Methyl formate (CH3OCHO, MF) is the simplest ester, a class of species that is of interest for combustion and atmospheric chemistry studies. H-abstraction reactions from the two sites of MF leads to radicals, R, that react with oxygen to form RO2 species that are important in the oxidation of MF under atmospheric and low temperature combustion conditions. This work reports the kinetics of the O2 reactions with R formed through the reaction of Cl with MF from 213 K to ∼420 K and in the range ∼5-100 Torr of N2 or Ar. The rate coefficients kR+O2 were determined by the analysis of the kinetic profiles of the OH radicals formed by the prompt dissociation of RO2, which were measured using the pulsed laser flash photolysis-laser induced fluorescence technique. At 294 K in N2 (5-100 Torr), kR+O2 was pressure independent, within experimental error, with a value of (5.0 ± 0.4) × 10-12 cm3 molecule-1 s-1. Isotopic studies using CH3OCDO were used to differentiate between the kinetics of the CH3OCO and CH2OCHO associations with O2. CH2OCDO reacted faster than CH3OCO by approximately 40% at 213 K, rising to 200% at 472 K. The enhanced reactivity of the CH2OCHO with O2 can be explained by a more attractive potential energy surface and a looser transition state. It was also possible to determine the rate coefficients of the unimolecular decomposition of CH3OCO, kdec (348-470 K). The experimental values of kdec were fitted with the master equation application, MESMER, with the literature (ANL0F″) barrier height of 57.3 kJ mol-1, and floating the average energy transfer parameter at 298 K, <ΔE>d, 298 K, and the temperature exponent of <ΔE>d. A good fit to the data was obtained with: <ΔE>d(Ar) = (110 ± 30) × (T/298 K)(0.0 ± 0.7) cm-1 and <ΔE>d(He) = (34.4 ± 6.2) × (T/298 K)(1.0 ± 0.5) cm-1.

The aroma compound ethyl butyrate (EB) and its methylated derivatives ethyl 2-methylbutyrate (EM), ethyl isovalerate (EI), and isopropyl butyrate (IB) are present in many consumer products. To evaluate the environmental and health impacts of these volatile organic compounds, a detailed understanding of their gas-phase photochemical reactivity is required. Here, we performed pulsed laser photolysis/laser-induced fluorescence (PLP-LIF) experiments to investigate the kinetics of their reactions with the hydroxyl radical (OH). Room temperature rate coefficients in units of 10-12molec.-1cm3s-1 with 2 sigma statistical errors were determined as: (5.5 +/- 0.2) for EB + OH, (7.0 +/- 0.3) for EM + OH, (11.2 +/- 0.4) for EI + OH, and (7.5 +/- 0.4) for IB + OH. All four reactions exhibited complex kinetics with distinct non-Arrhenius behaviour for temperatures up to about 400 K. This behaviour was attributed to pre-reaction complexes and is consistent with site-specific reactivities as predicted by an established structure-activity-relationship (SAR). In a second series of experiments, quasi-gas-phase UV-vis. spectroscopy and time-dependent density functional theory predictions were used to obtain absorption cross-sections. All four esters displayed an absorption band at around 213 nm (spin-forbidden pi*<- n transition), but did not absorb appreciably in the visible or UV-A part of the spectrum where light is abundant at ground level. Therefore, the reaction with OH was considered the main loss process, with lifetimes for tropospheric removal ranging from 22-45 h. Photochemical ozone creation potentials were estimated to be in a moderate range between 28 and 34.

Ozonolysis reactions are key oxidation mechanisms for unsaturated volatile organic compounds in the atmosphere, generating Criegee intermediates (CIs) that drive subsequent chemistry. Photolytic laboratory sources of certain CIs have demonstrated unexpectedly high CI reactivity but bypass the complex dynamics that occur in the atmosphere during ozonolysis, which affect yields and reactivity. Here, we report direct measurements of absolute concentrations of the CIs formaldehyde oxide (CH2OO) and acetone oxide [(CH3)2COO] during ozonolysis reactions in an atmospheric simulation chamber made in real-time using UV cavity-enhanced absorption spectroscopy (CEAS). The measurements enable direct determination of stabilized CI yields and reaction kinetics under atmospheric conditions. The technique has the potential to reduce uncertainties associated with ozonolysis chemistry in atmospheric models.

The kinetics between Cl and ethylene, R1, have been determined between 298 and 822 K in time-resolved experiments, where the Cl atoms were monitored for the first time via laser-induced fluorescence at 118.877 nm. A key advantage of this method of Cl detection is that there is limited absorption of 118 nm radiation by oxygen, and we report the first Cl reactivity measurements. The kinetics of Cl + C2H4 at room temperature are simple association, k1a([M]), and have been used in a master equation analysis (via the MESMER application) to show consistency with most of the literature. Between 393 and 490 K, the kinetics exhibited equilibrium behavior: Cl + C2H4 ⇔ Cl-C2H4 (k1a([M]),k-1a([M])). These forward and reverse rate coefficients have been used in van't Hoff and reaction rate theory (MESMER) analysis to determine the enthalpy of reaction, ΔrHR1ao. This analysis yields a ΔrH0,R1ao equal to -74.1 ± 0.6 kJ mol-1. Ab initio structure calculations provided input values for MESMER analysis of the equilibrium data. The range of ab initio calculations carried out returned consistent values for ΔrH0,R1ao, but the values are consistently more exothermic than the experimental value. Via comparison between theory and experiment, it is estimated that these ab initio calculations are good to ∼4 kJ mol-1. Above 500 K, the removal kinetics are dominated by abstraction: Cl + C2H4 → HCl + C2H3 (k1b). This reaction occurs on an endothermic potential surface, where the energy of the transition state is below that of the products. Analysis of this kinetic data and the literature highlights that the location of the transition state along the reaction coordinate varies with temperature, becoming more reagent-like with increased temperature.

Kinetics of the gas phase reaction between the stabilized Criegee intermediate formaldehyde oxide (CH2OO) and nitrogen dioxide (NO2) have been measured using laser flash photolysis of CH2I2/O2/N2/NO2 mixtures coupled with time-resolved broadband ultraviolet absorption spectroscopy. Experiments were performed in N2 under pseudo-first-order conditions at temperatures between 242 and 353 K and pressures in the range 25 to 300 Torr. The kinetics of CH2OO + NO2 are independent of pressure, with a mean rate coefficient of k1 = (1.24 ± 0.22) × 10-12 cm3 s-1 at 298 K, where the uncertainty represents a combination of the 1σ statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of NO2. Measurements indicate upper limits of <5% for production of NO3 and <5% for production of NO, and further studies of product yields are warranted. In contrast to expectations from theory, the kinetics of CH2OO + NO2 display a negative temperature dependence that can be described by k1 = (1.07 ± 0.02) × 10-12 × (T/298)-(2.9±0.2) cm3 s-1. Analysis using the Master Equation Solver for Multi-Energy well Reactions is able to reproduce a negative temperature dependence for the reaction if significant changes to barrier heights are made, but the overall agreement between the experiment and theory remains poor. This work highlights the challenges associated with calculations for systems with significant multi-reference character.

Formic acid (FA, HC(O)OH) is of interest as an e-fuel and hydrogen carrier, as well as a significant component of atmospheric acidification. Reaction with the OH radical is a major removal process for FA. We have measured the overall rate coefficient, k1, for the reaction of OH and FA over the temperature range of 300-850 K using laser flash photolysis to generate OH and monitoring the disappearance of OH under pseudo-first-order conditions using laser-induced fluorescence. The rate coefficient can be parameterized as k1(T) = 9.8 × 10-15 × (T/298 K)5.1 × exp(-14200/RT) cm3 molecule-1 s-1 where we estimate that the uncertainty increases from ∼ ±20% in the range 300-600 K to ±50% above 600 K. We have also determined the branching ratio to H2O + HCO2 (abstraction at the O-H site) by observing the H atoms produced by the fast decomposition of HCO2. H atom yields drop from ∼0.9 at 300 K to ∼0.5 at 600 K. The kinetics and branching ratios of k1 are in good agreement with theoretical calculations. Above 600 K, the observed H atom yield increases, and we show that this is due to the decomposition of the HOCO product. The implications of these studies on FA and HOCO in the combustion and pyrolysis of FA are considered.

The kinetics of the dimerization of NH3 in helium and nitrogen bath gas within the supersonic flow of a Laval nozzle were investigated at very low temperatures. Experimentally, the fraction of the NH3 monomer, fmonomer, remaining in the flow at 91 K in N2 and at 35 K in He for a total bath gas density [M]∼5 × 1016 molecules cm-3 was monitored using fluorescence from the electronically excited NH2 photofragment formed following NH3 photolysis at 213 nm. No dimerization was observed up to [NH3] = 1 × 1015 molecules cm-3 for 160 mm downstream of the 91 K N2 nozzle, nor up to [NH3] = 5 × 1014 molecules cm-3 for 150 mm downstream of the 35 K He nozzle. Dimerization was observed at higher [NH3], being more pronounced at lower temperatures. For the Cs and C2h conformers of the NH3 dimer, calculations at the CCSD(T)/aug-cc-pVTZ level gave a zero-point vibrational-energy corrected binding energy of -7.52 and -7.33 kJ mol-1, respectively. Energy-grained master equation calculations based on statistical rate theory using the open-source MESMER package were used to calculate rate coefficients for dimerization, kdimer, over the temperature range T = 25-300 K and [M] = 1013-1022 molecules cm-3 for He and N2. kdimer displayed a negative T dependence and a positive [M] dependence and was found to be sensitive to changes in the low-lying vibrational frequencies of the NH3 dimer, for example, the inclusion of a hindered rotor potential for the internal twisting mode, which alters the density of states. Using the axial profiles of T, [M], and velocity for the Laval nozzles, the calculated values of kdimer were used to calculate fmonomer in the flow for comparison with the experiment. At higher [NH3], when dimerization was observed, the calculations significantly underestimated the degree of dimerization taking place in the flow, with a significant increase in the calculated value of kdimer required to match the experiment. The reasons for the discrepancy are discussed, for example, errors in the calculation of the density of states for the NH3 dimer and the average energy removed per collision by the bath gas at very low temperatures.

Rate coefficients for the reaction of C2H with CH2O were measured for the first time over the temperature range of 37-603 K, with the C2H radicals produced by pulsed laser photolysis and detected by CH radical chemiluminescence following their reaction with O2. The low temperature measurements (<= 93 K) relevant to the interstellar medium were made within a Laval nozzle gas expansion, while higher temperature measurements (>= 308 K) were made within a temperature controlled reaction cell. The rate coefficients display a negative temperature dependence below 300 K, reaching (1.3 +/- 0.2) x 10-10 cm3 molecule-1 s-1 at 37 K, while only a slight positive temperature dependence is observed at higher temperatures above 300 K. Ab initio calculations of the potential energy surface (PES) were combined with rate theory calculations using the MESMER master-equation program in order to predict rate coefficients and branching ratios. The three lowest energy entrance channels on the PES all proceed via the initial formation of a weakly bound prereaction complex, bound by similar to 5 kJ mol-1, followed by either a submerged barrier on the route to the H-abstraction products (C2H2 + CHO), or emerged barriers on the routes to the C- or O-addition species. MESMER calculations indicated that over the temperature range investigated (10-600 K) the two addition channels were uncompetitive, accounting for less 0.3% of the total product yield even at 600 K. The PES containing only the H-abstraction product channel was fit to the experimentally determined rate coefficients, with only a minor adjustment to the height of the submerged barrier (from -2.6 to -5.9 kJ mol-1) required. Using this new submerged barrier height, and including the subsequent dissociation of the CHO product into CO + H in the PES, rate coefficients and branching ratios were calculated over a wide range of temperatures and pressures and these used to recommend best-fit modified Arrhenius expressions for use in astrochemical modeling. Inclusion of the new rate coefficients and branching ratios in a UMIST chemical model of an outflow from an asymptotic giant branch (AGB) star yielded no significant changes in the abundances of the reactants or the products of the reaction, however, removal of the C-addition channel currently in the UMIST Rate22 database did result in a significant reduction in the abundance of propynal (HCCCHO).

Methyl esters are an important component of combustion and atmospheric systems. Reaction with the OH radical plays an important role in the removal of the simplest methyl ester, methyl formate (MF, CH3OCHO). In this paper, the overall rate coefficients for the reactions of OH and OD with MF isotopologues, studied under pseudo-first-order conditions, are reported using two different laser flash photolysis systems with the decay of OH monitored by laser-induced fluorescence. The room-temperature rate coefficient for OH + MF, (1.95 +/- 0.34) x 10(-13) cm(3) molecule(-1) s(-1), is in good agreement with the literature. The rate coefficient exhibits curved Arrhenius behavior, and our results bridge the gap between previous low-temperature and shock tube studies. In combination with the literature, the rate coefficient for the reaction of OH with MF between 230 and 1400 K can be parametrized as k(OH+MF) = (3.2 x 10(-13)) x (T/300 K)(2.3) x exp(-141.4 K/T) cm(3) molecule(-1) s(-1) with an overall estimated uncertainty of similar to 30%. The reactions of OD with MF isotopologues show a small enhancement (inverse secondary isotope effect) compared to the respective OH reactions. The reaction of OH/OD with MF shows a normal primary isotope effect, a decrease in the rate coefficient when MF is partially or fully deuterated. Experimental studies have been supported by ab initio calculations at the CCSD(T)-F12/aug-cc-pVTZ//M06-2X/6-31+G** level of theory. The calculated, zero-point-corrected, barrier heights for abstraction at the methyl and formate sites are 1.3 and 6.0 kJ mol(-1), respectively, and the ab initio predictions of kinetic isotope effects are in agreement with experiment. Fitting the experimental isotopologue data refines these barriers to 0.9 +/- 0.6 and 4.1 +/- 0.9 kJ mol(-1). The branching ratio is approximately 50:50 at 300 K. Between 300 and 500 K, abstraction via the higher-energy, higher-entropy formate transition state becomes more favored (60:40). However, experiment and calculations suggest that as the temperature increases further, with higher energy, less constrained conformers of the methyl transition state become more significant. The implications of the experimental and theoretical results for the mechanisms of MF atmospheric oxidation and low-temperature combustion are discussed.

Activated chemistry in coupled reaction systems has broadened our understanding of the chemical kinetics. In the case of intermediates formed in gas phase abstraction reactions (e.g., OH + HC(O)C(O)H (glyoxal) →HC(O)CO + H2O), it is particularly crucial to understand how the reaction energy is partitioned between product species as this determines the propensity for a given product to undergo "prompt" dissociation (e.g., HC(O)CO → HCO + CO) before the excess reaction energy is removed. An example of such an activated system is the OH + glyoxal + O2 coupled reaction system. In this work, we develop a molecular dynamics pipeline, which, combined with a master equation analysis, accurately models previous experimental measurements. This new work resolves previous complexities and discrepancies from earlier master equation modeling for this reaction system. The detailed molecular dynamics approach employed here is a powerful new tool for modeling challenging activated reaction systems.

The kinetics of reactions between the simplest Criegee intermediate, CH2OO, and water vapour have been investigated at temperatures between 262 and 353 K at a total pressure of 760 Torr using laser flash photolysis of CH2I2–O2–N2–H2O mixtures coupled with broadband time-resolved UV absorption spectroscopy. Results indicate that the reaction with water monomers represents a minor contribution to the total loss of CH2OO under the conditions employed in this work, with an estimated rate coefficient for CH2OO + H2O (R1) of (9.8 ± 5.9) × 10−17 cm3 molecule−1 s−1 at 298 K and a temperature dependence described by k1 = (3.2 ± 1.1) × 10−13 exp(−(2410 ± 270)/T) cm3 molecule−1 s−1. The reaction of CH2OO with water dimers, CH2OO + (H2O)2 (R2), dominates under the conditions employed in this work. The rate coefficient for R2 has been measured to be k2 = (9.5 ± 2.5) × 10−12 cm3 molecule−1 s−1 at 298 K, with a negative temperature dependence described by k2 = (2.85 ± 0.40) × 10−15 exp((2420 ± 340)/T) cm3 molecule−1 s−1, where rateR2 = k2[CH2OO][(H2O)2]. For use in atmospheric models, we recommend description of the kinetics for R2 in terms of the product of the rate coefficient k2 and the equilibrium constant KDeq (k2,eff = k2KDeq) for water dimer formation to allow the rate of reaction to be expressed in terms of water monomer concentration as rateR2 = k2,eff[CH2OO][H2O]2 to avoid explicit calculation of dimer concentrations and impacts of differences in values of KDeq reported in the literature. Results from this work give k2,eff = (1.96 ± 0.51) × 10−32 cm6 molecule−2 s−1 at 298 K with a temperature dependence described by k2,eff = (2.78 ± 0.28) × 10−38 exp((4010 ± 400)/T) cm6 molecule−2 s−1. No significant impacts of a reaction between CH2OO and three water molecules were observed in this work, potentially as a result of the relative humidities used in this work (up to 57% at 298 K). Atmospheric implications of the results have been investigated using the global chemistry transport model GEOS-Chem. Model simulations indicate that the reaction with water dimers dominates the loss of CH2OO in the atmosphere and limits the impacts of other reactions of CH2OO, with the reaction with water dimers representing >98% of the total loss of CH2OO in the troposphere.

Kinetics of reactions between SO2 and CH3CHOO Criegee intermediate conformers have been measured at temperatures between 242 and 353 K and pressures between 10 and 600 Torr using laser flash photolysis of CH3CHI2/O-2/N-2/SO2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. The kinetics of syn-CH3CHOO + SO2 are pressure-dependent and exhibit a negative temperature dependence, with the observed pressure dependence reconciling apparent discrepancies between previous measurements performed at similar to 298 K. Results indicate a rate coefficient of (4.80 +/- 0.46) x 10(-11) cm(3) s(-1) for the reaction of syn-CH3CHOO with SO2 at 298 K and 760 Torr. In contrast to the behavior of the syn-conformer, the kinetics of anti-CH3CHOO + SO2 display no significant dependence on temperature or pressure over the ranges investigated, with a mean rate coefficient of (1.18 +/- 0.21) x 10(-10) cm(3) s(-1) over all conditions studied in this work. Results indicate that the reaction of syn-CH3CHOO with SO2 competes with unimolecular decomposition and reaction with water vapor in areas with high SO2 concentration and low humidity, particularly at lower temperatures.

The first theoretical results regarding the gas-phase reaction mechanism and kinetics of the CH (X2Π) + OCS reaction are presented here. This reaction has a proposed importance in the removal of OCS in regions of the interstellar medium (ISM) and has the potential to form the recently observed HCS/HSC isomers, with both constitutional isomers having recently been observed in the L483 molecular cloud in a 40:1 ratio. Statistical rate theory simulations were performed on stationary points along the reaction potential energy surface (PES) obtained from ab initio calculations at the RO-CCSD(T)/aug-cc-pV(Q+d)Z//M06-2X-D3/aug-cc-pV(Q+d)Z level of theory over the temperature and total density range of 150–3000 K and 1011–1024 cm–3, respectively, using a Master Equation analysis. Exploration of the reaction potential energy surface revealed that all three pathways identified to create CS + HCO products required surmounting barriers of 16.5 kJ mol–1 or larger when CH approached the oxygen side of OCS, rendering this product formation negligible below 1000 K, and certainly under low-temperature ISM conditions. In contrast, when CH approaches the sulfur side of OCS, only submerged barriers are found along the reaction potential energy surface to create HCCO + S or CO + HCS, both of which are formed via a strongly bound OCC(H)S intermediate (−358.9 kJ mol–1). Conversion from HCS to HSC is possible via a barrier of 77.8 kJ mol–1, which is still −34.1 kJ mol–1 below the CH + OCS entrance channel. No direct route from CH + OCS to H + CO + CS was found from our ab initio calculations. Rate theory simulations suggest that the reaction has a strong negative temperature dependence, in accordance with the barrierless addition of CH to the sulfur side of OCS. Product branching fractions were also determined from MESMER simulations over the same temperature and total density range. The product branching fraction of CO + HCS reduces from 79% at 150 K to 0.0% at 800 K, while that of HCS dissociation to H + CS + CO increases from 22% at 150 K to 100% at 800 K. The finding of CO + HCS as the major product at the low temperatures relevant to the ISM, instead of H + CS + CO, is in opposition to the current supposition used in the KIDA database and should be adapted in astrochemical models as another source of the HCS isomer.

The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH2 with acetaldehyde (CH3CHO) has been performed. Experiments were carried out using laser-flash photolysis and laser-induced fluorescence spectroscopy to create and monitor the temporal decay of NH2 in the presence of CH3CHO. Low temperatures relevant to the interstellar medium were achieved using a pulsed Laval nozzle expansion. Rate coefficients were measured over the temperature and pressure range of 29-107 K and 1.4-28.2 × 1016 molecules per cm3, with the reaction exhibiting a negative temperature dependence and a positive pressure dependence. The yield of CH3CO from the reaction has also been determined at 67.1 and 35.0 K, by observing OH produced from the reaction of CH3CO with added O2. Ab initio calculations of the potential energy surface (PES) were combined with Rice-Rampsberger-Kessel-Marcus (RRKM) calculations to predict rate coefficients and branching ratios over a broad range of temperatures and pressures. The calculated rate coefficients were shown to be sensitive to the calculated density of states of the stationary points, which in turn are sensitive to the inclusion of hindered rotor potentials for several of the vibrational frequencies. The experimentally determined rate coefficients and yields have been used to fit the calculated PES, from which low-pressure limiting rate coefficients relevant to the ISM were determined. These have been included in a single-point dark cloud astrochemical model, in which the reaction is shown to be a potential source of gas-phase CH3CO radicals under dark cloud conditions.

The first experimental study of the low-temperature kineticsofthe gas-phase reaction between NH2 and NO has been performed.A pulsed laser photolysis-laser-induced fluorescence technique wasused to create and monitor the temporal decay of NH2 inthe presence of NO. Measurements were carried out over the temperaturerange of 24-106 K, with the low temperatures achieved usinga pulsed Laval nozzle expansion. The negative temperature dependenceof the reaction rate coefficient observed at higher temperatures inthe literature continues at these lower temperatures, with the ratecoefficient reaching 3.5 x 10(-10) cm(3) molecule(-1) s(-1) at T = 26 K. Ab initio calculations of the potential energy surface werecombined with rate theory calculations using the MESMER software packagein order to calculate and predict rate coefficients and branchingratios over a wide range of temperatures, which are largely consistentwith experimentally determined literature values. These theoreticalcalculations indicate that at the low temperatures investigated forthis reaction, only one product channel producing N-2 +H2O is important. The rate coefficients determined in thisstudy were used in a gas-phase astrochemical model. Models were runover a range of physical conditions appropriate for cold to warm molecularclouds (10 to 30 K; 10(4) to 10(6) cm(-3)), resulting in only minor changes (<1%) to the abundances ofNH(2) and NO at steady state. Hence, despite the observedincrease in the rate at low temperatures, this mechanism is not adominant loss mechanism for either NH2 or NO under darkcloud conditions.

Rate coefficients for the reaction of CN with CH2O were measured for the first time below room temperature in the range 32-103 K using a pulsed Laval nozzle apparatus together with the Pulsed Laser Photolysis-Laser-Induced Fluorescence technique. The rate coefficients exhibited a strong negative temperature dependence, reaching (4.62 ± 0.84) × 10-11 cm3 molecule-1 s-1 at 32 K, and no pressure dependence was observed at 70 K. The potential energy surface (PES) of the CN + CH2O reaction was calculated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, with the lowest energy channel to reaction characterized by the formation of a weakly-bound van der Waals complex, bound by 13.3 kJ mol-1, prior to two transition states with energies of -0.62 and 3.97 kJ mol-1, leading to the products HCN + HCO or HNC + HCO, respectively. For the formation of formyl cyanide, HCOCN, a large activation barrier of 32.9 kJ mol-1 was calculated. Reaction rate theory calculations were performed with the MESMER (Master Equation Solver for Multi Energy well Reactions) package on this PES to calculate rate coefficients. While this ab initio description provided good agreement with the low-temperature rate coefficients, it was not capable of describing the high-temperature experimental rate coefficients from the literature. However, increasing the energies and imaginary frequencies of both transition states allowed MESMER simulations of the rate coefficients to be in good agreement with data spanning 32-769 K. The mechanism for the reaction is the formation of a weakly-bound complex followed by quantum mechanical tunnelling through the small barrier to form HCN + HCO products. MESMER calculations showed that channel generating HNC is not important. MESMER simulated the rate coefficients from 4-1000 K which were used to recommend best-fit modified Arrhenius expressions for use in astrochemical modelling. The UMIST Rate12 (UDfa) model yielded no significant changes in the abundances of HCN, HNC, and HCO for a variety of environments upon inclusion of rate coefficients reported here. The main implication from this study is that the title reaction is not a primary formation route to the interstellar molecule formyl cyanide, HCOCN, as currently implemented in the KIDA astrochemical model.

Literature rate coefficients for the prototypical radical-radical reaction OH + HO2 (k1)-> H2O O-2 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) similar to 1 x 10(-11) cm(3) 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) x 10(-28) cm(6) molecule(-2) s(-1), where the error is statistical at the 1 sigma 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 k1,298K is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO2 -> HCl + O-2. The implications of these results in atmospheric models are discussed.

The reaction of CH radicals with H2 has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k1 shows complex behavior: at low pressures, the main route for the CH3* formed from the insertion of CH into H2 is the formation of 3CH2 + H, and as the pressure is increased, CH3* is increasingly stabilized to CH3. The kinetic data on CH + H2 have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, 3CH2 + H → CH + H2. The resulting parametrization is kCH2+H(T) = (1.69 ± 0.11) × 10-10 × (T/298 K)(0.05±0.010) cm3 molecule-1 s-1, where the errors are 1σ.