Product distributions from the OH radical-induced oxidation of but-1-ene, methyl-substituted but-1-enes and isoprene in NOx-free air

Product distributions resulting from the OH-induced oxidation of but-1-ene, 2-methylbut-1-ene, 3-methylbut-1-ene and isoprene in air were measured in the absence of nitrogen oxides and compared with predictions based on curre ntly accepted oxidation mechanisms. In the case of butenes, the observed di stributions of carbonyl compounds, hydroxyketones, hydroxyalkanals and diol s were evaluated to obtain probabilities for the initial attack of OH radic al on the outer position of the double bond (gamma=0.90+/-0.03 for 2-Me-but -1-ene and gamma=0.76+/-0.05 for both but-1-ene and 3-Me-but-1-ene), for th e probability of formation of stable products in the self-reaction of secon dary beta-hydroxyperoxyl radicals (k(ssb)/k(ss)=0.29+/-0.07 for but-1-ene a nd k(ssb)/k(ss)=0.19+/-0.06 for 3-Me-but-1-ene), and for the ratio of the r eaction with oxygen vs. decomposition of beta-hydroxyalkoxyl radicals, k(3) [O-2]/(k(4)+k(3)[O-2])=0.25+/-0.04 for but-1-ene and =0.38+/-0.04 for 3-Me- but-1-ene. The last two values disagree with other published data, which su ggest a smaller effect of oxygen. The oxidation of isoprene produced methac rolein and methyl vinyl ketone with a ratio 0.93+/-0.10, the ratio of methy l vinyl ketone and 3-methylfuran was 7.3+/-1.0. Other products were 1-hydro xy-3-methylbut-3-en-2-one (identified by mass spectrometry) and 3-methyl-3- oxo-butane (tentatively identified). The overall product distribution was c omplex and could not be fully elucidated. Computer simulations based on sev eral mechanisms applied the relative probabilities for OH addition found fo r the but-1-enes. Comparison with the experimental data suggests probabilit ies for OH addition to the methylated double bond of 0.504+/-0.027 (outer p osition) and 0.056+/-0.003 (inner position), and to the non-methylated doub le bond of 0.335+/-0.023 (outer position) and 0.105+/-0.008 (inner position ).