Unimolecular Reaction of Hydroxyperoxyl Radicals in the Troposphere PDF Download

Author: Sui So
Publisher:
ISBN:
Category : Atmospheric chemistry
Languages : en
Pages : 346

Get Book Here

Book Description
[Beta]-Hydroxyperoxyl radicals are formed during atmospheric oxidation of unsaturated volatile organic compounds (VOCs) such as isoprene. They are also important intermediates in the combustion of alcohols. In these environments the unimolecular isomerisation and decomposition of [beta]-hydroxyperoxyl radicals may be of importance. Results of ion-trap mass spectrometry generating a prototypical distonic charge-tagged [beta]-hydroxyalkyl radical anion •CH2C(OH)(CH3)CH2C(O)O- have been obtained by a collaborating research group. The subsequent reaction of the radical anion with O2 in the gas phase has been investigated under conditions that are devoid of complicating radical-radical reactions. In this thesis, quantum chemical calculations and master equation/RRKM theory modelling are used to rationalise the results and discern a reaction mechanism. Reaction is found to proceed via initial hydrogen abstraction from the [gamma]-methylene group and [beta]-hydroxyl group, with both reaction channels eventually forming isobaric product ions due to loss of either •OH + HCHO or •OH + CO2. Isotope labelling studies confirm that a 1,5-hydrogen shift from the [beta]-hydroxyl functionality results in a hydroperoxyalkoxyl radical intermediate that can undergo further unimolecular dissociation. Furthermore, facile decomposition of [beta]-hydroxyperoxyl radicals has been confirmed to yield •OH in the gas phase. Moreover, the influence of an anionic charge on the reaction chemistry of [beta]-hydroxyperoxyl radicals has been investigated by examining the molecular orbitals of a distonic [beta]-hydroxyperoxyl radical anion analogue •OOCH2CH(OH)CH2C(O)O-. Instead of following the conventional Aufbau principle, the radical anion exhibits a peculiar electronic arrangement, where the singly occupied molecular orbital (SOMO) is no longer the frontier orbital and carries energy lower than other doubly occupied molecular orbitals (HOMOs). This phenomenon is manifested as SOMO-HOMO conversion and is caused by the through space stabilisation between the interaction of the anion and radical site. Further investigation of the other C4H6O5•- isomers involved in the unimolecular reaction mechanisms of the hydroxyperoxyl radical anion •OOCH2CH(OH)CH2C(O)O- revealed that these radical anion isomers exhibit different extent of orbital conversion. As a result, the reaction chemistry of this radical anion is influenced by various additional stabilities associated with the unconventional electron arrangement, switching the dominant reaction pathway from [beta]-OH abstraction in the relevant neutral radical to C-H abstraction at the [beta]-carbon in the radical anion analogue. Despite the change in product distribution, all reaction pathways remain the same in both the neutral radical and radical anion analogues. Enols are emerging as trace atmospheric components that may play a significant role in the formation of organic acids in the atmosphere. They are unsaturated VOCs and their oxidation involves hydroxyperoxyl radicals as key intermediates. It has recently been discovered that acetaldehyde can undergo UV-induced isomerisation to vinyl alcohol (the enol counterparts) under atmospheric conditions. The •OH-initiated oxidation chemistry of vinyl alcohol has been investigated in this thesis, using quantum chemical calculations and energy-grained master equation simulations. The reaction proceeds by •OH addition at both the [alpha]-carbon (66%) and [beta]-carbon (33%) of the [pi] system, yielding the C-centred radicals •CH2CH(OH)2 and HOCH2C•HOH respectively. Subsequent trapping by O2 leads to the respective peroxyl radicals. About 90% of the chemically activated population of the major peroxyl radical adduct •O2CH2CH(OH)2 is predicted to undergo fragmentation to produce formic acid and formaldehyde, with regeneration of •OH. The minor peroxyl radical CH2(OH)CH(OH)O2• is even less stable and almost exclusively undergoes HO2• elimination to form glycolaldehyde. The •OH-initiated oxidation of vinyl alcohol ultimately leads to three main product channels, being (i) •O2CH2CH(OH)2 (8%), (ii) HC(O)OH + HCHO + •OH (56%) and (iii) HOCH2CHO + HO2• (37%). This study supports previous findings that vinyl alcohol should be rapidly removed from the atmosphere by reaction with •OH and O2, with glycolaldehyde being identified as a previously unconsidered product. Moreover, it is also shown that direct chemically activated reactions can lead to •OH and HO2• (HOx) recycling. Following the study on the acetaldehyde-vinyl alcohol pair, the photo-isomerisation of glycolaldehyde to 1,2-ethenediol has been studied. The keto-enol isomerisation is associated with a barrier of 66 kcal mol-1 and involves a double hydrogen shift mechanism to give the lower energy Z isomer. This barrier lies below the energy of the UV/Vis absorption band of glycolaldehyde and is also considerably below the energy of the products resulting from photolytic decomposition. The atmospheric oxidation of 1,2-ethenediol by •OH is initiated by radical addition to the [pi] system to give the •CH(OH)CH(OH)2 radical, which is subsequently trapped by O2 to form the peroxyl radical •O2CH(OH)CH(OH)2. According to kinetic simulations, collisional deactivation of the latter is negligible and cannot compete with rapid fragmentation reactions, which lead to (i) formation of glyoxal hydrate and HO2• through an [alpha]-hydroxyl mechanism (96%) and (ii) two molecules of formic acid with release of •OH through a [beta]-hydroxyl pathway (4%). The lifetime of the two enols in the presence of tropospheric levels of •OH is determined to be around 4 hours and 68 hours respectively. Phenomenological rate coefficients for these two oxidation reactions are obtained for use in atmospheric chemical modelling. Finally, photo-induced dissociation and isomerisation of other common tropospheric carbonyl compounds, namely methyl vinyl ketone (MVK) and methacrolein (MACR), has been reinvestigated. The reaction of both molecules proceeds through dissociation, cyclisation and hydrogen shift (including keto-enol isomerisation) pathways. From the simulation of reaction dynamics, MACR photolysis is significantly less efficient than MVK photolysis, which is consistent with the experimental data in the literature. Isomerisation dominates dissociation in the actinic spectrum at longer wavelengths for both MVK and MACR photolysis. The total photolysis rate of MVK and MACR is calculated to be 3.8 x 10-5 s-1 and 8.6 x 10-7 s-1 respectively. The study reveals that MVK and MACR photolysis may lead to formation of new atmospheric VOCs such as hydroxylbutadiene from MVK and dimethylketene from MACR.