Theoretical Study of the Reactions of H Atoms with CH3I and CH2I2.

Abstract

High level ab initio methods have been used to provide reliable kinetic data for the H + CH3I and H + CH2I2 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: kH+CH3I( T) = 1.07 × 10-17 × T2.13 exp(2.68 (kJ mol-1)/ RT) and kH+CH2I2( T) = 5.73 × 10-21 × T2.97 exp(3.15 (kJ mol-1)/ RT). The I-abstraction is predicted to be the major pathway for both H + CH3I and H + CH2I2 reactions. The obtained kinetic parameters for the H + CH3I 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 CH2I and CHI2 has been provided. Obtained values are Δf H°298K (CH2I) = 219.5 kJ mol-1 and Δf H°298K(CHI2) = 296.3 kJ mol-1.