Decomposition kinetics of stabilised CH2OO and CD2OO Criegee intermediates have been investigated as a function of temperature (450-650 K) and pressure (2-350 Torr) using flash photolysis coupled with time-resolved cavity-enhanced broadband UV absorption spectroscopy. Decomposition of CD2OO was observed to be faster than CH2OO under equivalent conditions. Production of OH radicals following CH2OO decomposition was also monitored using flash photolysis with laser-induced fluorescence (LIF), with results indicating direct production of OH in the v = 0 and v = 1 states in low yields. Master equation calculations performed using the Master Equation Solver for Multi-Energy well Reactions (MESMER) enabled fitting of the barriers for the decomposition of CH2OO and CD2OO to the experimental data. Parameterisations of the decomposition rate coefficients, calculated by MESMER, are provided for use in atmospheric models and implications of the results are discussed. For CH2OO, the MESMER fits require an increase in the calculated barrier height from 78.2 kJ mol-1 to 81.8 kJ mol-1 using a temperature-dependent exponential down model for collisional energy transfer with ΔEdown = 32.6(T/298 K)1.7 cm-1 in He. The low- and high-pressure limit rate coefficients are k1,0 = 3.2 × 10-4(T/298)-5.81exp(-12 770/T) cm3 s-1 and k1,∞ = 1.4 × 1013(T/298)0.06exp(-10 010/T) s-1, with median uncertainty of ∼12% over the range of experimental conditions used here. Extrapolation to atmospheric conditions yields k1(298 K, 760 Torr) = 1.1+1.5-1.1 × 10-3 s-1. For CD2OO, MESMER calculations result in ΔEdown = 39.6(T/298 K)1.3 cm-1 in He and a small decrease in the calculated barrier to decomposition from 81.0 kJ mol-1 to 80.1 kJ mol-1. The fitted rate coefficients for CD2OO are k2,0 = 5.2 × 10-5(T/298)-5.28exp(-11 610/T) cm3 s-1 and k2,∞ = 1.2 × 1013(T/298)0.06exp(-9800/T) s-1, with overall error of ∼6% over the present range of temperature and pressure. The extrapolated k2(298 K, 760 Torr) = 5.5+9.2-5.5 × 10-3 s-1. The master equation calculations for CH2OO indicate decomposition yields of 63.7% for H2 + CO2, 36.0% for H2O + CO and 0.3% for OH + HCO with no significant dependence on temperature between 400 and 1200 K or pressure between 1 and 3000 Torr.

The reaction of the OH radical with isoprene, CH (R1), has been studied over the temperature range 298-794 K and bath gas pressures of nitrogen from 50 to 1670 Torr using laser flash photolysis (LFP) to generate OH and laser-induced fluorescence (LIF) to observe OH removal. Measurements have been made using both a conventional LFP/LIF apparatus and a new high pressure system. The measured rate coefficient at 298 K ( k = (9.90 ± 0.09) × 10 cm molecule s) in the high pressure apparatus is in excellent agreement with the literature, confirming the accuracy of measurements made with this instrument. Above 700 K, the OH decays were no longer single exponentials due to regeneration of OH from adduct decomposition and the establishment of the OH + CH ⇌ HOCH equilibrium (R1a, R-1a). This equilibrium was analyzed by comparison to a master equation model of reaction R1 and determined the well depth for OH addition to carbon C and C to be equal to 153.5 ± 6.2 and 143.4 ± 6.2 kJ mol, respectively. These well depths are in excellent agreement with the present ab initio-CCSD(T)/CBS//M062X/6-311++G(3df,2p)-calculations (154.1 kJ mol for the C adduct). Addition to the less stable C and C adducts is not important. The data above 700 K also indicated that a minor but significant direct abstraction channel, R1b, was also operating with k = (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s. Additional support for the presence of this abstraction channel comes from our ab initio calculations and from room-temperature proton transfer mass spectrometry product analysis. The literature data on reaction R1 together with the present data were assessed using master equation analysis, using the MESMER package. This analysis produced a refined data set that generates our recommended k( T, [ M]). An analytical representation of k( T, [ M]) and k( T, [ M]) is provided via a Troe expression. The reported experimental data (the sum of addition and abstraction), k = (9.5 ± 0.2) × 10( T/298 K) + (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s, significantly extend the measured temperature range of R1.

The supplementary information (369 pages) contains: Pages S2 -S6 Fits to all 96 traces showing the raw data and final fit. At the top of each trace are the associated experimental conditions. Pages S7 -S99 An example MESMER input file for the analysis. The majority of the information, from page S12 onwards, are the experimental input data for each trace. Pages S100 -S369 The associated MESMER log file. The data from page S104 onwards are the experimental data and associated fit. S2 Trace FitsPlots of the trace fits are shown below. Blue lines are the MESMER traces the red lines connect experimental points. For clarity only every other experimental point is plotted. S3 S4 S5 S6 S7 MESMER Input file Following is the MESMER input file for the fitting of the OH + C2H4 trace data. Ethylene + OH Gaussian 09, Revision D.01 6-311++G(3df,2p) (5D, 7F) g03 -112.133 2.00 0.964 5.

Fast radical reactions are central to the chemistry of planetary atmospheres and combustion systems. Laser-induced fluorescence is a highly sensitive and selective technique that can be used to monitor a number of radical species in kinetics experiments, but is typically limited to low pressure systems owing to quenching of fluorescent states at higher pressures. The design and characterisation of an instrument are reported using laser-induced fluorescence detection to monitor fast radical kinetics (up to 25 000 s(-1)) at high temperatures and pressures by sampling from a high pressure reaction region to a low pressure detection region. Kinetics have been characterised at temperatures reaching 740 K and pressures up to 2 atm, with expected maximum operational conditions of up to ∼900 K and ∼5 atm. The distance between the point of sampling from the high pressure region and the point of probing within the low pressure region is critical to the measurement of fast kinetics. The instrumentation described in this work can be applied to the measurement of kinetics relevant to atmospheric and combustion chemistry.

The rate coefficients of the reactions of OH radicals and Cl atoms with three alkylcyclohexanes compounds, methylcyclohexane (MCH), trans-1,4-dimethylcyclohexane (DCH), and ethylcyclohexane (ECH) have been investigated at (293 ± 1) K and 1000 mbar of air using relative rate methods. A majority of the experiments were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC), a stainless steel chamber using in situ FTIR analysis and online gas chromatography with flame ionization detection (GC-FID) detection to monitor the decay of the alkylcyclohexanes and the reference compounds. The studies were undertaken to provide kinetic data for calibrations of radical detection techniques in HIRAC. The following rate coefficients (in cm 3 molecule −1 s −1 ) were obtained for Cl reactions: k (Cl+MCH) = (3.51 ± 0.37) × 10 -10 , k (Cl+DCH) = (3.63 ± 0.38) × 10 −10 , k (Cl+ECH) = (3.88 ± 0.41) × 10 −10 , and for the reactions with OH radicals: k (OH+MCH) = (9.5 ± 1.3) × 10 -12 , k (OH+DCH) = (12.1 ± 2.2) × 10 −12 , k (OH+ECH) = (11.8 ± 2.0) × 10 −12 . Errors are a combination of statistical errors in the relative rate ratio (2 ) and the error in the reference rate coefficient. Checks for possible systematic errors were made by the use of two reference compounds, two different measurement techniques, and also three different sources of OH were employed in this study: photolysis of CH 3 ONO with black lamps, photolysis of H 2 O 2 at 254 nm, and nonphotolytic trans-2-butene ozonolysis. For DCH, some direct laser flash photolysis studies were also undertaken, producing results in good agreement with the relative rate measurements. Additionally, temperature-dependent rate coefficient investigations were performed for the reaction of methylcyclohexane with the OH radical over the range 273-343 K using the relative rate method; the resulting recommended Arrhenius expression is k (OH + MCH) = (1.85 ± 0.27) × 10 -11 exp((-1.62 ± 0.16) kJ mol −1 /RT) cm 3 molecule −1 s −1 . The kinetic data are discussed in terms of OH and Cl reactivity trends, and comparisons are made with the existing literature values and with rate coefficients from structure-activity relationship methods. This is the first study on the rate coefficient determination of the reaction of ECH with OH radicals and chlorine atoms, respectively. K E Y W O R D Scalibration of OH detector, kinetics, OH and Cl structure activity relationships, substituent effect 544

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Literature rate coefficients for the prototypical radical–radical reaction at 298 K vary by close to an order of magnitude; such variations challenge our understanding of fundamental reaction kinetics. We have studied the title reaction at room temperature via the use of laser flash photolysis to generate OH and HO2 radicals, monitoring OH by laser-induced fluorescence using two different approaches, looking at the direct reaction and also the perturbation of the slow OH + H2O2 reaction with radical concentration, and over a wide range of pressures. Both approaches give a consistent measurement of k 1,298K ∼1 × 10–11 cm3 molecule–1 s–1, at the lowest limit of previous determinations. We observe, experimentally, for the first time, a significant enhancement in the rate coefficient in the presence of water, k 1,H2O, 298K = (2.17 ± 0.09) × 10–28 cm6 molecule–2 s–1, where the error is statistical at the 1σ level. This result is consistent with previous theoretical calculations, and the effect goes some way to explaining some, but not all, of the variation in previous determinations of k 1,298K. Supporting master equation calculations, using calculated potential energy surfaces at the RCCSD­(T)-F12b/CBS//RCCSD/aug-cc-pVTZ and UCCSD­(T)/CBS//UCCSD/aug-cc-pVTZ levels, are in agreement with our experimental observations. However, realistic variations in barrier heights and transition state frequencies give a wide range of calculated rate coefficients showing that the current precision and accuracy of calculations are insufficient to resolve the experimental discrepancies. The lower value of k 1,298K is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO2 → HCl + O2. The implications of these results in atmospheric models are discussed.

Ab initio theory has been used to identify the pre-reaction complex in the atmospherically important reaction between OH + SO2, (R1), where the binding energy of the pre-reaction complex was determined to be 7.2 kJ mol-1. Using reaction rate theory, implemented with the master equation package MESMER, the effects of this complex on the kinetics of R1 at temperatures above 250 K have been investigated. From simulations and fitting to the experimental kinetic data, it is clear that the influence of this pre-reaction complex is negligible and that the kinetics are controlled by the inner transition-state that leads to the product, HOSO2. While the effect of this complex on the thermal kinetics is small it potentially provides an efficient route to remove energy from vibrationally excited OH. The fitting to the past experimental data reveals that this inner transition-state is submerged with a barrier -0.25 kJ mol-1 below the entrance channel, which is outside the range predicted from the best theoretical calculations. The data fitting also yielded ΔR1H0K equal to -(109 ± 5.6) kJ mol-11 and a more precise expression for k∞1(T), (5.95 ± 0.83) × 10-13 × (T/298)-0.11±0.27.