Can the Methoxyradical CH3O Act as Sink for Cl and ClO in the Atmosphere?

Abstract

Several reaction pathways on the potential energy surface for the reaction CH3O + ClO are investigated using ab intio methods. As a result we find two reaction pathways to be most probable: (i) the formation of CH2O + HOCl via hydrogen abstraction from the methoxy radical and (ii) the formation of the low-lying adduct CH3OOCl. The other adduct CH3OClO is calculated to lie about 25 kcal/mol higher in energy than CH3OOCl. CH3OOCl can be considered to be a sink for Cl and ClO radicals on the ground-state surface. We have also performed MRD-CI calculations for the excited states of CH3OOCl and CH3OClO to study their photochemistry. In CH3OOCl we calculated a strong transition (11a → 14a) at 4.80 eV corresponding to a σ(Cl−O)−σ*(Cl−O) excitation. We find all low-lying excited singlet and triplet states in the energy range between 2.5 and 5.5 eV to be highly repulsive toward Cl−O and O−O cleavage, and thus photodissociation pathways forming the reactive radicals Cl and ClO from CH3OOCl are very probable.