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Aminoacids play fundamental roles both as building blocks of proteinsand as intermediates in metabolism. Proline, one of the 20 naturalamino acids, has a primordial function in enzymes, peptide hormones,and proteins. The energetic characterization of these molecules providesinformation concerning stability and reactivity and has great importancein understanding the activity and behavior of larger molecules containingthese structures as fragments. In the present work, parallel experimentaland computational studies have been performed. The experimental studieshave been based on calorimetric and effusion techniques, from whichthe enthalpy of formation in the crystalline phase and the enthalpyof sublimation of the sterioisomers l-, d-, andthe dl-mixture of proline have been derived. Additionally,vapor pressure measurements have also enabled the determination ofthe entropies and Gibbs energies of sublimation, at T= 298.15 K. From the former results, the experimental standard (po= 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 calculatedas −388.6 ± 2.3, −391.9 ± 2.0, and −391.5± 2.4 kJ·mol–1, respectively. A computationalstudy at the G3 and G4 levels has been carried out. Conformationalanalysis has been done and the enthalpy of formation of proline aswell as other intrinsic properties such as acidity, basicity, adiabaticionization enthalpy, electron and proton affinities, and bond dissociationenthalpies have been calculated. There is a very good agreement betweencalculated and experimental values, when they are available. [ABSTRACT FROM AUTHOR]

Thermochemical and thermodynamic properties of 2,5-dimethyl-1-phenylpyrrole and 2,5-dimethyl-1-(4-nitrophenyl)pyrrole have been determined by using a combination of calorimetric and effusion techniques as well as high-level ab initio molecular orbital calculations. The standard (p° = 0.1 MPa) molar enthalpies of formation, in the crystalline state, ΔfHm°(cr), at T= 298.15 K, were derived from the standard molar enthalpies of combustion, ΔcHm°, which were obtained from static bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to determine the standard molar enthalpies of sublimation, ΔcrgHm°, at T= 298.15 K. From the experimental results, the standard molar enthalpies of formation, in the gaseous phase, at T= 298.15 K, were derived. The results were analyzed and interpreted in terms of enthalpic increments and molecular structure. For comparison purposes, standard ab initio molecular calculations at the G3(MP2)//B3LYP level were performed, using a set of working reactions and the gas-phase enthalpies of formation of both compounds were estimated; the results are in excellent agreement with experimental data. The computational study was also extended to the determination of proton and electron affinities, basicities and adiabatic ionization enthalpies. [ABSTRACT FROM AUTHOR]

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° = 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 CO2(g) and H2SO4·115H2O, 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° = 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. [ABSTRACT FROM AUTHOR]

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° = 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. [ABSTRACT FROM AUTHOR]