The pressure dependence of the OH radical yield from ozone-alkene reactions

The OH radical is the key oxidizing agent in the troposphere, and ozone-alk ene reactions appear to be a significant and sometimes dominant source of n ew HOx radicals in urban and rural air. In this work, we report the first s tudy of the pressure dependence of the OH radical yield for the ozonolysis of ethene, propene, I-butene, trans-2-butene, and 2,3-dimethyl-2-butene ove r the range 20-760 Torr and of trans-3-hexene and cyclopentene over the ran ge 200-760 Torr. Low-pressure experiments were performed in a long-path eva cuable FTIR cell or a steady-state flow-tube reactor in series with a gas c hromatograph/flame ionization detector and FTIR cell. We have also investig ated the effect of adding SF6 at atmospheric pressure for ethene, I-butene, and trans-2-butene, in a collapsible Teflon chamber. OH formation increase d almost 3-fold for ethene at low pressures, from 0.22 +/- 0.06 at 760 Torr to 0.61 +/- 0.18 at 20 Torr, and increased somewhat for propene from 0.33 +/- 0.07 at 760 Torr to 0.46 +/- 0.11 at 20 Torr. A pressure dependence of the OH formation yield was not observed for I-butene, trans-2-butene, 2,3-d imethyl-2-butene, trans-3-hexene, or cyclopentene over the ranges studied. Density functional theory calculations at the B3LYP/6-31G(d,p) level are pr esented to aid in understanding the trends observed. They lead to the propo sal that the formation of a hydroperoxide via a diradical pathway can compe te with the formation of the carbonyl oxide for the ethene primary ozonide.