Kinetic and Theoretical Investigation of the Gas-Phase Ozonolysis of Isoprene:  Carbonyl Oxides as an Important Source for OH Radicals in the Atmosphere

Abstract

Kinetic measurements as well as B3LYP/ and MP2/6-31G(d,p) calculations provide evidence that carbonyl oxides formed in the gas-phase ozonolysis of alkylated alkenes are an important source of OH radicals. In the gas-phase ozonolysis of propene, cis-2-butene, trans-2-butene, tetramethylethene, and isoprene, 18, 17, 24, 36, and 19% OH radicals (relative to reacted ozone, error margin ≤4%) are measured using CO as a scavenger for OH. The quantum chemical calculations show that OH radical production depends on syn positioned methyl (alkyl) groups and their interaction with the terminal O atom of a carbonyl oxide. For example, in the gas-phase ozonolysis of ethene only 5% OH radicals are measured while for a carbonyl oxide with syn-positioned methyl (alkyl) group, a much larger amount of OH radicals is formed. This is due to the fact that 1,4 H migration and the formation of an intermediate hydroperoxy alkene, that is prone to undergo OO bond cleavage, is energetically more favorable than isomerization to dioxirane. In the case of syn-methyl, dimethyl, and isopropenyl carbonyl oxide calculated activation enthalpies at 298 K are 14.8, 14.4, and 15.5 kcal/mol compared to the corresponding dioxirane isomerization barriers of 23.8, 21.4, and 23.0 kcal/mol, respectively. The OO cleavage reactions of the hydroperoxy alkenes formed in these cases are just 11, 12.8, and 10.3 kcal/mol.

First-principles-derived kinetics of the reactions involved in low-temperature dimethyl ether oxidation

“…Experimental highpressure limit rates are unknown for these reactions. Conditions for known experiments are around the standard conditions of 298 K and 1 atm [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]. Considerations at higher temperatures and pressures have never been made for these reactions, but will be considered here in the low-temperature combustion of dimethyl ether.…”

“…Many theoretical studies have been devoted to the unimolecular rearrangement/decomposition of CH 2 OO [14,15,43,[72][73][74][75][76][77][78][79][80]. We have based our calculations of the two primary pathways (see Figure 8) on the most recent theoretical work of Cremer and coworkers [15,74].…”

Measurements of OH and HO<sub>2</sub> yields from the gas phase ozonolysis of isoprene

“…OH yield Technique Aschmann et al (1996) 0.27 Cyclohexane scavenger Donahue et al (1998) 0.50 LIF of OH at 5 Torr Gutbrod et al (1997) 0 Neeb and Moortgat (1999) 0.26 Cyclohexane scavenger Paulson et al (1992) 0.68 Methylcyclohexane scavenger Paulson et al (1998) 0.26 Tracer pair Rickard et al (1999) 0.44 TMB tracer Zhang et al (2002) 0.25 Statistical-dynamical master equation…”

Theoretical investigation of reactions between ammonia and precursors from the ozonolysis of ethene

“…The Criegee intermediate can be stabilized by collisions with the surrounding molecules or it can decompose into (i) an unsaturated hydro-peroxide by fragmentation of a HO radical, (ii) an ester, or (iii) a carbonyl oxide by elimination of an oxygen atom. In the gas phase the reaction mechanisms for ozonolysis of ethene, isoprene and various terpenes have been investigated both with experimental studies [6][7][8][9][10] and theoretical calculations [11][12][13][14][15][16]. The fate of the stabilized Criegee intermediate is manifold, it can undergo unimolecular decomposition [11,16,17] or react with surrounding compounds in the atmosphere such as water [7,[17][18][19][20], HO x (=HO + HO 2 ) [21], NO x (=NO + NO 2 ) [22], SO 2 [17,22], CO 2 [17], H 2 SO 4 [23], aldehydes [9,10,17,24], or carboxylic acid [9,24,25].…”