Direct Observation of Aliphatic Peroxy Radical Autoxidation and Water Effects: An Experimental and Theoretical Study

Abstract

The autoxidation of organic peroxy radicals (RO₂ ) into hydroperoxy-alkyl radicals (QOOH), then hydroperoxy-peroxy radicals (HOOQO₂ ) is now considered to be important in the Earth's atmosphere. To avoid mechanistic uncertainties these reactions are best studied by monitoring the radicals. But for the volatile and aliphatic RO₂ radicals playing key roles in the atmosphere this has long been an instrumental challenge. This work reports the first study of the autoxidation of aliphatic RO₂ radicals and is based on monitoring RO₂ and HOOQO₂ radicals. The rate coefficients, k_iso (s^-1 ), were determined both experimentally and theoretically using MC-TST kinetic theory based on CCSD(T)//M06-2X quantum chemical methodologies. The results were in excellent agreement and confirmed that the first H-migration is strongly rate-limiting in the oxidation of non-oxygenated volatile organic compounds (VOCs). At higher relative humidity (2-30 %) water complexes were evidenced for HOOQO₂ radicals, which could be an important fate for HOO-substituted RO₂ radicals in the atmosphere.

Keywords: atmospheric chemistry; autoxidation; mass spectrometry; peroxy radicals; water complexation.