Experimental and computational study on the molecular energetics of 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid.

This paper reports a combined thermochemical experimental and computational study of 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid. Static bomb combustion calorimetry and Knudsen mass-loss effusion technique were used to determine the standard (p degrees = 0.1 MPa) molar enthalpies of combustion, Delta(c)H(m) degrees, and sublimation, Delta(cr)(g)H(m) degrees, respectively, from which the standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were derived. The values obtained were -(286.3 +/- 1.7) and -(291.6 +/- 1.7) kJ x mol for 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid, respectively. For comparison purposes, the gas-phase enthalpies of formation of these two compounds were estimated by G3(MP2)//B3LYP and MP2 approaches, using a set of gas-phase working reactions; the results are in excellent agreement with experimental data. G3(MP2)//B3LYP computations were also extended to the calculation of N-H bond dissociation enthalpies, gas-phase acidities and basicities, proton and electron affinities and adiabatic ionization enthalpies. Moreover, the results are also discussed in terms of the energetic effects of the addition of a carboxylic and of a methyl groups to the pyrrole ring and compared with structurally similar compounds.