A theoretical study of H- and I-abstraction reactions from CH3I molecule by I (2P3/2) atom and IO radical

The rate constants of the H- and I-abstraction reactions from CH3I molecules by I (2P3/2) atom and IO radical have been estimated over the range 600–2500 K using three levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies have been performed using MP2 method combined with the cc-pVTZ basis set. Single-point energy calculations have been carried out with the highly-correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using the cc-pVTZ and cc-pVQZ basis sets. The CCSD(T) calculated potential energies were extrapolated to estimate the complete basis limit (CBS). Canonical transition-state theory combined with an Eckart tunneling correction and a hindered rotor treatment has been used to predict the rate constants as a function of temperature. In order to choose the appropriate levels of theory with iodine-containing species, the reference pathway CH3I + I (2P3/2) → CH3 + I2 was theoretically studied because its kinetic parameters have been established from numerous experimental and evaluation studies. The rate constants of the reference pathway calculated at the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level of theory are in good agreement with the literature counterparts. This level of theory has been used to calculate the rate constant of the pathways CH3I + I (2P3/2) → CH2I + HI, CH3I + IO → CH2I + HOI (cis and trans), CH3I + IO → CH2I + HIO, CH3I + IO → CH3 + IOI, and CH3I + IO → CH3 + IIO. For the seven pathways, three-parameter Arrhenius expressions have been obtained by fitting to the calculated rate constants over the range 600–2500 K. For the reaction CH3I + I (2P3/2), the pathway CH3I + I (2P3/2) → CH3 + I2 is the fastest one over the range 600–2500 K. For the reaction CH3I + IO, the cis pathway CH3I + IO → CH2I + HOI is the fastest one over the range 600–2500 K.