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

Abstract

Calorimetric techniques, namely static bomb calorimetry and high temperature Calvet microcalorimetry were used to derive, respectively, the standard (po=0.1MPa) molar enthalpy of formation, in the liquid phase, ΔfHmo(l), and the standard molar enthalpy of vaporization, at T=298.15K, ΔlgHmo, of the methyl 1-methyl-2-pyrrolecarboxylate (M1M2PC). These experiments allowed the determination of its standard (po=0.1MPa) molar enthalpy of formation, in the gaseous phase, ΔfHmo(g)=−(270.3±2.2)kJ·mol−1, at T=298.15K.Additionally, the standard molar enthalpy of formation of M1M2PC was 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 one, giving us support to estimate the gas-phase enthalpies of formation of the methyl 1-methyl-3-pyrrolecarboxylate (M1M3PC), ethyl 1-methyl-2-pyrrolecarboxylate (E1M2PC) and ethyl 1-methyl-3-pyrrolecarboxylate (E1M3PC), that were not studied experimentally. The molecular structures of the four molecules were established and the structural parameters were determined at the B3LYP/6-31G(d) level of theory. Furthermore, all the results were interpreted in terms of enthalpic increments.