Ab initio quantum-chemical study of the unimolecular pyrolysis mechanisms of acetic acid

The mechanisms of unimolecular dehydration and decarboxylation reactions occurring during the pyrolysis of acetic acid above 700°C have been investigated by ab initio methods. The atomic basis set influence as well as the electron correlation effects are analyzed by using a variety of basis sets, ranging from minimal to polarized split-valence, and by introducing the Møller-Plesset (MP) perturbation theory. With an activation barrier of 76.0 kcal mol^?1, the concerted dehydration process occurs via a four-centre transition state. On the other hand, the decarboxylation process could be described by two different mechanisms depending on the nature of the kinetic experiments. While in flow systems, the decarboxylation of acetic acid takes place by a concerted mechanism via a four-centre transition state with an activation energy of 77.3 kcal mol^?, the results suggest rather a water-catalyzed concerted mechanism via a six-membered transition state for the reaction carried out in batch systems, the activation barrier amounting to 64.0 kcal mol^?1.