Reactions of chlorine atoms and hydroxyl radicals with trichloroethanol: a mechanistic and kinetic study

The multi-channel reactions (1) CCl3CH2OH+Cl -> products and (2) CCl3CH2OH+OH -> products have been investigated by using the dual-level direct dynamics method. Two reaction channels, i.e., methylene- and hydroxyl-hydrogen abstraction, are identified for each reaction. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) levels. Higher-level energies are obtained at the MC-QCISD and G3(MP2) levels based on the B3LYP and MP2 geometries, respectively, as well as by the CCSD(T)/6-31G(d)+CF method using the B3LYP geometries. Complexes with energies lower than those of the reactants are located at the entrance of each reaction channel. The rate constants for each reaction channel are evaluated by using the canonical variational transition state theory (CVT) incorporating the small-curvature tunneling (SCT) correction in a temperature range of 200-2000 K at the MC-QCISD//B3LYP/6-311G(d,p) level. The agreement of the calculated rate constants and experimental values for two reactions is seen to be remarkably good. Theoretical results indicate that in a low temperature range, the branching ratio to the hydroxyl-H-abstraction channel for both reactions is found negligible. The reactions proceed practically via methylene-H-abstraction yielding the products of CCl3CHOH+HCl and CCl3CHOH+H2O, respectively; while for reaction of CCl3CH2OH+Cl, hydroxyl-H-abstraction channel appears to be probable with the increase of temperature. The enthalpies of formation for the CCl3CH2OH, CCl3CHOH, and CCl3CH2O species are evaluated via isodesmic reactions at several levels.