Theoretical Study of the Elimination Kinetics of Carboxylic Acid Derivatives in the Gas Phase. Decomposition of 2-Chloropropionic Acid

Abstract

The reaction mechanism for the decomposition of 2-chloropropionic acid in the gas phase to form hydrogen chloride, carbon monoxide, and acetaldehyde has been theoretically characterized. Analytical gradients have been used by means of AM1 and PM3 semiempirical procedures and ab initio methods at HF and DFT (BLYP) levels with the 6-31G** basis set. The correlation effects were also included by using the perturbational approach at the MP2 level with the 6-31G** and 6-31++G** basis sets and the variational approach at the CISD/6-31G** level and by means of MCSCF wave functions with a (6,6) complete active space and the 6-31G** basis set. The global potential energy surface has been studied, and the stationary points were localized and characterized. The geometries, electronic structure, and transition vector associated with the transition structures have been analyzed and the dependence of these properties upon theoretical methods is discussed. The present study points out, in agreement with the experimental data, that the decomposition process occurs through a two-step mechanism involving the formation of the α-propiolactone intermediate. The transition structure associated with the first step can be described as a five-membered ring with participation of leaving chloride and hydrogen, assisted by the carbonyl oxygen of the carboxyl group. The second transition structure, controlling the α-propiolactone decomposition step, yields the formation of CO and CH3CHO molecules. The rate constants and the Arrhenius preexponential factors for the different interconversion steps have been calculated in terms of the transition state theory. The comparison of experimental and theoretical values for these parameters allows us to prove the validity of theoretical methods. The results suggest that the process must be considered as essentially irreversible, the first step being the rate-determining step. From a computational point of view, the inclusion of the correlation energy at the MP2/6-31G** level is necessary to obtain an accurate calculation of the kinetic parameters.