Robin J. Shannon

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

Understanding the abundances of molecules in dense interstellar clouds requires knowledge of the rates of gas-phase reactions between uncharged species. However, because of the low temperatures within these clouds, reactions with an activation barrier were considered too slow to play an important role. Here we show that, despite the presence of a barrier, the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol--one of the most abundant organic molecules in space--is almost two orders of magnitude larger at 63 K than previously measured at ∼200 K. We also observe the formation of the methoxy radical product, which was recently detected in space. These results are interpreted by the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum-mechanical tunnelling to form products. We postulate that this tunnelling mechanism for the oxidation of organic molecules by OH is widespread in low-temperature interstellar environments.

Analysis of the kinetics and yields of OH radical production from the CH3OCH2 + O2 reaction in the temperature range 195-650 K: an experimental and computational study.

The methoxymethyl radical, CH3OCH2, is an important intermediate in the low temperature combustion of dimethyl ether. The kinetics and yields of OH from the reaction of the methoxymethyl radical with O2 have been measured over the temperature and pressure ranges of 195-650 K and 5-500 Torr by detecting the hydroxyl radical using laser-induced fluorescence following the excimer laser photolysis (248 nm) of CH3OCH2Br. The reaction proceeds via the formation of an energized CH3OCH2O2 adduct, which either dissociates to OH + 2 H2CO or is collisionally stabilized by the buffer gas. At temperatures above 550 K, a secondary source of OH was observed consistent with thermal decomposition of stabilized CH3OCH2O2 radicals. In order to quantify OH production from the CH3OCH2 + O2 reaction, extensive relative and absolute OH yield measurements were performed over the same (T, P) conditions as the kinetic experiments. The reaction was studied at sufficiently low radical concentrations (∼10(11) cm(-3)) that secondary (radical + radical) reactions were unimportant and the rate coefficients could be extracted from simple bi- or triexponential analysis. Ab initio (CBS-GB3)/master equation calculations (using the program MESMER) of the CH3OCH2 + O2 system were also performed to better understand this combustion-related reaction as well as be able to extrapolate experimental results to higher temperatures and pressures. To obtain agreement with experimental results (both kinetics and yield data), energies of the key transition states were substantially reduced (by 20-40 kJ mol(-1)) from their ab initio values and the effect of hindered rotations in the CH3OCH2 and CH3OCH2OO intermediates were taken into account. The optimized master equation model was used to generate a set of pressure and temperature dependent rate coefficients for the component nine phenomenological reactions that describe the CH3OCH2 + O2 system, including four well-skipping reactions. The rate coefficients were fitted to Chebyshev polynomials over the temperature and density ranges 200 to 1000 K and 1 × 10(17) to 1 × 10(23) molecules cm(-3) respectively for both N2 and He bath gases. Comparisons with an existing autoignition mechanism show that the well-skipping reactions are important at a pressure of 1 bar but are not significant at 10 bar. The main differences derive from the calculated rate coefficient for the CH3OCH2OO → CH2OCH2OOH reaction, which leads to a faster rate of formation of O2CH2OCH2OOH.

Measurements of Rate Coefficients for Reactions of OH with Ethanol and Propan-2-ol at Very Low Temperatures

The low temperature kinetics of the reactions of OH with ethanol and propan-2-ol have been studied using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) spectroscopy. The rate coefficients for both reactions have been found to increase significantly as the temperature is lowered, by approximately a factor of 18 between 293 and 54 K for ethanol, and by ∼10 between 298 and 88 K for OH + propan-2-ol. The pressure dependence of the rate coefficients provides evidence for two reaction channels: a zero pressure bimolecular abstraction channel leading to products and collisional stabilization of a weakly bound OH-alcohol complex. The presence of the abstraction channel at low temperatures is rationalized by a quantum mechanical tunneling mechanism, most likely through the barrier to hydrogen abstraction from the OH moiety on the alcohol.

The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

Recent laboratory experiments using a pulsed Laval nozzle apparatus have shown that reactions between a neutral molecule and the radical OH can occur efficiently at low temperatures despite activation energy barriers if there is a hydrogen-bonded complex in the entrance channel which allows the system to tunnel efficiently under the barrier. Since OH is a major radical in the interstellar medium, this class of reactions may well be important in the chemistry that occurs in the gas phase of interstellar clouds. Using a new gas-grain chemical network with both gas-phase reactions and reactions on the surfaces of dust particles, we studied the role of OH–neutral reactions in dense interstellar clouds at 10, 50, and 100 K. We determined that at least one of these reactions can be significant, especially at the lowest temperatures studied, where the rate constants are large. It was found in particular that the reaction between CH3OH and OH provides an effective and unambiguous gas-phase route to the production of the gaseous methoxy radical (CH3O), which has been recently detected in cold, dense interstsellar clouds. The role of other reactions in this class is explored.

Comment on “When Rate Constants Are Not Enough”

We discuss “phenomenological” reactions, rate constants, and flux coefficients in the context of their relationship to master-equation methods for obtaining rate constants. The isomerizationdissociation of 2-butene is used as an example for illustrating some of the pitfalls that can exist in trying to obtain rate constants from population-time histories alone (with associated, derivative quantities such as flux coefficients). We attach a considerable amount of Supporting Information that elaborates on some of the key points made in the text. Included in the Supporting Information is a description of two methods for obtaining rate constants from the master equation when its transition matrix is not readily available.

Kinetic Study of the OH + Glyoxal Reaction: Experimental Evidence and Quantification of Direct OH Recycling

The kinetics of the OH + glyoxal, (HCO)2, reaction have been studied in N2 and N2/O2 bath gas from 5-80 Torr total pressure and 212-295 K, by monitoring the OH decay via laser induced fluorescence (LIF) in excess (HCO)2. The following rate coefficients, kOH+(HCO)2 = (9.7 ± 1.2), (12.2 ± 1.6), and (15.4 ± 2.0) × 10(-12) cm(3) molecule(-1) s(-1) (where errors represent a combination of statistical errors at the 2σ level and estimates of systematic errors) were measured in nitrogen at temperatures of 295, 250, and 212 K, respectively. Rate coefficient measurements were observed to be independent of total pressure but decreased following the addition of O2 to the reaction cell, consistent with direct OH recycling. OH yields, ΦOH, for this reaction were quantified experimentally for the first time as a function of total pressure, temperature, and O2 concentration. The experimental results have been parametrized using a chemical scheme where a fraction of the HC(O)CO population promptly dissociates to HCO + CO, the remaining HC(O)CO either dissociates thermally or reacts with O2 to give CO2, CO, and regenerate OH. A maximum ΦOH of (0.38 ± 0.02) was observed at 212 K, independent of total pressure, suggesting that ∼60% of the HC(O)CO population promptly dissociates upon formation. Qualitatively similar behavior is observed at 250 K, with a maximum ΦOH of (0.31 ± 0.03); at 295 K, the maximum ΦOH decreased further to (0.29 ± 0.03). From the parametrization, an OH yield of ΦOH = 0.19 is calculated for 295 K and 1 atm of air. It is shown that the proposed mechanism is consistent with previous chamber studies. While the fits are robust, experimental evidence suggests that the system is influenced by chemical activation and cannot be fully described by thermal rate coefficients. The atmospheric implications of the measurements are briefly discussed.

Mechanism of the reaction of OH with alkynes in the presence of oxygen.

Previous work has shown that the branching ratio of the reaction of OH/C2H2/O2 to glyoxal and formic acid is dependent on oxygen fraction, and a significant component of the product yield under atmospheric conditions is formed from reaction of chemically activated OH-C2H2 adduct. In this article, isotopic substitution is used to determine the mechanism of the OH/C2H2/O2 reaction resolving previous contradictory observations in the literature. Using laser flash photolysis and probing OH concentrations via laser induced fluorescence, a rate coefficient of kHO-C2H2+O2 = (6.17 ± 0.68) × 10(-12) cm(3) molecule(-1) s(-1) is determined at 298 K from the analysis of biexponential OH decays in the presence of C2H2 and low concentrations of O2. The studies have been extended to propyne and but-2-yne. The reactions of OH with propyne and but-2-yne have been studied as a function of pressure in the absence of oxygen. The reaction of OH with propyne is in the fall off region from 2-25 Torr of nitrogen at room temperature. A pressure independent value of (4.21 ± 0.47) × 10(-12) cm(3) molecule(-1) s(-1) was obtained from averaging the eight independent measurements at 25 and 75 Torr. The reaction of OH with but-2-yne at 298 K is pressure independent (5-25 Torr N2) with a value of (1.87 ± 0.19) × 10(-11) cm(3) molecule(-1) s(-1). Analysis of biexpontial OH decays in alkyne/low O2 conditions gives the following rate coefficients at 298 K: kHO-C3H4+O2 = (8.00 ± 0.82) × 10(-12) cm(3) molecule(-1) s(-1) and kHO-C4H6+O2 = (6.45 ± 0.68) × 10(-12) cm(3) molecule(-1) s(-1). The branching ratio of bicarbonyl to organic acid in the presence of excess oxygen also shows an oxygen fraction dependence for propyne and but-2-yne, qualitatively similar to that for acetylene. For an oxygen fraction of 0.2 at 298 K, pressure independent yields of methylglyoxal (0.70 ± 0.03) and biacetyl (0.74 ± 0.03) were determined for the propyne and but-2-yne systems, respectively. The yield of acid increases with temperature from 212-500 K. Master equation calculations show that, under atmospheric conditions, the acetyl cofragment of organic acid production will dissociate, consistent with experimental observations.

The fast C(3P) + CH3OH reaction as an efficient loss process for gas-phase interstellar methanol

Rate constants for the C(3P) + CH3OH reaction have been measured in a continuous supersonic flow reactor over the range 50 K ≤ T ≤ 296 K. C(3P) was created by the in situ pulsed laser photolysis of CBr4, a multiphoton process which also produced some C(1D), allowing us to investigate simultaneously the low temperature kinetics of the C(1D) + CH3OH reaction. C(1D) atoms were followed by an indirect chemiluminescent tracer method in the presence of excess CH3OH. C(3P) atoms were detected by the same chemiluminescence technique and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF). Secondary measurements of product H(2S) atom formation have been undertaken allowing absolute H atom yields to be obtained by comparison with a suitable reference reaction. In parallel, statistical calculations have been performed based on ab initio calculations of the complexes, adducts and transition states (TSs) relevant to the title reaction. By comparison with the experimental H atom yields, the preferred reaction pathways could be determined, placing important constraints on the statistical calculations. The experimental and theoretical work are in excellent agreement, predicting a negative temperature dependence of the rate constant increasing from 2.2 × 10−11 cm3 molecule−1 s−1 at 296 K to 20.0 × 10−11 cm3 molecule−1 s−1 at 50 K. CH3 and HCO are found to be the major products under our experimental conditions. As this reaction is not considered in current astrochemical networks, its influence on interstellar methanol abundances is tested using a dense interstellar cloud model.

Global Uncertainty Propagation and Sensitivity Analysis in the CH3OCH2 + O2 System: Combining Experiment and Theory To Constrain Key Rate Coefficients in DME Combustion

Statistical rate theory calculations, in particular formulations of the chemical master equation, are widely used to calculate rate coefficients of interest in combustion environments as a function of temperature and pressure. However, despite the increasing accuracy of electronic structure calculations, small uncertainties in the input parameters for these master equation models can lead to relatively large uncertainties in the calculated rate coefficients. Master equation input parameters may be constrained further by using experimental data and the relationship between experiment and theory warrants further investigation. In this work, the CH3OCH2 + O2 system, of relevance to the combustion of dimethyl ether (DME), is used as an example and the input parameters for master equation calculations on this system are refined through fitting to experimental data. Complementing these fitting calculations, global sensitivity analysis is used to explore which input parameters are constrained by which experimental conditions, and which parameters need to be further constrained to accurately predict key elementary rate coefficients. Finally, uncertainties in the calculated rate coefficients are obtained using both correlated and uncorrelated distributions of input parameters.

Observation of a new channel, the production of CH3, in the abstraction reaction of OH radicals with acetaldehyde

Using laser flash photolysis coupled to photo-ionization time-of-flight mass spectrometry (PIMS), methyl radicals (CH3) have been detected as primary products from the reaction of OH radicals with acetaldehyde (ethanal, CH3CHO) with a yield of ∼15% at 1-2 Torr of helium bath gas. Supporting measurements based on laser induced fluorescence studies of OH recycling in the OH/CH3CHO/O2 system are consistent with the PIMS study. Master equation calculations suggest that the origin of the methyl radicals is from prompt dissociation of chemically activated acetyl products and hence is consistent with previous studies which have shown that abstraction, rather than addition/elimination, is the sole route for the OH + acetaldehyde reaction. However, the observation of a significant methyl product yield suggests that energy partitioning in the reaction is different from the typical early barrier mechanism where reaction exothermicity is channeled preferentially into the newly formed bond. The master equation calculations predict atmospheric yields of methyl radicals of ∼9%. The implications of the observations in atmospheric and combustion chemistry are briefly discussed.