The influence of methyl groups on the torsion angle and on the energetics of 1-phenylpyrrole derivatives: a thermodynamic and computational study

Abstract

Calorimetric and effusion techniques, complemented by computational calculations were combined to determine the standard (p o = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, $$\Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {\text{g}} \right)$$ , at T = 298.15 K, of 1-(3,5-dichlorophenyl)-2,5-dimethylpyrrole and 2,5-dimethyl-1-phenyl-3-pyrrolecarboxaldehyde, as (107.2 ± 2.7) and (25.9 ± 3.2) kJ mol−1, respectively. These values were derived from the respective standard molar enthalpies of formation, in the crystalline phase, $${{\Updelta}}_{\text{f}} H_{\text{m}}^{\text{o}} \left( {\text{cr}} \right)$$ , at T = 298.15 K, obtained from combustion calorimetry measurements, and from the standard molar enthalpies of sublimation, at T = 298.15 K, determined by the Knudsen effusion mass-loss method. The gas-phase enthalpies of formation of both experimentally studied compounds were also estimated by G3(MP2)//B3LYP computations, using a set of working reactions; the results obtained are in good agreement with the experimental data. With this computational approach, the enthalpies of formation of 1-(3,5-dichlorophenyl)pyrrole, 1-(3,5-dichlorophenyl)-2-methylpyrrole, 1-phenyl-3-pyrrolecarboxaldehyde and 2-methyl-1-phenyl-3-pyrrolecarboxaldehyde were also estimated and a value for their $${{\Updelta}}_{\text{f}} H_{\text{m}}^{\text{o}} \left( {\text{g}} \right)$$ has been defined. Moreover, the molecular structures of the six molecules were established, their geometrical parameters were determined and the influence of methyl groups in the pyrrole ring (2 and 5 positions) on the phenyl/pyrrole torsion angle was analyzed. All the results were also interpreted in terms of enthalpic increments.