Calorimetric and computational study of 2- and 3-acetyl-1-methylpyrrole isomers.

This work reports the enthalpies of formation in the condensed and gas phases of 2-acetyl-1-methylpyrrole and 3-acetyl-1-methylpyrrole, derived from the standard (p(o) = 0.1 MPa) molar enthalpies of combustion, in oxygen, Delta(c)H(m)(o), measured by static bomb combustion calorimetry and the standard molar enthalpies of vaporization, Delta(l)(g)H(m)(o), at T = 298.15 K, obtained by high-temperature Calvet microcalorimetry. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level; the computed values compare very well with the experimental results obtained in this work and show that the 2-acetyl-1-methylpyrrole is thermodynamically more stable than the 3-isomer. Furthermore, this composite method was also applied in the calculation of bond dissociation enthalpies, gas-phase basicities, proton and electron affinities, and adiabatic ionization enthalpies.