Computational insights into the reactivity of chlorpyrifos and chlorpyrifos-methyl toward singlet oxygen.

Abstract

A complete mechanism for the •OH-initiated atmospheric decomposition of the pesticides chlorpyrifos and chlorpyrifos-methyl is proposed, incorporating additional studies on the competing reaction with singlet oxygen. The computational study is based on density functional theory (DFT) at the double-hybrid functional level to treat static correlation in the calculations of energy barriers. Reaction of the P-bonded intermediate with 1O2 has a small energy barrier of ~ 2kcalmol-1, generating the Oxone compound and the HOSO• radical, with a reaction free energy of - 49.8kcal/mol for the chlorpyrifos reaction pathway. Direct reaction of the pesticides with singlet oxygen is unlikely to happen due to the exceedingly high energy barrier of ~ 52kcal/mol. However, in aqueous solution, the activation energy reduces dramatically and changes the reaction thermodynamics, making it kinetically accessible and thermodynamically viable.