A Promising Thermodynamic Study of Hole Transport Materials to Develop Solar Cells: 1,3-Bis(N-carbazolyl)benzene and 1,4-Bis(diphenylamino)benzene.

The thermochemical study of the 1,3-bis(N-carbazolyl)benzene (NCB) and 1,4-bis(diphenylamino)benzene (DAB) involved the combination of combustion calorimetric (CC) and thermogravimetric techniques. The molar heat capacities over the temperature range of (274.15 to 332.15) K, as well as the melting temperatures and enthalpies of fusion were measured for both compounds by differential scanning calorimetry (DSC). The standard molar enthalpies of formation in the crystalline phase were calculated from the values of combustion energy, which in turn were measured using a semi-micro combustion calorimeter. From the thermogravimetric analysis (TGA), the rate of mass loss as a function of the temperature was measured, which was then correlated with Langmuir's equation to derive the vaporization enthalpies for both compounds. From the combination of experimental thermodynamic parameters, it was possible to derive the enthalpy of formation in the gaseous state of each of the title compounds. This parameter was also estimated from computational studies using the G3MP2B3 composite method. To prove the identity of the compounds, the 1H and 13C spectra were determined by nuclear magnetic resonance (NMR), and the Raman spectra of the study compounds of this work were obtained.

Corrections to standard state in combustion calorimetry: an update and a web-based tool.

Combustion calorimetry is the predominant method for determination of enthalpies of formation for organic compounds. Both initial and final states of the calorimeter deviate significantly from the standard conditions. Correction of the obtained results to the standard state must be applied as accurately as possible to determine the combustion energy with an acceptable uncertainty, which is typically a few hundredths of a percent. The correction procedures in their current form were introduced in 1956 with simplifications to allow application in a pre-computer era. In this work, the procedures have been updated with respect to both the equations and reference values. The most reliable data sources are identified, and the updated algorithm is presented in the form of a Web-based tool available through the NIST TRC Web site.

Experimental and Theoretical Investigation on the Thermochemistry of 3-Methyl-2-benzoxazolinone and 6-Nitro-2-benzoxazolinone.

The determination of the reliable thermodynamic properties of 2-benzoxazolinone derivatives is the main goal of this work. Some correlations are established between the energetic properties determined and the structural characteristics of the title compounds, and the reactivity of this class of compounds is also evaluated. Static-bomb combustion calorimetry and high-temperature Calvet microcalorimetry were used to determine, respectively, the standard molar enthalpies of formation in the solid state and the standard molar enthalpies of sublimation, both at T = 298.15 K. Using the results obtained for each compound, the respective gas-phase standard molar enthalpy of formation was derived. High-level quantum chemical calculations were performed to estimate the same property and the results evidence good accordance. Moreover, the gas-phase relative thermodynamic stability of 2-benzoxazolinone derivatives was also evaluated using the respective gas-phase standard molar Gibbs energy of formation. In addition, the relationship between the energetic and structural characteristics of the benzoxazolinones is presented, evidencing the enthalpic increments associated with the presence of a methyl and a nitro groups in the molecule, and this effect is compared with similar ones in other structurally related compounds.

The Relative Thermodynamic Stability of Diamond and Graphite.

Recent density-functional theory (DFT) calculations raised the possibility that diamond could be degenerate with graphite at very low temperatures. Through high-accuracy calorimetric experiments closing gaps in available data, we reinvestigate the relative thermodynamic stability of diamond and graphite. For T<400 K, graphite is always more stable than diamond at ambient pressure. At low temperatures, the stability is enthalpically driven, and entropy terms add to the stability at higher temperatures. We also carried out DFT calculations: B86bPBE-25X-XDM//B86bPBE-XDM and PBE0-XDM//PBE-XDM results overlap with the experimental -TΔS results and bracket the experimental values of ΔH and ΔG, displaced by only about 2× the experimental uncertainty. Revised values of the standard thermodynamic functions for diamond are Δf Ho =-2150±150 J mol-1 , Δf So =3.44±0.03 J K-1 mol-1 and Δf Go =-3170±150 J mol-1 .

Influence of Hydroxyl Functional Group on the Structure and Stability of Xanthone: A Computational Approach.

The present work addresses computational research focused on the energetic and structural properties of four isomers monohydroxyxanthone, using the G3(MP2)//B3LYP method, in order to evaluate the influence of the hydroxyl (-OH moiety) functional group on the xanthone molecule. The combination of these computational results with previous experimental data of these compounds enabled the determination of their enthalpies, entropies and Gibbs energies of formation, in the gaseous phase, and consequently to infer about the relative thermodynamic stability of the four isomers. Other issues were also addressed for the hydroxyxanthone isomers, namely the conformational and the tautomeric equilibrium analysis of the optimized molecular structures, the frontier orbitals, and the electrostatic potential energy maps. Complementarily, an energetic study of the intramolecular O - H ⋯ O hydrogen bond for 1-hydroxanthone was also performed.

Energetic and Structural Properties of Two Phenolic Antioxidants: Tyrosol and Hydroxytyrosol.

Theoretical and experimental studies on the energetic, structural and some other relevant physicochemical properties of the antioxidant tyrosol (1), hydroxytyrosol (1OH) molecules and the corresponding radicals 1rad• and 1Orad• are reported in this work. The experimental values of the gas-phase enthalpy of formation, Δf Hm0(g), in kJ·mol-1, of 1 (-302.4 ± 3.4) and 1OH (-486.3 ± 4.1) have been determined. Quantum chemical calculations, at DFT (M05-2X) and composite ab initio G3 and G4 levels of theory, provided results that served to (i) confirm the excellent consistency of the experimental measurements performed, (ii) establish that the stabilizing effect of H-bond of hydroxyethyl chain and aromatic ring (OH···π interaction) is smaller in radicals than in parent molecules, (iii) deduce-combining experimental data in isodesmic reactions-Δf Hm0(g) of radicals 1rad• (-152.3 ± 4.4 kJ·mol-1) and 1Orad• (-370.6 ± 3.8 kJ·mol-1), (iv) estimate a reliable O-H bond dissociation enthalpy, BDE of 1 (368.1 ± 5.6 kJ·mol-1) and of 1OH (333.7 ± 5.6 kJ·mol-1), and (v) corroborate-using "BDE criteria"-than 1OH is a more effective antioxidant than 1.

Energetic Effect of the Carboxylic Acid Functional Group in Indole Derivatives.

The standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, was calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, yielding -(222.2 ± 3.5) kJ·mol-1 and -(234.1 ± 2.1) kJ·mol-1 for indole-3-carboxylic acid and 1-methylindole-3-carboxylic acid, respectively. Computational studies, at the G3(MP2) composite level, were conducted for indole-3-carboxylic acid and 1-methylindole-3-carboxylic acid as a complement of the experimental work, and they were also extended to the remaining isomers, indole-2-carboxylic acid, 1-methylindole-2-carboxylic acid, 3-methylindole-2-carboxylic acid, and 2-methylindole-3-carboxylic acid, to provide reliable estimates of the corresponding thermochemical parameters. The agreement of the estimates of the standard gas-phase enthalpy of formation so obtained, indole-2-carboxylic acid -(223.6 ± 0.8) kJ·mol-1, 1-methylindole-2-carboxylic acid -(223.7 ± 0.8) kJ·mol-1, 3-methylindole-2-carboxylic acid -(251.6 ± 1.0) kJ·mol-1, indole-3-carboxylic acid -(227.1 ± 1.1) kJ·mol-1, 1-methylindole-3-carboxylic acid -(238.0 ± 1.0) kJ·mol-1, and 2-methylindole-3-carboxylic acid -(267.2 ± 1.0) kJ·mol-1, with the available experimental data gives us additional confidence for the situations not studied experimentally. The enthalpic effect resulting from the entrance of the carboxyl group into the indole ring was discussed, and an enthalpic stabilization was found for indole and pyrrole derivatives when compared with other similar systems.

Study on the volatility of halogenated fluorenes.

This work reports the experimental determination of relevant thermophysical properties of five halogenated fluorenes. The vapor pressures of the compounds studied were measured at different temperatures using two different experimental techniques. The static method was used for studying 2-fluorofluorene (liquid and crystal vapor pressures between 321.04 K and 411.88 K), 2-iodofluorene (liquid and crystal vapor pressures between 362.63 K and 413.86 K), and 2,7-dichlorofluorene (crystal vapor pressures between 364.64 K and 394.22 K). The Knudsen effusion method was employed to determine the vapor pressures of 2,7-difluorofluorene (crystal vapor pressures between 299.17 K and 321.19 K), 2,7-diiodofluorene (crystal vapor pressures between 393.19 K and 415.14 K), and (again) 2-iodofluorene (crystal vapor pressures between 341.16 K and 361.12 K). The temperatures and the molar enthalpies of fusion of the five compounds were determined using differential scanning calorimetry. The application to halogenated fluorenes of recently developed methods for predicting vapor pressures and enthalpies of sublimation and vaporization of substituted benzenes is also discussed.

Vapor pressures, thermodynamic stability, and fluorescence properties of three 2,6-alkyl naphthalenes.

This work reports the experimental determination of relevant thermodynamic properties and the characterization of luminescence properties of the following polycyclic aromatic hydrocarbons (PAHs): 2,6-diethylnaphthalene, 2,6-diisopropylnaphthalene and 2,6-di-tert-butylnaphthalene. The standard (p(o) = 0.1 MPa) molar enthalpies of combustion, ΔcHm(o), of the three compounds were determined using static bomb combustion calorimetry. The vapor pressures of the crystalline phase of 2,6-diisopropylnaphthalene and 2,6-di-tert-butylnaphthalene were measured at different temperatures using the Knudsen effusion method and the vapor pressures of both liquid and crystalline phases of 2,6-diethylnaphthalene were measured by means of a static method. The temperatures and the molar enthalpies of fusion of the three compounds were determined using differential scanning calorimetry. The gas-phase molar heat capacities and absolute entropies of the three 2,6-dialkylnaphthalenes studied were determined computationally. The thermodynamic stability of the compounds in both the crystalline and gaseous phases was evaluated by the determination of the Gibbs energies of formation and compared with the ones reported in the literature for 2,6-dimethylnaphthalene. From fluorescence spectroscopy measurements, the optical properties of the compounds studied and of naphthalene were evaluated in solution and in the solid state.

Thermochemical insights on the conformational energetics of azepan and azepan-1-ylacetonitrile.

This paper is concerned with computational and experimental thermochemical studies of azepan and azepan-1-ylacetonitrile, molecules whose flexible ring structure provides several conformational forms with low energy barriers among them. The computational study describes the energetic analysis of the six most stable conformers on the potential energy surfaces and the determination of their gas-phase standard enthalpy of formation at the reference temperature of 298.15 K. The same gas-phase enthalpic parameters are also derived from the enthalpies of formation in the liquid phase and the enthalpies of vaporization, at T = 298.15 K, determined experimentally using the combustion calorimetry and the Calvet microcalorimetry techniques, respectively. The experimental data reported in this work for the two titled compounds together with other available in the literature for related molecules enabled the establishment of an increments scheme, providing a reliable approach on the prevision of gas-phase enthalpy of formation of cyclic/acyclic hydrocarbons and amines. Complementary, natural bond orbital (NBO) calculations were also performed, allowing an advance on the analysis of the structural and reactivity behavior of these type of compounds.

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.

Effects of methoxy and formyl substituents on the energetics and reactivity of α-naphthalenes: a calorimetric and computational study.

A combined experimental and computational study was developed to evaluate and understand the energetics and reactivity of formyl and methoxy α-naphthalene derivatives. Static bomb combustion calorimetry and the Calvet microcalorimetry were the experimental techniques used to determine the standard (p(o)=0.1 MPa) molar enthalpies of formation, in the liquid phase, ΔfHm(o)(l), and of vaporization, Δl(g)Hm(o), at T=298.15K, respectively, of the two liquid naphthalene derivatives. Those experimental values were used to derive the values of the experimental standard molar enthalpies of formation, in the gaseous phase, ΔfHm(o)(g), of 1-methoxynaphthalene, (-3.0 ± 3.1)kJmol(-1), and of 1-formylnaphthalene, (36.3 ± 4.1)kJ mol(-1). High-level quantum chemical calculations at the composite G3(MP2)//B3LYP level were performed to estimate the values of the ΔfHm(o)(g) of the two compounds studied resulting in values in very good agreement with experimental ones. Natural bond orbital (NBO) calculations were also performed to determine more about the structure and reactivity of this class of compounds.

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.

Energetic studies and phase diagram of thioxanthene.

The molecular stability of thioxanthene, a key species from which very important compounds with industrial relevance are derived, has been studied by a combination of several experimental techniques and computational approaches. The standard (p degrees = 0.1 MPa) molar enthalpy of formation of crystalline thioxanthene (117.4 +/- 4.1 kJ x mol(-1)) was determined from the experimental standard molar energy of combustion, in oxygen, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The enthalpy of sublimation was determined by a direct method, using the vacuum drop microcalorimetric technique, and also by an indirect method, using a static apparatus, where the vapor pressures at different temperatures were measured. The latter technique was used for both crystalline and undercooled liquid samples, and the phase diagram of thioxanthene near the triple point was obtained (triple point coordinates T = 402.71 K and p = 144.7 Pa). From the two methods, a mean value for the standard (p degrees = 0.1 MPa) molar enthalpy of sublimation, at T = 298.15 K (101.3 +/- 0.8 kJ x mol(-1)), was derived. From the latter value and from the enthalpy of formation of the solid, the standard (p degrees = 0.1 MPa) enthalpy of formation of gaseous thioxanthene was calculated as 218.7 +/- 4.2 kJ x mol(-1). Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several homodesmotic reactions in order to derive the standard molar enthalpy of formation of thioxanthene. The ab initio results are in excellent agreement with the experimental data.

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.

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 the N-O bonds in 2-hydroxyphenazine-di-N-oxide.

The standard enthalpy of formation and the enthalpy of sublimation of crystalline 2-hydroxyphenazine-di-N-oxide, at T = 298.15 K, were determined from isoperibol static bomb combustion calorimetry and from Knudsen effusion experiments, as -76.7 +/- 4.2 kJ.mol(-1) and 197 +/- 5 kJ.mol(-1), respectively. The sum of these two quantities gives the standard enthalpy of formation in the gas-phase for this compound, delta(f)H(m)degrees(g) = 120 +/- 6 kJ.mol(-1). This value was combined with the gas-phase standard enthalpy of formation for 2-hydroxyphenazine retrieved from a group estimative method yielding the mean (N-O) bond dissociation enthalpy, in the gas-phase, for 2-hydroxyphenazine-di-N-oxide. The result obtained with this strategy is (DH(m)degrees (N - O)) = 263 +/- 4 kJ.mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computed value, 265 kJ.mol(-1).

Thermochemistry of 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide. Evaluation of the mean dissociation enthalpy of the (N-O) bond.

The standard enthalpy of formation of the 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide compound in the gas-phase was derived from the enthalpies of combustion of the crystalline solid measured by static bomb combustion calorimetry and its enthalpy of sublimation determined by Knudsen mass-loss effusion at T= 298.15 K. This value is (383.8 +/- 5.4) kJ mol(-1) and was subsequently combined with the experimental gas-phase enthalpy of formation of atomic oxygen and with the computed gas-phase enthalpy of formation of 2-amino-3-quinoxalinecarbonitrile, (382.0 +/- 6.3) kJ mol(-1), in order to estimate the mean (N-O) bond dissociation enthalpy in the gas-phase of 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide. The result obtained is (248.3 +/- 8.3) kJ mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computed value.