Theoretical Study of the Reactions of H Atoms with CHI and CHI.

High level ab initio methods have been used to provide reliable kinetic data for the H + CHI and H + CHI gas-phase reactions. The (H, I)-abstraction and I-substitution reaction pathways were identified. The structures were determined on the potential energy surface at the MP2/aug-cc-pVTZ level of theory. The energetics was then refined using the coupled cluster theory. For the iodinated species, the spin-orbit coupling was calculated using the MRCI approach. The core valence and the scalar relativistic corrections were considered. Thermal rate constants were reported using the canonical transition-state theory (TST) and compared to computed values with the canonical variational transition-state theory (CVT) using the zero curvature tunneling (ZCT) and the small curvature tunneling (SCT) corrections over a wide temperature range (250-2500 K) to show the importance of quantum tunneling effects at low temperatures. They are given by the following expressions for the overall reactions using the CVT/SCT method: k( T) = 1.07 × 10 × T exp(2.68 (kJ mol)/ RT) and k( T) = 5.73 × 10 × T exp(3.15 (kJ mol)/ RT). The I-abstraction is predicted to be the major pathway for both H + CHI and H + CHI reactions. The obtained kinetic parameters for the H + CHI reaction are in excellent agreement with their experimental counterparts over the temperature range 300-750 K. On the basis of our calculated reaction enthalpies, a new evaluation of the standard enthalpy of formation at 298 K of CHI and CHI has been provided. Obtained values are Δ H° (CHI) = 219.5 kJ mol and Δ H°(CHI) = 296.3 kJ mol.