Molecular Energetics of Alkyl Pyrrolecarboxylates: Calorimetric and Computational Study

Abstract

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.

Energetics and molecular structure of alkyl 1-methylpyrrolecarboxylates (alkyl=methyl or ethyl)

“…The NÀH,O-anti conformation is 8.4, 0.3 and 8.0 kJ Á mol À1 less stable than the NÀH,O-syn, for M1M2PC, M1M3PC and E1M2PC, respectively. For E1M3PC, the NÀH,O-anti conformation is slightly more stable than the syn structure, by 0.7 kJ Á mol À1 ; the same conformation has already been found for the E3PC [12].…”

“…At the G3(MP2)//B3LYP level, the NÀH,O-syn is the preferred conformation for M1M2PC, M1M3PC and E1M2PC, in agreement with the corresponding non-methylated analogues in the N-position, the methyl 2-pyrrolecarboxylate (M2PC), methyl 3-pyrrolecarboxylate (M3PC) and ethyl 2-pyrrolecarboxylate (E2PC) [12], as well as with other substituted pyrroles [2,3,10]. The NÀH,O-anti conformation is 8.4, 0.3 and 8.0 kJ Á mol À1 less stable than the NÀH,O-syn, for M1M2PC, M1M3PC and E1M2PC, respectively.…”

“…In the optimized structures of M1M2PC, M1M3PC and E1M3PC obtained in this work, the alkoxycarbonyl group is syn with respect to the pyrrole N atom of the heterocyclic ring, whereas for E1M2PC the ethoxycarbonyl group is anti, similarly with the analogues M2PC, M3PC, E2PC and E3PC [12].…”

“…Experimental and computational thermodynamic studies of pyrrole derivatives, with different substituent groups [1][2][3][4][5][6][7][8][9][10][11][12][13], have been developed in our Research Group during the last few years, with the main goal to correlate the effects of the corresponding molecular structures on the energetic characteristics of the compounds. Pyrrole moiety is the structural core of the naturallyoccurring porphyrins, the tetrapyrrolic ''pigments of life'', which include the hemoglobin, porphyrins, corrins, vitamin B 12 , the bile pigments and chlorophyll, playing an important role in respiration and photosynthesis processes [14][15][16][17][18].…”

A Benchmark Study on the Properties of Unsubstituted and Some Substituted Polypyrroles

“…The thermodynamic properties, especially enthalpy of formation and zero point energy are vital to establish structural, energetic and reactivity relationships. These properties are useful in chemistry, medicine, and industries [17]. These properties are necessary in evaluating the stability, reactivity and nature of reactions.…”