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

Abstract

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 600–2500K 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 600–2500K. For the reaction CH3I+I (2P3/2), the pathway CH3I+I (2P3/2)→CH3+I2 is the fastest one over the range 600–2500K. For the reaction CH3I+IO, the cis pathway CH3I+IO→CH2I+HOI is the fastest one over the range 600–2500K.