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Ozone−alkene reactions form vibrationally excited Criegee intermediates (of the form R1R2COO), some of which, once thermalized, are thought to react with SO2, H2O, NOx, aldehydes, and alcohols. Several studies using relative rate techniques or ab initio calculations have resulted in estimates for the rate coefficients of reactions of the thermalized biradicals. The ranges of measured and estimated rate coefficients span 2−6 orders of magnitude, depending on the reaction partner. Using an atmospheric pressure flow reactor, we have made the first absolute rate coefficient determinations for reactions of a thermalized Criegee intermediate, measuring rates for unimolecular decomposition and reaction with acetaldehyde. For the thermalized CH3CHOO formed in trans-2-butene ozonolysis, values for kdec = 76 s^-1 and kald = 1.0 × 10^-12 cm³ molecule^-1 s^-1, accurate to within a factor of 3 and 6, respectively, were obtained.

OH radical formation yields from the reaction of ozone with several cycloalkenes were measured using small amounts of fast-reacting aromatics and aliphatic ethers to trace OH formation. Measured OH yields are much higher than for acyclic analogues. The yields are 0.62 ± 0.15, 0.54 ± 0.13, 0.36 ± 0.08, and 0.91 ± 0.20 for cyclopentene, cyclohexene, cycloheptene, and 1-methylcyclohexene, respectively. Density functional theory calculations at the B3LYP/6-31G(d,p) level are presented to aid in understanding the trends observed. Theory indicates that the OH production from cycloalkenes is largely controlled by the transition states for the cycloreversion of the primary ozonide.

The OH radical is the key oxidizing agent in the troposphere, and ozone−alkene reactions appear to be a significant and sometimes dominant source of new HOx radicals in urban and rural air. In this work, we report the first study of the pressure dependence of the OH radical yield for the ozonolysis of ethene, propene, 1-butene, trans-2-butene, and 2,3-dimethyl-2-butene over the range 20−760 Torr and of trans-3-hexene and cyclopentene over the range 200−760 Torr. Low-pressure experiments were performed in a long-path evacuable FTIR cell or a steady-state flow-tube reactor in series with a gas chromatograph/flame ionization detector and FTIR cell. We have also investigated the effect of adding SF6 at atmospheric pressure for ethene, 1-butene, and trans-2-butene, in a collapsible Teflon chamber. OH formation increased 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 1-butene, trans-2-butene, 2,3-dimethyl-2-butene, trans-3-hexene, or cyclopentene over the ranges studied. Density functional theory calculations at the B3LYP/6-31G(d,p) level are presented to aid in understanding the trends observed. They lead to the proposal that the formation of a hydroperoxide via a diradical pathway can compete with the formation of the carbonyl oxide for the ethene primary ozonide.

A small-ratio relative-rate technique to derive OH yields from O3 reactions with alkenes is described. OH tracer compounds&sbd;aromatic and aliphatic hydrocarbons&sbd;are added to O3−alkene mixtures in small quantities. Under these conditions, a large fraction of the tracer is consumed, resulting in a large signal that allows accurate determination of OH formation yields. Analytical approximations, numerical analysis, and sources of random and systematic uncertainties are discussed. This method is applied to measuring the OH formation yield from the O3 reaction with propene and ethene, finding 0.35 ± 0.07 and 0.18 ± 0.06, respectively.

Arginine has been considered as the major energy source of nonglycolytic arginine-utilizing mycoplasmata. When three strains of Mycoplasma arginini, and one strain each of Mycoplasma arthritidis, Mycoplasma fermentans, Mycoplasma gallinarum, Mycoplasma gallisepticum and Mycoplasma hominis were grown in the medium with high arginine concentration (34 mM) compared with low arginine (4 mM), both the protein content of the organisms and the specific activity of arginine deiminase increased. M. fermentans, the one arginine-utilizing species included in the survey which is also glycolytic, showed an increase in protein content but no increase in specific activity of the enzyme. The glycolytic non-arginine-utilizing M. gallisepticum did not show an increase in either parameter. The Km for arginine deiminase from crude cell extracts was 1.66 X 10(-4)M. The enzyme demonstrated a hyperbolic activation curve subject to substrate inhibition and was not affected by the presence of L-histidine. When mycoplasmic protein and arginine deiminase were determined for M. hominis under aerobic and anaerobic conditions, aerobically grown cells exhibited no detectable enzymatic increases until late in log phase. Higher levels of arginine deiminase were observed earlier in the anaerobic growth cycle. The rate of 14CO2 evolution from [guanido-14C]arginine was not altered in arginine-supplemented cells compared with cells grown in low arginine. In addition, CO2 production did not parallel increased arginine deiminase activity. These observations argue that arginine is used only as an alternate energy source in these organisms.