A Combined Experimental and Computational Thermodynamic Study of the Isomers of Pyrrolecarboxaldehyde and 1-Methyl- pyrrolecarboxaldehyde

The present paper reports an experimental calorimetric study of 2-pyrrolecarboxaldehyde and 1-methyl-2-pyrrolecarboxaldehyde, which aims to determine their standard (<i>p</i>° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at <i>T</i> = 298.15 K. These values were derived from the standard (<i>p</i>° = 0.1 MPa) molar enthalpies of formation, in the condensed phase, Δ_f<i>H</i>_m°(cr,l), at <i>T</i> = 298.15 K, obtained from the standard molar enthalpies of combustion, Δ_c<i>H</i>_m°, measured by static bomb combustion calorimetry, and from the standard molar enthalpies of phase transition, Δ_cr,l^g<i>H</i>_m^o at <i>T</i> = 298.15 K, obtained by high temperature Calvet microcalorimetry. Additionally, the standard enthalpies of formation of these two compounds were estimated by computations based on standard ab initio molecular calculations at the G3(MP2)//B3LYP level. The estimated values are in very good agreement with experimental data, giving us support to estimate the gas-phase enthalpies of formation of the 3-pyrrolecarboxaldehyde and 1-methyl-3-pyrrolecarboxaldehyde that were not studied experimentally. N–H bond dissociation enthalpies, gas-phase acidities and basicities, proton and electron affinities, and adiabatic ionization enthalpies were also calculated. Furthermore, the molecular structure of the four molecules was established and the structural parameters were determined at the B3LYP/6-31G(d) level of theory

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 (<i>p</i>^o = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at <i>T</i> = 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 (<i>p</i>^o = 0.1 MPa) molar enthalpies of formation, in the condensed phase, Δ_f<i>H</i>_m^o (cr,l), at <i>T</i> = 298.15 K, obtained by static bomb combustion calorimetry, and from the standard molar enthalpies of phase transition, Δ_cr,l^g<i>H</i>_m^o, at <i>T</i> = 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

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 <i>T</i> = 298.15 K. From the former results, the experimental standard (<i>p</i> ^o = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at <i>T</i> = 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