Chemical reactivity analysis of the CO + OH and CO + HO₂ reactions

The kinetics and reactivity associated with the CO + OH and CO + HO₂ reactions were analyzed based on ab initio molecular orbital theory, with the interest of identifying the difference in their reaction dynamics that leads to the slower rate of the latter. Two hydrogen-bonded pre-reactive complexes, OOH···OC and OOH···CO, were found on the potential energy surface of CO + HO₂, having energies that are higher than those of the entrance reactants by 1.2 and 2.9 kcal mol^-1, respectively. Through intrinsic reaction coordinate analysis, it was further found that these two complexes hinder the forward reaction, HO₂ + CO → products, by forcing a reorientation of the reactants. The configurations of the transition state structures in forming the trans/cis intermediates in the title reactions were rationalized by virtual representation of the electrostatic potentials of the reactants from self-consistent field calculations at the CCSD(T)/AUG-cc-pVTZ level, with the different reactivities of the two reactions ascribed to spatial interactions between the local electrostatic potentials of the entrance reactants. The energy gap of the frontier molecular orbitals for the CO + OH entrance channel was found to be ca. 29 kcal mol^-1 lower than that of the CO + HO₂ entrance channel by time-dependent coupled cluster theories, which quantitatively explains the difference in their reactivities. Furthermore, the condensed Fukui functions were calculated using Mulliken atomic charges through natural population analysis to identify the most reactive sites of the entrance reactants, with the results again supporting the slower reaction rate of CO + HO₂.