Theoretically derived mechanisms of HPALD photolysis in isoprene oxidation. Issue 13 (20th March 2017)

• Record Type: Journal Article
• Authors: Liu, Zhen; Nguyen, Vinh Son; Harvey, Jeremy; Müller, Jean-François; Peeters, Jozef
• Is Part Of: Physical chemistry chemical physics. Volume 19:Issue 13(2017)
• Page Start: 9096
• Page End: 9106
• Publication Date: 2017-03-20
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c7cp00288b
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Two theoretically derived efficient mechanisms for the atmospheric photolysis of Z -HPALDs, and the subsequent chemistry of the additional first-generation OH formed. Abstract : In this work we identified and theoretically quantified two photolysis mechanisms of HPALDs (hydroperoxy aldehydes) that result from the isomerization of peroxy radicals in the atmospheric oxidation of isoprene at low/moderate NO x . As a first photolysis mechanism, we show that a fraction of the initially excited S1 -state HPALDs isomerizes by a near-barrierless 1, 5 H-shift at a rate approaching 10 12 s −1 – competing with the ∼equally fast intersystem crossing to the T2 triplet state – forming an unstable biradical that spontaneously expels an OH (hydroxyl) radical. A second mechanism is shown to proceed through the activated T2 triplet biradical – formed from S1 – undergoing a concerted ring-closure and OH-expulsion, yielding an oxiranyl-type co-product radical that quickly ring-opens to enoxy radicals. In both mechanisms, subsequent chemistry of the co-product radicals yields additional first-generation OH. The combined HPALD-photolysis quantum yield by these two mechanisms – which may not be the only photolysis routes – is estimated at 0.55 and the quantum yield of OH generation at 0.9, in fair accordance with experimental data on an HPALD proxy (Wolfe et al., Phys. Chem. Chem. Phys., 2012, 14, 7276–7286).