Thermochemical Research on Furfurylamine and 5-Methylfurfurylamine: Experimental and Computational Insights.

The need to transition from fossil fuels to renewables arises from factors such as depletion, price fluctuations, and environmental considerations. Lignocellulosic biomass, being abundant, and quickly renewable, and not interfering with food supplies, offers a standout alternative for chemical production. This paper explores the energetic characteristics of two derivatives of furfural-a versatile chemical obtained from biomass with great potential for commercial sustainable chemical and fuel production. The standard (p° = 0.1 MPa) molar enthalpies of formation of the liquids furfurylamine and 5-methylfurfurylamine were derived from the standard molar energies of combustion, determined in oxygen and at T = 298.15 K, by static bomb combustion calorimetry. Their standard molar enthalpies of vaporization were also determined at the same temperature using high-temperature Calvet microcalorimetry. By combining these data, the gas-phase enthalpies of formation at T = 298.15 K were calculated as -(43.5 ± 1.4) kJ·mol-1 for furfurylamine, and -(81.2 ± 1.7) kJ·mol-1 for 5-methylfurfurylamine. Furthermore, a theoretical analysis using G3 level calculations was performed, comparing the calculated enthalpies of formation with the experimental values to validate both results. This method has been successfully applied to similar molecules. The discussion looks into substituent effects in terms of stability and compares them with similar compounds.

Thermodynamic and conformational study of proline stereoisomers.

Amino acids play fundamental roles both as building blocks of proteins and as intermediates in metabolism. Proline, one of the 20 natural amino acids, has a primordial function in enzymes, peptide hormones, and proteins. The energetic characterization of these molecules provides information concerning stability and reactivity and has great importance in understanding the activity and behavior of larger molecules containing these structures as fragments. In the present work, parallel experimental and computational studies have been performed. The experimental studies have been based on calorimetric and effusion techniques, from which the enthalpy of formation in the crystalline phase and the enthalpy of sublimation of the sterioisomers L-, D-, and the DL-mixture of proline have been derived. Additionally, vapor pressure measurements have also enabled the determination of the entropies and Gibbs energies of sublimation, at T = 298.15 K. From the former results, the experimental standard (p(o) = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of L-proline, D-proline, and DL-proline have been calculated as -388.6 ± 2.3, -391.9 ± 2.0, and -391.5 ± 2.4 kJ·mol(-1), respectively. A computational study at the G3 and G4 levels has been carried out. Conformational analysis has been done and the enthalpy of formation of proline as well as other intrinsic properties such as acidity, basicity, adiabatic ionization enthalpy, electron and proton affinities, and bond dissociation enthalpies have been calculated. There is a very good agreement between calculated and experimental values, when they are available.

From 2,4-dimethoxypyrimidine to 1,3-dimethyluracil: isomerization and hydrogenation enthalpies and noncovalent interactions.

An enthalpic value for the N-methyllactam/O-methyllactim isomerization, in the gaseous phase, is reported in this work for the conversion between 2,4-dimethoxypyrimidine and 1,3-dimethyluracil. For this purpose, the enthalpy of formation of 2,4-dimethoxypyrimidine, in the gaseous phase, was obtained experimentally combining results from combustion calorimetry and Calvet microcalorimetry, and the enthalpy of formation of 1,3-dimethyluracil, in the gaseous phase, reported previously in the literature, is also discussed. The enthalpy of hydrogenation of 1,3-dimethyluracil is compared with the enthalpy of hydrogenation of uracil and interpreted in terms of aromaticity, considering the influence of the hyperconjugation and the hindrance of the solvation of the ring by the methyl groups. The enthalpy of sublimation of 2,4-dimethoxypyrimidine was obtained combining Calvet microcalorimetry and differential scanning calorimetry results. This enthalpy is compared with the enthalpy of sublimation of 1,3-dimethyluracil previously reported in the literature and analyzed herein. From the interplay between the experimental results and the theoretical simulation of dimers of these molecules, the influence of stereochemical hindrance on the in-plane intermolecular contacts and aromaticity on the π···π interactions is analyzed.

Energetic study of 4(3H)-pyrimidinone: aromaticity of reactions, hydrogen bond rules, and support for an anomeric effect.

4(3H)-Pyrimidinone is observed in nature in equilibrium with other tautomeric forms, mimicking the tautomeric equilibrium in pyrimidine nucleobases. In this work, the enthalpy of formation in the gaseous phase of 4(3H)-pyrimidinone was derived from the combination of the enthalpy of formation in the crystalline phase, obtained by static bomb combustion calorimetry, and the enthalpy of sublimation, obtained by Knudsen effusion. The gaseous phase enthalpy of formation of 4(3H)-pyrimidinone was interpreted in terms of isodesmic reactions that consider the enthalpic effects of hydroxypyridines and pyrimidine. After comparison of the experimental and computational results, the same type of isodesmic reactions was used to study the substituent effects of the hydroxyl functional group of 2-, 4-, and 5-hydroxypyrimidines. The influence of aromaticity on the energetics of hydroxypyrimidines was evaluated using the variation of nucleus-independent chemical shifts for several reactions. The influence of intramolecular hydrogen bonds was investigated using the quantum theory of atoms in molecules and the geometric rule of Baker and Hubbard to identify hydrogen bonds. The energetic results obtained were also interpreted in terms of an in plane anomeric effect in the pyrimidine ring.

Thermodynamic study of chlorobenzonitrile isomers: a survey on the polymorphism, pseudosymmetry, and the chloro···cyano interaction.

The relationships among structural and thermodynamic properties of 2-, 3-, and 4-chlorobenzonitrile were investigated, in the present work, using several experimental techniques (Knudsen effusion, differential scanning calorimetry, and combustion calorimetry) and computational studies. The CN···Cl intermolecular interactions are weaker in 2-chlorobenzonitrile, reflecting a lower enthalpy of sublimation. The two polymorphic forms of 4-chlorobenzonitrile were observed by differential scanning calorimetry and interpreted in terms of the strength of CN···Cl intermolecular interactions. The entropic differentiation due to the pseudosymmetry observed in the crystalline packing of 2-chlorobenzonitrile was evaluated. Using adequate working reactions and the respective standard molar enthalpies of formation, in the gaseous phase, the halogen-cyano intramolecular interaction was also evaluated. The theoretically estimated gas-phase enthalpies of formation were calculated using high-level ab initio molecular orbital calculations at the G3MP2B3 and MP2/cc-pVTZ levels of theory. The computed values support very well the experimental results obtained in this work.

From 2-hydroxypyridine to 4(3H)-pyrimidinone: computational study on the control of the tautomeric equilibrium.

In this work is investigated why the entrance of a nitrogen atom in the ring of cis-2-hydroxypyridine and 2-pyridinone, resulting in cis-4-hydroxypyrimidine and 4(3H)-pyrimidinone, respectively, shifts the tautomeric equilibrium from the hydroxyl form, in the pyridine derivative, to the ketonic form, in the pyrimidine derivative. The conclusions obtained for these model systems allow us to understand how to control the gaseous-phase keto-enol tautomeric equilibrium in nitrogen heterocyclic rings and justify the tautomeric preference in pyrimidine nucleobases. The experimental and computational energetics of tautomeric equilibrium were interpreted in terms of the aromaticity, intramolecular hydrogen bonds, and electronic delocalization, evaluated using nucleus independent chemical shifts, quantum theory of atoms in molecules, natural bond orbital analysis, and the thermodynamic changes of appropriate reactions.

Molecular energetics of alkyl pyrrolecarboxylates: calorimetric and computational study.

The pyrrole subunit plays an important role in material science as the building block of polypyrroles, an important representative class of conducting polymers, which found widely applications in the area of new materials due to their chemical, thermal, and electrical properties associated with their easiness and low cost of production, making them especially promising for commercial applications. The energetic characterization of this kind of molecules provides information concerning stability, reactivity, and biodegrability of chemical compounds in environment being, for example, helpful in choosing the most adequate method for their elimination by converting the waste into harmless compounds or even decreasing the production of toxic substances in industrial processes. This work reports a combination of calorimetric and computational determinations of several alkyl pyrrolecarboxylates (alkyl = methyl or ethyl) whose main purpose is the calculation of their standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K. Experimentally, for methyl 1-pyrrolecarboxylate (M1PC), methyl 2-pyrrolecarboxylate (M2PC), and ethyl 2-pyrrolecarboxylate (E2PC), these values were derived from the standard (p° = 0.1 MPa) molar enthalpies of formation, in the condensed phase, ΔfHm° (cr,l), at T = 298.15 K, obtained by static bomb combustion calorimetry, and from the standard molar enthalpies of phase transition, Δcr,l(g)Hm°, at T = 298.15 K, determined by high-temperature Calvet microcalorimetry. Standard ab initio molecular calculations, at the G3(MP2)//B3LYP level, were performed, and the standard enthalpies of formation of these three compounds were estimated. A very good agreement between the calculated and the experimental data was obtained. Thereby, we have extended these calculations to other alkyl pyrrolecarboxylates, namely, ethyl 1-pyrrolecarboxylate (E1PC), methyl 3-pyrrolecarboxylate (M3PC), and ethyl 3-pyrrolecarboxylate (E3PC), whose study was not performed experimentally. The computational analysis, at the B3LYP/6-31G(d) level of theory, of the six molecules allowed a detailed inspection and a better knowledge about their molecular structure and geometrical parameters.

Energetic study applied to the knowledge of the structural and electronic properties of monofluorobenzonitriles.

The present work reports an energetic and structural study of 2-fluoro-, 3-fluoro-, and 4-fluorobenzonitrile. The standard molar enthalpies of formation, in the condensed phase, of the three isomers were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K. The standard molar enthalpies of vaporization or sublimation (for 4-fluorobenzonitrile), at T = 298.15 K, were measured using high-temperature Calvet microcalorimetry. The combination of these two parameters yields the standard molar enthalpies of formation in the gaseous phase. The vapor-pressure study of the referred compounds was performed by a static method, and the enthalpies of phase transition derived from the application of the Clarke and Glew equation. Theoretically estimated gas-phase enthalpies of formation, basicities, proton and electron affinities, and adiabatic ionization enthalpies were calculated from the G3MP2B3 level of theory. In order to evaluate the electronic properties, the geometries were reoptimized at MP2/cc-pVTZ level, and the QTAIM and NICS were computed. On the basis of the donor-acceptor system, another approach for evaluating the electronic effect for these compounds, using the NBO is suggested. The UV-vis spectroscopy study for the three isomers was performed. The intensities and the band positions were correlated with the thermodynamic properties calculated computationally.

Diaminobenzenes: an experimental and computational study.

In the present work, the values of the standard (p(o) = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of 1,2-diaminobenzene, 1,3-diaminobenzene, and 1,4-diaminobenzene are reported as 86.6 ± 1.6, 89.6 ± 1.6, and 99.7 ± 1.7 kJ·mol⁻¹, respectively. These values were derived from experimental thermodynamic parameters, namely the standard (p(o) = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, Δf H(m)(o)(cr), at T = 298.15 K, obtained from the standard molar enthalpies of combustion, Δ(c) H(m)(o), measured by static bomb combustion calorimetry, and the standard molar enthalpies of sublimation, at T = 298.15 K, derived from the temperature-vapor pressure dependence, determined by the Knudsen mass loss effusion method. The results were compared with estimates obtained by standard ab initio molecular calculations at the G3(MP2)//B3LYP level. Experimental and calculated data are in very good agreement and show that the 1,2-diaminobenzene is, thermodynamically, the most stable isomer. Finally, proton and electron affinities, basicities and adiabatic ionization enthalpies were also computed at the same level.

Calorimetric and computational study of the thermochemistry of phenoxyphenols.

Thermodynamic properties of 3- and 4-phenoxyphenol have been determined by using a combination of calorimetric and effusion techniques as well as by high-level ab initio molecular orbital calculations. The standard (p° = 0.1 MPa) molar enthalpies of formation in the condensed and gas states, Δ(f)H(m)°(cr or l) and Δ(f)H(m)°(g), at T = 298.15 K, of 3- and 4-phenoxyphenol were derived from their energies of combustion in oxygen, measured by a static bomb calorimeter, and from the enthalpies of vaporization or sublimation derived respectively by Calvet microcalorimetry for the 3-phenoxyphenol and by Knudsen effusion technique for the 4-phenoxyphenol. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. 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. The good agreement between the G3MP2B3-derived values and the experimental gas-phase enthalpies of formation for the 3- and 4-phenoxyphenol gives confidence to the estimate concerning the 2-phenoxyphenol isomer, which was not experimentally studied, and to the estimates concerning the radical and the anion. Additionally, the experimental values of gas-phase enthalpies of formation were also compared with estimates based on the empirical scheme developed by Cox.

A combined experimental and computational thermodynamic study of difluoronitrobenzene isomers.

This work reports the experimental and computational thermochemical study performed on three difluorinated nitrobenzene isomers: 2,4-difluoronitrobenzene (2,4-DFNB), 2,5-difluoronitrobenzene (2,5-DFNB), and 3,4-difluoronitrobenzene (3,4-DFNB). The standard (p° = 0.1 MPa) molar enthalpies of formation in the liquid phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. A static method was used to perform the vapor pressure study of the referred compounds allowing the construction of the phase diagrams and determination of the respective triple point coordinates, as well as the standard molar enthalpies of vaporization, sublimation, and fusion for two of the isomers (2,4-DFNB and 3,4-DFNB). For 2,5-difluoronitrobenzene, only liquid vapor pressures were measured enabling the determination of the standard molar enthalpies of vaporization. Combining the thermodynamic parameters of the compounds studied, the following standard (p° = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived: Δ(f)H(m)° (2,4-DFNB, g) = -(296.3 ± 1.8) kJ · mol⁻¹, Δ(f)H(m)° (2,5-DFNB, g) = -(288.2 ± 2.1) kJ · mol⁻¹, and Δ(f)H(m)° (3,4-DFNB, g) = -(302.4 ± 2.1) kJ · mol⁻¹. Using the empirical scheme developed by Cox, several approaches were evaluated in order to identify the best method for estimating the standard molar gas phase enthalpies of formation of these compounds. The estimated values were compared to the ones obtained experimentally, and the approach providing the best comparison with experiment was used to estimate the thermodynamic behavior of the other difluorinated nitrobenzene isomers not included in this study. Additionally, the enthalpies of formation of these compounds along with the enthalpies of formation of the other isomers not studied experimentally, i.e., 2,3-DFNB, 2,6-DFNB, and 3,5-DFNB, were estimated using the composite G3MP2B3 approach together with adequate gas-phase working reactions. Furthermore, we also used this computational approach to calculate the gas-phase basicities, proton and electron affinities, and, finally, adiabatic ionization enthalpies.

Computational thermochemistry of six ureas, imidazolidin-2-one, N,N'-trimethyleneurea, benzimidazolinone, parabanic acid, barbital (5,5'-diethylbarbituric acid), and 3,4,4'-trichlorocarbanilide, with an extension to related compounds.

A computational study of the structural and thermochemical properties of N-phenyl (open) and N-alkyl (cyclic) ureas, through the use of M05-2X and B3LYP density functional theory calculations has been carried out. The consistency of the literature experimental results has been confirmed, and using mainly isodesmic reactions, the unknown Delta(f)H(0)(g) of the other urea derivatives were estimated. The experimental results together with the theoretical information have permitted the study of the effect of phenyl, p- and m-chlorophenyl, alkyl, and carbonyl substitutions on the thermodynamical stability of urea and its cyclic derivatives. The peculiar behavior of the N-tert-butyl substituent in cyclic ureas has been related to geometric deformations.

Experimental and computational thermodynamic study of three monofluoronitrobenzene isomers.

The present work reports the thermodynamic study performed on three monofluorinated nitrobenzene derivatives by a combination of experimental techniques and computational approaches. The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the liquid phase of the three isomers of fluoronitrobenzene were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapor pressure study of the referred compounds was done by a static method and, from the obtained results, the phase diagrams were elaborated, and the respective triple point coordinates, as well as the standard molar enthalpies of vaporization, sublimation and fusion, at T = 298.15 K, were determined. The combination of some of the referred thermodynamic parameters yielded the standard (p degrees = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K, of the studied compounds: Delta(f)H(m)(o) (2-fluoronitrobenzene, g) = -(102.4 +/- 1.5) kJ x mol(-1), Delta(f)H(m)(o) (3-fluoronitrobenzene, g) = -(128.0 +/- 1.7) kJ x mol(-1), and Delta(f)H(m)(o) (4-fluoronitrobenzene, g) = -(133.9 +/- 1.4) kJ x mol(-1). Using the empirical scheme developed by Cox, values of standard molar enthalpies of formation in the gaseous phase were estimated and afterwards compared with the ones obtained experimentally, and both were interpreted in terms of the molecular structure of the compounds. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that 4-fluoronitrobenzene is the most stable isomer from the thermodynamic point of view. Furthermore, this composite approach was also used to obtain information about the gas-phase basicities, proton and electron affinities and, finally, adiabatic ionization enthalpies.

Calorimetric and computational study of 2- and 3-acetyl-1-methylpyrrole isomers.

This work reports the enthalpies of formation in the condensed and gas phases of 2-acetyl-1-methylpyrrole and 3-acetyl-1-methylpyrrole, derived from the standard (p(o) = 0.1 MPa) molar enthalpies of combustion, in oxygen, Delta(c)H(m)(o), measured by static bomb combustion calorimetry and the standard molar enthalpies of vaporization, Delta(l)(g)H(m)(o), at T = 298.15 K, obtained by high-temperature Calvet microcalorimetry. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level; the computed values compare very well with the experimental results obtained in this work and show that the 2-acetyl-1-methylpyrrole is thermodynamically more stable than the 3-isomer. Furthermore, this composite method was also applied in the calculation of bond dissociation enthalpies, gas-phase basicities, proton and electron affinities, and adiabatic ionization enthalpies.

Thermochemistry of bithiophenes and thienyl radicals. A calorimetric and computational study.

The relative stabilities of 2,2'- and 3,3'-bithiophenes were evaluated by experimental thermochemistry and the results compared with data obtained from state of the art calculations, which were also extended to 2,3'-bithiophene. The standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline 2,2'-bithiophene and 3,3'-bithiophene were calculated from the standard molar energies of combustion, in oxygen, to yield CO(2) (g) and H(2)SO(4) x 115 H(2)O, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of these two compounds were measured as a function of temperature by Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation, at T = 298.15 K, were derived from the Clausius-Clapeyron equation. The experimental values were used to calculate the standard (p(o) = 0.1 MPa) enthalpies of formation of the title compounds in the gaseous phase; the results were analyzed and interpreted in terms of enthalpic increments and molecular structure. Standard ab initio molecular orbital calculations at the G3(MP2)//B3LYP level were performed. Enthalpies of formation, using homodesmotic reactions, were calculated and compared with experimental data. The computational study was also extended to the isomeric compound 2,3'-bithiophene. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out. Finally, bond dissociation enthalpies (BDE) and enthalpies of formation of thienyl radicals were also computed.

Experimental and computational study on the molecular energetics of 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid.

This paper reports a combined thermochemical experimental and computational study of 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid. Static bomb combustion calorimetry and Knudsen mass-loss effusion technique were used to determine the standard (p degrees = 0.1 MPa) molar enthalpies of combustion, Delta(c)H(m) degrees, and sublimation, Delta(cr)(g)H(m) degrees, respectively, from which the standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were derived. The values obtained were -(286.3 +/- 1.7) and -(291.6 +/- 1.7) kJ x mol for 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid, respectively. For comparison purposes, the gas-phase enthalpies of formation of these two compounds were estimated by G3(MP2)//B3LYP and MP2 approaches, 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 N-H bond dissociation enthalpies, gas-phase acidities and basicities, proton and electron affinities and adiabatic ionization enthalpies. Moreover, the results are also discussed in terms of the energetic effects of the addition of a carboxylic and of a methyl groups to the pyrrole ring and compared with structurally similar compounds.

Molecular energetics of cytosine revisited: a joint computational and experimental study.

A static bomb calorimeter has been used to measure the standard molar energy of combustion, in oxygen, at T = 298.15 K, of a commercial sample of cytosine. From this energy, the standard (p degrees = 0.1 MPa) molar enthalpy of formation in the crystalline state was derived as -(221.9 +/- 1.7) kJ.mol(-1). This value confirms one experimental value already published in the literature but differs from another literature value by 13.5 kJ.mol(-1). Using the present standard molar enthalpy of formation in the condensed phase and the enthalpy of sublimation due to Burkinshaw and Mortimer [J. Chem. Soc., Dalton Trans. 1984, 75], (155.0 +/- 3.0) kJ.mol(-1), results in a value for the gas-phase standard molar enthalpy of formation for cytosine of -66.9 kJ.mol(-1). A similar value, -65.1 kJ.mol(-1), has been estimated after G3MP2B3 calculations combined with the reaction of atomization on three different tautomers of cytosine. In agreement with experimental evidence, the hydroxy-amino tautomer is the most stable form of cytosine in the gas phase. The enthalpies of formation of the other two tautomers were also estimated as -60.7 kJ.mol(-1) and -57.2 kJ.mol(-1) for the oxo-amino and oxo-imino tautomers, respectively. The same composite approach was also used to compute other thermochemical data, which is difficult to be measured experimentally, such as C-H, N-H, and O-H bond dissociation enthalpies, gas-phase acidities, and ionization enthalpies.

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.

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.

Energetics of C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols. enthalpies of formation of XCH(2)CH(2)OH (X = F, Cl, Br, I) compounds and of the 2-hydroxyethyl radical.

The energetics of the C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-chloro-, 2-bromo-, and 2-iodoethanol, at 298.15 K, were determined as Delta(f)H(degree)m(CH2CH2OH, l) = -315.5 +/- 0.7 kJ.mol-1, Delta(f)H(degree)mBrCH2CH2OH, l) = -275.8 +/- 0.6 kJ.mol-1, Delta(f)H(degree)m(ICH2CH2OH, l) = -207.3 +/- 0.7 kJ.mol-1, by rotating-bomb combustion calorimetry. The corresponding standard molar enthalpies of vaporization, Delta(vap)H(degree)m(ClCH2CH2OH) = 48.32 +/- 0.37 kJ.mol-1, Delta(vap)H(degree)m(BrCH2CH2OH) = 54.08 +/- 0.40 kJ.mol-1, and Delta(vap)H(degree)m(ICH2CH2OH) = 57.03 +/- 0.20 kJ.mol-1 were also obtained by Calvet-drop microcalorimetry. The condensed phase and vaporization enthalpy data lead to Delta(f)H(degree)m(ClCH2CH2OH, g) = -267.2 +/- 0.8 kJ.mol-1, Delta(f)H(degree)m(BrCH2CH2OH, g) = -221.7 +/- 0.7 kJ.mol-1, and Delta(f)H(degree)m(ICH2CH2OH, g) = -150.3 +/- 0.7 kJ.mol-1. These values, together with the enthalpy of selected isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3LYP/cc-pVTZ) and CBS-QB3 calculations were used to derive the enthalpies of formation of gaseous 2-fluoroethanol, Delta(f)H(degree)m(FCH2CH2OH, g) = -423.6 +/- 5.0 kJ.mol-1, and of the 2-hydroxyethyl radical, Delta(f)H(degree)m(CH2CH2OH, g) = -28.7 +/- 8.0 kJ.mol-1. The obtained thermochemical data led to the following carbon-halogen bond dissociation enthalpies: DHo(X-CH2CH2OH) = 474.4 +/- 9.4 kJ.mol-1 (X = F), 359.9 +/- 8.0 kJ.mol-1 (X = Cl), 305.0 +/- 8.0 kJ.mol-1 (X = Br), 228.7 +/- 8.1 kJ.mol-1 (X = I). These values were compared with the corresponding C-X bond dissociation enthalpies in XCH2COOH, XCH3, XC2H5, XCH=CH2, and XC6H5. In view of this comparison the computational methods mentioned above were also used to obtain Delta(f)H(degree)m-594.0 +/- 5.0 kJ.mol-1 from which DHo(F-CH2COOH) = 435.4 +/- 5.4 kJ.mol-1. The order DHo(C-F) > DHo(C-Cl) > DHo(C-Br) > DHo(C-I) is observed for the haloalcohols and all other RX compounds. It is finally concluded that the major qualitative trends exhibited by the C-X bond dissociation enthalpies for the series of compounds studied in this work can be predicted by Pauling's electrostatic-covalent model.