high temperature was found. In the solid state, two anhydrous forms are known displaying two out of the six hydrogen-bonding patterns observed in the alkyl/alkenyl derivatives retrieved from the Cambridge Crystallographic Database. The stability of these motifs has been analyzed by theoretical calculations. X-ray powder diffraction technique was used to establish to which polymorphic form corresponds to the commercial sample used in this study and to characterize the new form at high temperature.     

Experimental and computational studies on the molecular energetics of chlorobenzophenones

The standard (p degrees = 0.1 MPa) molar enthalpies of formation, Delta(f)H(m)degrees, of crystalline 2-, 3- and 4-chlorobenzophenone and 4,4'-dichlorobenzophenone were derived from the standard molar energies of combustion, Delta(c)U(m)degrees, in oxygen, to yield CO(2)(g), N(2)(g), and HCl x 600H(2)O(l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The Calvet high-temperature vacuum sublimation technique was used to measure the enthalpy of sublimation, Delta(cr)(g)H(m)degrees, of the compound 2-chlorobenzophenone. For the other three compounds, the standard molar enthalpies of sublimation, at T = 298.15 K were derived by the Clausius-Clapeyron equation, from the temperature dependence of the vapor pressures of these compounds, measured by the Knudsen-effusion technique. From the values of Delta(f)H(m)degrees and Delta(cr)(g)H(m)degrees, the standard molar enthalpies of formation of all the compounds, in the gaseous phase, Delta(f)H(m)degrees (g), at T = 298.15 K, were derived. These values were also calculated by using the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computational approach.     

Experimental thermochemical study of two polymethylpyrazine N,N′-dioxide derivatives

The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline 2,3,5-trimethylpyrazine-1,4-dioxide and 2,3,5,6-tetramethylpyrazine-1,4-dioxide were measured, at T = 298.15 K, by static bomb calorimetry and the standard molar enthalpies of sublimation, at T = 298.15 K, were obtained from Calvet microcalorimetric measurements. These values were used to derive the respective standard molar enthalpies of formation in gaseous phase. The mean N–O bond dissociation enthalpy has been calculated for both compounds.