Energetics and molecular structure of 2,5-dimethyl-1-phenylpyrrole and 2,5-dimethyl-1-(4-nitrophenyl)pyrrole.

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, Δ(f)H(m)°(cr), at T = 298.15 K, were derived from the standard molar enthalpies of combustion, Δ(c)H(m)°, which were obtained from static bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to determine the standard molar enthalpies of sublimation, Δ(cr)(g)H(m)°, 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.