Ultrafast photochemistry of methyl hydroperoxide on ice particles

Abstract

Simulations show that photodissociation of methyl hydroperoxide, CH 3 OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH 3 OOH is found to prefer the exterior of the cluster, with the CH 3 O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH 3 O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH 3 O to form CH 3 OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH 2 O, H 2 O, H 2 , CO, CH 3 OH, and H 2 O 2 .air-water interface | molecular dynamics | peroxides | photodissociation P hotoinduced processes at surfaces of water or ice are of interest in atmospheric chemistry and other areas. However, molecular-level understanding of such processes is largely lacking, because the systems and the interactions involved are complex. In this article, we explore, as a fundamental prototype, the photodissociation of the simplest organoperoxide, CH 3 OOH, on an ice particle. A major motivation for studying this peroxide is its relevance to atmospheric chemistry, as discussed in refs. 1-11 and references therein. Methyl hydroperoxide is typically present in cloud water droplets in micromolar concentrations (7,12,13) and is also likely to occur in aerosols (4,14). The fate of CH 3 OOH in water droplets may influence its role as a reservoir for HO x because this peroxide is known to be a major source of OH at high tropospheric altitudes (10, 15). Fenton-like reactions involving CH 3 OOH and Fe(II) serve as an important source of aqueous HO 2 radicals (16). Finally, recent evidence suggests that reactions between peroxides and isoprene oxidation products lead to the formation of tetrols, thus contributing to the production of secondary organic aerosols (17).It is well established that gas-phase CH 3 OOH photolyzes under UV-visible excitation to form CH 3 O and OH with a quantum yield approaching unity (8,(18)(19)(20)(21)(22)(23). Because of the moderate solubility of CH 3 OOH, it is also likely to undergo condensed-phase photolysis after being taken up by aqueous particles and ice particles and contribute to the production of aqueous free radicals (24). To the best of our knowledge, only two prior experiments examined photolysis of CH 3 OOH in liquid water (9) and ice particles.† Each of these studies identified CH 2 O and H 2 O as major products, suggesting that the photochemistry of CH 3 OOH is conside...

Molecular Characterization of Organosulfates in Organic Aerosols from Shanghai and Los Angeles Urban Areas by Nanospray-Desorption Electrospray Ionization High-Resolution Mass Spectrometry

“…78 Even if these free radicals manage to escape from the solvent cage, secondary reactions would generally result in smaller products relative to the size of the precursor peroxide compound, which would consequentially also reduce the average C value.…”

Perspective: Water cluster mediated atmospheric chemistry

“…90,[186][187][188] Radical recombination reactions are strongly affected by surface water and ice where reactions are enhanced. 189 The mechanism and role of water in some of these instances remains uncertain as pointed out for photochemical reactions of aromatic molecules on ice which however do not occur on liquid water. 190 Adsorbed water on surfaces of oxide and mineral dust aerosols were characterized in insightful experiments by Grassian et al and shown to alter and enhance reactivity of O 3 , H 2 CO 3 and HNO 3 .…”

Adsorption of Methyl Hydroperoxide (CH3OOH) on Water Ice. Theoretical Study with Systematic Assessment of Coordination Modes

“…7 Moreover, among the complexes formed by MHP, only several representatives are considered. From experimental study, 8 it follows that the water solubility of MHP is not high in comparison with H 2 O 2 (the corresponding Henry's constant of MHP is approximately 200 times lower than for H 2 O 2 ).…”