A combined experimental and computational study on the thermochemistry of 2- and 3-acetylpyrroles was performed. The enthalpies of combustion and sublimation were measured by static bomb combustion calorimetry and Knudsen effusion mass-loss technique, respectively, and the standard (p(o) = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were determined. Additionally, the gas-phase enthalpies of formation were estimated by G3(MP2)//B3LYP calculations, using several gas-phase working reactions, and were compared with the experimental ones. N-H bond dissociation enthalpies, gas-phase acidities and basicities, proton and electron affinities and ionization enthalpies were also calculated. Experimental and theoretical results are in good agreement and show that 2-acetylpyrrole is thermodynamically more stable than the 3-isomer. The substituent effects of the acetyl group in pyrrole, thiophene and pyridine rings were also analyzed.

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.

Density functional theory has been used to investigate gas-phase thermodynamic properties of phenol and dichlorophenols. Molecular geometries, energies, and vibrational frequencies were computed at the B3LYP and BP86 levels of theory. At T = 298.15 K, calculated standard enthalpies of formation are in excellent agreement with experimental data. The average deviation between calculated and experimental values is of about 2.3 kJ/mol, and in some cases, theoretical values fall within experimental uncertainty. Other properties for which only a few experimental results were available in the literature were also calculated, namely, O−H homolytic bond dissociation energies, gas-phase acidities, ionization energies, and proton and electron affinities.

There are conflicting reports on the origin of the effect of Y substituents on the S-H bond dissociation enthalpies (BDEs) in 4-Y-substituted thiophenols, 4-YC(6)H(4)S-H. The differences in S-H BDEs, [4-YC(6)H(4)S-H] - [C(6)H(5)S-H], are known as the total (de)stabilization enthalpies, TSEs, where TSE = RSE - MSE, i.e., the radical (de)stabilization enthalpy minus the molecule (de)stabilization enthalpy. The effects of 4-Y substituents on the S-H BDEs in thiophenols and on the S-C BDEs in phenyl thioethers are expected to be almost identical. Some S-C TSEs were therefore derived from the rates of homolyses of a few 4-Y-substituted phenyl benzyl sulfides, 4-YC(6)H(4)S-CH(2)C(6)H(5), in the hydrogen donor solvent 9,10-dihydroanthracene. These TSEs were found to be -3.6 +/- 0.5 (Y = NH(2)), -1.8 +/- 0.5 (CH(3)O), 0 (H), and 0.7 +/- 0.5 (CN) kcal mol(-1). The MSEs of 4-YC(6)H(4)SCH(2)C(6)H(5) have also been derived from the results of combustion calorimetry, Calvet-drop calorimetry, and computational chemistry (B3LYP/6-311+G(d,p)). The MSEs of these thioethers were -0.6 +/- 1.1 (NH(2)), -0.4 +/- 1.1 (CH(3)O), 0 (H), -0.3 +/- 1.3 (CN), and -0.8 +/- 1.5 (COCH(3)) kcal mol(-1). Although all the enthalpic data are rather small, it is concluded that the TSEs in 4-YC(6)H(4)SH are largely governed by the RSEs, a somewhat surprising conclusion in view of the experimental fact that the unpaired electron in C(6)H(5)S(*) is mainly localized on the S. The TSEs, RSEs, and MSEs have also been computed for a much larger series of 4-YC(6)H(4)SH and 4-YC(6)H(4)SCH(3) compounds by using a B3P86 methology and have further confirmed that the S-H/S-CH(3) TSEs are dominated by the RSEs. Good linear correlations were obtained for TSE = rho(+)sigma(p)(+)(Y), with rho(+) (kcal mol(-1)) = 3.5 (S-H) and 3.9 (S-CH(3)). It is also concluded that the SH substituent is a rather strong electron donor with a sigma(p)(+)(SH) of -0.60, and that the literature value of -0.03 is in error. In addition, the SH rotational barriers in 4-YC(6)H(4)SH have been computed and it has been found that for strong electron donating (ED) Ys, such as NH(2), the lowest energy conformer has the S-H bond oriented perpendicular to the aromatic ring plane. In this orientation the SH becomes an electron withdrawing (EW) group. Thus, although the OH group in phenols is always in-plane and ED irrespective of the nature of the 4-Y substituent, in thiophenols the SH switches from being an ED group with EW and weak ED 4-Ys, to being an EW group for strong ED 4-Ys.

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The standard (p ¼ 0:1 MPa) molar enthalpies of formation of 2-, 3-, and 4-bromoaniline, 2,4-, 2,5-, and 2,6-dibromoaniline, and 2,4,6-tribromoaniline were derived from the standard molar enthalpies of combustion, in oxygen, which yields CO 2 (g), N 2 (g), and HBr Á 600H 2 O(l), at T ¼ 298:15 K, measured by using rotating-bomb calorimetry.The standard molar enthalpies of sublimation, or vaporization, of these compounds at T ¼ 298:15 K were measured by using Calvet microcalorimetry, and so their standard molar enthalpies of formation in the gaseous state were derived. The enthalpies of fusion of the solid compounds were determined by DSC. The gas-phase enthalpies of formation were also estimated by density functional theory calculations performed at the BP86/6-31+G Ã level. As a result it is confirmed that thermodynamic data is consistent with intramolecular N-HÁÁÁBr hydrogen bonding.Aromatic amines are of great importance in biological and material sciences as well as in pharmaceutical and chemical industries. Halogenated anilines have gained the most focus of all of the aromatic amines, due to their high toxicity and wide uses. For example, they may be used as reagents or precursors for synthesizing organic dyes, in pigments, in agricultural agents, in pharmaceuticals and in the rubber industries.1,2 Haloanilines are frequently found in both effluents of waste water treatment plants and surface water, due to chemical degradation or biotransformation of pesticides and herbicides, or due to accidental spills and illegal release of industrial and municipal wastewater. 3 These compounds have been determined to induce several kinds of toxicity, like methemoglobinemia, splenotoxicity, hepatotoxicity, and nephrotoxicity. 2,4Because of their stability in the presence of air and humidity, and potentially attractive economics, 5,6 aniline and haloanilines have been also used in the synthesis of conducting polymers with applications in rechargeable batteries, electromagnetic interference shielding, electrochromic display devices, sensors, and electrocatalysis.The current work presents the results of calorimetric and computational thermochemistry studies on bromoanilines. The experimental investigation includes the determination of the standard massic energies of combustion in oxygen at T ¼ 298:15 K of the different bromoanilines using a rotatingbomb combustion calorimeter, from which the values of the standard molar enthalpies of formation in the condensed phase were derived. The determination of the standard molar enthalpies of sublimation or vaporization at T ¼ 298:15 K was done by using Calvet microcalorimetry with the high-temperature vacuum sublimation technique; these values allowed the derivation of the standard molar enthalpies of formation, in the gaseous state, of 2-, 3-, and 4-bromoaniline, 2,4-, 2,5-, and 2,6-dibromoaniline, and 2,4,6-tribromoaniline, which were compared with values estimated by the Cox scheme 7 and those obtained by density functional theory calculations. Experimental Materials.The three mono...

To understand the influence of the methyl group in the stability and conformational behavior of the piperidine ring, the standard (p 0 ) 0.1 MPa) molar enthalpies of formation of 1-methylpiperidine, 3-methylpiperidine, 4-methylpiperidine, 2,6-dimethylpiperidine, and 3,5-dimethylpiperidine, both in the liquid and in the gaseous states, were determined at the temperature of 298.15 K. The numerical values of the enthalpies of formation in the liquid and in the gaseous state are, respectively, -(95.9 ( 1.6) and -(59.1 ( 1.7) kJ‚mol -1 for 1-methylpiperidine; -(123.6 ( 1.4) and -(79.2 ( 1.6) kJ‚mol -1 for 3-methylpiperidine; -(123.5 ( 1.5) and -(82.9 ( 1.7) kJ‚mol -1 for 4-methylpiperidine; -(153.6 ( 2.1) and -(111.2 ( 2.2) kJ‚mol -1 for 2,6-dimethylpiperidine; and -(155.0 ( 1.7) and -(105.9 ( 1.8) kJ‚mol -1 for 3,5-dimethylpiperidine. In addition, and to be compared with the experimental results, theoretical calculations were carried out considering different ab initio and density functional theory based methods. The standard molar enthalpies of formation of the four isomers of methylpiperidine and of the 12 isomers of dimethylpiperidine have been computed. The G3MP2B3-derived numbers are in excellent agreement with experimental data, except in the case of 2,6-dimethylpiperidine for which a deviation of 9 kJ‚mol -1 was found. Surprisingly, the DFT methods fail in the prediction of these properties with the exception of the most approximated SVWN functional.

Density Functional Theory was used to investigate several gas-phase thermodynamic parameters of the o-, m-, and p-aminophenol isomers. Within the DFT approach, the B3LYP method and the 6-31G(d) and 6-311ϩG(2d,2p) basis sets were used to compute standard enthalpies of formation. Calculated data are in excellent agreement with the experimental work of Nuñ ez et al. [J Chem Thermodyn 1996, 18, 575-579] but differs significantly from the values of Sabbah et al. [Can J Chem 1996, 74, 500 -507]. In this work, other properties such as homolytic O-H and N-H bond dissociation energies, gas-phase acidities, and proton or electron affinities were also obtained and confirm the few experimental results available for these properties in these kind of compounds, except the O-H homolytic dissociation energy of 2-aminophenol.