Matrix Isolation FT-IR Study The Reaction Mechanism Of Ozone And Alkenes

The reaction of alkenes with ozone has important application in organic synthesis and purifying sewage and industrial waste water. Alkenes, which are the most active VOCs, reaction with ozone will form the atmospheric oxidant, OH radical and contribute to the secondary organic aerosol (SOA) in the tropospheric atmosphere. So it is significant to study mechanism of this reaction in gas phase or in condensed phase. Using the self-constructed matrix isolation system and Quantum Chemistry Calculation, this work studied the ozonolysis mechanism of ethene, propene, tetra-methylethene, isoprene, methacrolein (MAC), and tetra-methylethene in the gas phase for its high OH radical yield. Temperature programmed technique was used to detect and idectify the intermediates by comparison with literature and theoretical calculations. The main conclusions are summarized as follows:1) For the simple symmetrical ethene and asymmetrical propene, the results showed that the peaks assigned to intermediateds: POZ and SOZ, were identical with peaks in the literatures, and well consistent with the vibrational frequencies predicted by theoretical calculation on the peak position and intensity. All the suggested that we successfully detected the important intermediateds:POZ and SOZ due to ethene and propene, moreover, the results unambiguously revealled overall process of the generation of POZ and subsequently converting into SOZ, which strongly supported Criegee mechanism, so matrix isolation technique combining temperature programming is a good method to study the reaction mechanism of alkenes and ozone.2) Using same method we study the ozonolysis of tetra-methylethene. The findings indicated that its intermediate POZ was identified. However, not SOZ but tetra-methyl epoxide was produced after POZ splitting. It is deduced that the Criegee intermediate did not react with the acetone for the inert of ketone calbonyl but with the reactant tetra-methylethene to produce the tetra-methyl epoxide.3) Likewise, two primary ozonides of isoprene:ISPI (five-membered ring substituted by methyl group) and ISPⅡ(five-membered ring not substituted by methyl group) and according two secondary ozonides: ISSI and ISSⅡwere obtained using above method. The results indicated that ISPI and ISSI were main ozonids, and ozone preferentially cyclo-added to double-bond which was not substituted by methyl group.4) The yield of MACP, the POZ of MAC, was clearly observed from the reaction results of MAC and ozone. Subsequently bulk of MACP split into methylglyoxal and CH2OO radical at 155K. Finally CH2OO radical recombined with the aldehyde carbonyl of methylglyoxal into MACSII, the main SOZ of MAC, which suggested the inert of ketone carbonyl again.5) The intermediate, isopropenyl hydrogen peroxide, which can produce OH radical, was not detected in the gas-phase ozonolysis of tetra-methylethene. Only some stable compounds were observed, such as acetone, methylglyoxal, hydroxyl acetone, formic acid, acetic acid, formaldehyde. It was concluded that the reaction rate of tetra-methylethene and ozone in gas phase was very fast, and the lifetime of reactive intermediates were very short, and smaller than the shortest mixing time 0.54s.6) The structures of reaction intermediates POZ and SOZ of each alkene was optimized by B3LYP/6-311++G (2d,2p) level. It was found that All the POZs had an O-envelop conformation in which the unique oxygen was bent out of the plane defined by the remaining four ring atoms, and all the SOZ possessed an O-O half hair conformation in which the two adjacent oxygen atoms were twisted out of the C-O-C plane. The good consistence of harmonic vibrational frequencies calculated on these structures with the experimental results demonstrated that the results obtained was reasonable, and in turn suggested the most stable conformation of POZ was O-envelop and the most stable one of SOZ was O-O half hair.In a word, the findings of the present work will provide essential information for organic synthesis and sewage treatment, and help to understand the formation mechanism of OH radical and SOA in the tropospheric atmosphere.