Energetics and kinetics of the reaction of HOCO with hydrogen atoms

Abstract

The potential energy surface for the reaction of HOCO radicals with hydrogen atoms has been explored using the CCSD(T)/aug-cc-pVQZ ab initio method. Results show that the reaction occurs via a formic acid (HOC(O)H) intermediate, and produces two types of products: H2O+CO and H2+CO 2. Reaction enthalpies (0 K) are obtained as -102.0 kcalmol for the H2+CO2 products, and -92.7 kcalmol for H2O+CO. Along the reaction pathways, there exists a nearly late transition state for each product channel. However, the transition states locate noticeably below the reactant asymptote. Direct ab initio dynamics calculations are also carried out for studying the kinetics of the H+HOCO reaction. At room temperature, the rate coefficient is predicted to be 1.07× 10-10 cm3 molec-1 s-1 with a negligible activation energy Ea =0.06 kcalmol, and the branching ratios are estimated to be 0.87 for H2+CO2, and 0.13 for H2O+CO. In contrast, the product branching ratios have a strong T dependence. The branching ratio for H2O+CO could increase to 0.72 at T=1000 K. © 2008 American Institute of Physics.