Theoretical study of the reaction OH + acetone: a possible kinetic effect of the presence of water?

Abstract

The effect of water on the molecular mechanism of the reaction of the OH radical with acetone in the homogeneous gas-phase has been studied by quantum chemical computations. The three-molecular reaction system of OH + acetone + H₂O has been characterised using molecular parameters, electronic energies and Gibbs free energies computed for the stationary points of the potential energy surface. The MP2 method with a 6-31G(d,p) basis set was employed for geometry optimisation. The electronic energies were obtained at the MP4 and the CCSD(T) level of theory using the 6-311G(d,p) basis set. We have found that the presence of a water molecule changes significantly both the energy profile and free energy profiles of the reaction. A “water-assisted” reaction mechanism has been established in which both the H-abstraction channel and the CO-addition channel occur via intermolecular complexes and transition state structures that involve the water molecule. The activation free energy for the out-of-plane abstraction channel at low temperatures has been found to be significantly smaller than that for the “water-free” system indicating a possible catalytic rate enhancement effect. Abstraction is the predominant reaction route also for the water-assisted reaction as shown by the much larger activation free energy computed for the addition channel. In order to estimate atmospheric concentrations of some intermolecular complexes, we have validated our employed level of theory by computing the equilibrium constant of HO₂ + H₂O ⇄ HO₂⋯H₂O at three temperatures and compared them to the values derived from experiments available in the literature. Then, using our theoretical results, we have estimated the tropospheric concentration of OH⋯acetone⋯H₂O complexes to be very small, but they are probably detectable under laboratory conditions.