KINETIC AND THEORETICAL INVESTIGATION OF THE GAS-PHASE OZONOLYSIS OF ISOPRENE - CARBONYL OXIDES AS AN IMPORTANT SOURCE FOR OH RADICALS IN THE ATMOSPHERE

Kinetic measurements as well as B3LYP/ and MP2/6-31G(d,p) calculations provide evidence that carbonyl oxides formed in the gas-phase ozonoly sis of alkylated alkenes are an important source of OH radicals. In th e gas-phase ozonolysis of propene, cis-2-butene, trans-2-butene, tetra methylethene, and isoprene, 18, 17, 24, 36, and 19% OH radicals (relat ive to reacted ozone, error margin less than or equal to 4%) are measu red using CO as a scavenger for OH. The quantum chemical calculations show that OH radical production depends on syn positioned methyl (alky l) 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 m ethyl (alkyl) group, a much larger amount of OH radicals is formed. Th is is due to the fact that 1,4 H migration and the formation of an int ermediate hydroperoxy alkene, that is prone to undergo OO bond cleavag e, is energetically more favorable than isomerization to dioxirane. In the case of syn-methyl, dimethyl, and isopropenyl carbonyl oxide calc ulated activation enthalpies at 298 K are 14.8, 14.4, and 15.5 kcal/mo l compared to the corresponding dioxirane isomerization barriers of 23 .8, 21.4, and 23.0 kcal/mol, respectively. The OO cleavage reactions o f the hydroperoxy alkenes formed in these cases are just 11, 12.8, and 10.3 kcal/mol.