Hydroxyl Reaction with HCF₃

James Tyrrell

Chemistry and Biochemistry
Southern Illinois University at Carbondale

The reactants, products, and the transition state for the reaction HCF₃ + OH = CF₃ + H₂O have been investigated using ab initio molecular orbital theory at the MP2 level. Geometry optimizations and vibrational frequency calculations have been performed on all reactants, products, and the transition state at both the MP2/3-21G** and the MP2/6-311G** levels. Single-point energy calculations at the MP2/6-311G** level using the MP2/3-21G** optimized geometries and at the QCISD/6-311G** level using the MP2/6-311G** optimized geometries have also been carried out on all species. Classical barriers corresponding to these different calculations have been determined and corrected for zero-point energy effects. Transition state theory including tunneling contributions has been used to determine a rate constant which was compared with available experimental data. The QCISD/6-311G**//MP2/6-311G** calculations lead to a classical barrier of 9.589 kcal/mol and a rate constant at 298K (using a non-symmetric Eckart barrier to compute the tunneling correction) of 1.034 x 10^-16 cm Molecule^-1 s^-1, in excellent agreement with the best experimental value of 2.1 x 10^-16 cm³ molecule^-1s^-1. This leads to an estimated lifetime for HCF₃, based on its reaction with OH radicals in the troposphere, of 65.5 yr.