Measurements of the rate constant for the reaction of OH radicals with propionaldehyde as a function of temperature were performed using low-pressure discharge-flow tube techniques coupled with laser-induced fluorescence detection of OH radicals. The measured room-temperature rate constant of (1.51 ± 0.22) × 10–11 cm3 molecules–1 s–1 at 4 Torr was generally lower but in reasonable agreement with previous absolute and relative rate studies at higher pressures. Measurements as a function of temperature resulted in an Arrhenius expression of (2.3 ± 0.4) × 10–11 exp[(−110 ± 50)/T] cm3 molecules–1 s–1 between 277 and 375 K at 4 Torr. The observed temperature dependence at low pressure is in contrast to previous measurements of a negative temperature dependence at higher pressures. Ab initio calculations of the potential energy surface for this reaction suggest that the primary reaction pathway involves the formation of a hydrogen-bonded prereactive complex, which could account for the difference in the observed temperature dependence at lower and higher pressures.

Formation yields of methacrolein (MAC), methyl vinyl ketone (MVK), and 3-methyl furan (3MF) from the hydroxyl radical (OH) initiated oxidation of isoprene were investigated under NOx-free conditions (NOx = NO + NO2) at 50 °C and 1 atm in a quartz reaction chamber coupled to a mass spectrometer. Yields of the primary products were measured at various OH and hydroperoxy (HO2) radical concentrations and were found to decrease as the HO2-to-isoprene-based peroxy radical (ISORO2) concentration ratio increases. This is likely the result of a competition between ISORO2 self- and cross-reactions that lead to the formation of the primary products, with reactions between these peroxy radicals and HO2 which can lead to the formation of peroxides. Under conditions with HO2/ISORO2 ratios close to 0.1, yields of MVK (15.5% ± 1.4%) and MAC (13.0% ± 1.2%) were higher than the yields of MVK (8.9% ± 0.9%) and MAC (10.9% ± 1.1%) measured under conditions with HO2/ISORO2 ratios close to 1. This radical dependence of the yields was reproduced reasonably well by an explicit model of isoprene oxidation, suggesting that the model is able to reproduce the observed products yields under a realistic range of atmospheric HO2/ISORO2 ratios.

The rate constant for the reaction of the OH radical with hydroxyacetone was measured between 2 and 5 Torr and over the temperature range of 280−350 K, using a discharge-flow system coupled with resonance fluorescence detection of the OH radical. At 298 K the rate constant was found to be (3.02 ± 0.28) × 10−12 cm3 molecule−1 s−1, in excellent agreement with several previous studies. A positive temperature dependence was measured over the temperature range 280−350 K, described by the Arrhenius expression k = (1.88 ± 0.75) × 10−11 exp[−(545 ± 60)/T] cm3 molecule−1 s−1, in contrast to previous measurements of the temperature dependence for this reaction and suggesting that the atmospheric lifetime of hydroxyacetone may be greater than previously estimated. Theoretical calculations of the potential energy surface for this reaction suggest that the mechanism for this reaction involves hydrogen abstraction through a hydrogen-bonded prereactive complex similar to the OH + acetone reaction, with a calculated barrier height between −1 and 1 kcal mol−1 depending on the level of theory.

The rate constants for the reaction of the OH radical with 1,3-butadiene and its deuterated isotopomer has been measured at 1−6 Torr total pressure over the temperature range of 263−423 K using the discharge flow system coupled with resonance fluorescence/laser-induced fluorescence detection of OH. The measured rate constants for the OH + 1,3-butadiene and OH + 1,3-butadiene-d6 reactions at room temperature were found to be (6.98 ± 0.28) × 10−11 and (6.94 ± 0.38) × 10−11 cm3 molecule−1 s−1, respectively, in good agreement with previous measurements at higher pressures. An Arrhenius expression for this reaction was determined to be k1II(T) = (7.23 ± 1.2) ×10−11exp[(664 ± 49)/T] cm3 molecule−1 s−1 at 263−423 K. The reaction was found to be independent of pressure between 1 and 6 Torr and over the temperature range of 262− 423 K, in contrast to previous results for the OH + isoprene reaction under similar conditions. To help interpret these results, ab initio molecular dynamics results are presented where the intramolecular energy redistribution is analyzed for the product adducts formed in the OH + isoprene and OH + butadiene reactions.

The kinetics of the reactions of OH and OD with acetone and acetone-d6 were studied from 2–5 Torr and 258–402 K using a discharge flow system with laser induced fluorescence or resonance fluorescence detection of the OH radical. The rate constants at 300 K for the reaction of OH with acetone and acetone-d6 were (1.73 ± 0.06) × 10−13 and (3.36 ± 0.32) × 10−14 cm3 molecule−1 s−1, respectively. The rate constants at 300 K for the reaction of OD with acetone and acetone-d6 were (2.87 ± 0.22) × 10−13 and (3.69 ± 0.12) × 10−14 cm3 molecule−1 s−1, respectively. Above room temperature, the temperature dependence of the rate constants for the OH + acetone and acetone-d6 display Arrhenius behavior and are described by the equations kH(T) = (3.92 ± 0.81) × 10−12 exp(−938 ± 70/T) and kD(T) = (8.19 ± 1.45) × 10−12 exp(−1647 ± 58/T) cm3 molecule−1 s−1 for acetone and acetone-d6, respectively. Measurements of kH and kD below room temperature begin to display non-Arrhenius behavior, consistent with previous measurements at higher pressures. Theoretical calculations of the kinetic isotope effect as a function of temperature are in good agreement with the experimental measurements using a hydrogen abstraction mechanism that proceeds through a hydrogen-bonded complex.