CH3O Radicals Research Articles

Determination of the Branching Ratio of CH3OH + OH Reaction on Water Ice Surface at 10 K

The CH3O and CH2OH radicals can be important precursors of complex organic molecules (COMs) in interstellar dust. The COMs presumably originating from these radicals were abundantly found in various astronomical objects. Because each radical leads to different types of COMs, determining the abundance ratio of CH3O to CH2OH is crucial for a better understanding of the chemical evolution into various COMs. Recent work suggested that the reaction between CH3OH and OH on ice dust plays an important role in forming CH3O and CH2OH radicals. However, quantitative details on the abundance of these radicals have not been presented to date. Herein, we experimentally determined the branching ratio (CH3O/CH2OH) resulting from the CH3OH + OH reaction on the water ice surface at 10 K to be 4.3 ± 0.6. Furthermore, the CH3O product in the reaction would participate in subsequent diffusive reactions even at a temperature as low as 10 K. This fact should provide critical information for COMs formation models in cold molecular clouds.

A molecular investigation on the effects of OMEX addition on soot inception of diesel pyrolysis

This study aimed at revealing the atomic-level chemical pathways involved in the soot inception inhibition of diesel pyrolysis with oxymethylene dimethyl ether (OMEX) addition. Using the reactive force field parameters in molecular dynamics simulation, the results effectively identified the specific pathways of OME3 pyrolysis and soot inception, via the analysis of the conversion of three main species: CH2O, CH3O, and CH3, generated from the initial decomposition of OME3. It has been found that CH2O molecule would be converted to CO through continuous dehydrogenation, and the carbon atoms convertered into CO would not participate in forming soot precursors, thus reducing soot formation. The oxidizing groups (mainly OH) are primarily produced from the CH3O radicals. These oxidizing species would react with small gaseous soot precursors to form stable oxides, thus inhibiting soot formation. However, this influence is relatively small under conditions where no oxygen is involved. The CH3 radicals would participate in the formation of soot precursors. In this study, it was observed that the addition of OME3 did not reduce the number of main gaseous precursors of polycyclic aromatic hydrocarbons such as C2H2, C2H3, and C3H3 during diesel-OME3 co-pyrolysis, because there were reactions between CH3 radicals and C1 and C2 species to form C2 and C3 hydrocarbon products. Based on the simulation results, the process from diesel decomposition to incipient soot production under the effects of OME3 is panoramically demonstrated. It is also found that the chain length increase of OMEX with a fixed molar fraction does not influence soot inhibition noticeably. Increasing the proportion of OMEX added to diesel helps reduce soot as carbon atoms involved in the formation of soot precursors are reduced, and more oxidizing species would be produced to slow the formation of gaseous soot precursors.

A systematic mechanistic survey on the reactions between OH radical and CH3OH on ice.

A systematic mechanistic survey was performed for the CH3OH + OH reaction on ice. ONIOM(ωB97X-D/Def2-TZVP:AMOEBA09) calculations suggested a range of binding energies for the CH2OH radical (0.29-0.69 eV) and CH3OH (0.15-0.72 eV) molecule on hexagonal water ice (Ih) and amorphous solid water (ASW). Computed average binding energies of CH2OH radical (0.49 eV) and CH3OH (0.41 eV) are relatively stronger compared to the CH3O radical binding energies (0.32 eV, Sameera et al., J. Phy. Chem. A, 2021, 125, 387-393). Thus, the CH3OH molecule, CH2OH and CH3O radicals can adsorb on ice, where the binding energies follow the order CH2OH > CH3OH > CH3O. The multi-component artificial force-induced reaction (MC-AFIR) method systematically determined the reaction mechanisms for the CH3OH + OH reaction on ice, where two reaction paths, giving rise to CH2OH and CH3O radicals, were confirmed. A range of reaction barriers, employing the ωB97X-D/Def2-TZVP level of theory, was found for each reaction (0.03-0.11 eV for CH2OH radical formation, and 0.03-0.44 eV for CH3O radical formation). Based on the lowest energy reaction paths, we suspect that both reactions operate on ice. The computed data in this study evidence that the nature of the binding site or the reaction site has a significant effect on the computed binding energies or reaction barriers. Thus, the outcomes of the present study will be very useful for the computational astrochemistry community to determine reliable binding energies and reaction barriers on ice.

Modelling the Radical Chemistry on Ice Surfaces: An Integrated Quantum Chemical and Experimental Approach

Heterogeneous radical processes on ice surfaces play a vital role in the formation of building blocks of the biologically relevant molecules in space. Therefore, quantitative mechanistic details of the radical binding and radical reactions on ices are crucial in rationalizing the chemical evolution in the Universe. The radical chemistry on ice surfaces was explored at low temperatures by combining quantum chemical calculations and laboratory experiments. A range of binding energies was observed for OH, HCO, CH3, and CH3O radicals binding on ices. Computed reaction paths of the radical reactions on ices, OCS + H and PH3 + D, explained the experimentally observed products. In both radical reactions, quantum tunnelling plays a key role in achieving the reactions at low temperatures. Our findings give quantitative insights into radical chemistry on ice surfaces in interstellar space and the planetary atmospheres.

Methoxymethanol formation starting from CO hydrogenation

Context. Methoxymethanol (CH3OCH2OH) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. Like several other C-, O-, and H-containing complex organic molecules (COMs), this molecule is expected to form upon hydrogen addition and abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products. Aims. The goal of this work is to experimentally and theoretically investigate the most likely solid-state methoxymethanol reaction channel – the recombination of CH2OH and CH3O radicals – for dark interstellar cloud conditions and to compare the formation efficiency with that of other species that were shown to form along the CO-hydrogenation line. We also investigate an alternative hydrogenation channel starting from methyl formate. Methods. Hydrogen atoms and CO or H2CO molecules were co-deposited on top of predeposited H2O ice to mimic the conditions associated with the beginning of “rapid” CO freeze-out. The formation of simple species was monitored in situ using infrared spectroscopy. Quadrupole mass spectrometry was used to analyze the gas-phase COM composition following a temperature-programmed desorption. Monte Carlo simulations were used for an astrochemical model comparing the methoxymethanol formation efficiency with that of other COMs. Results. The laboratory identification of methoxymethanol is found to be challenging, in part because of diagnostic limitations, but possibly also because of low formation efficiencies. Nevertheless, unambiguous detection of newly formed methoxymethanol has been possible in both CO+H and H2CO+H experiments. The resulting abundance of methoxymethanol with respect to CH3OH is about 0.05, which is about six times lower than the value observed toward NGC 6334I and about three times lower than the value reported for IRAS 16293B. Astrochemical simulations predict a similar value for the methoxymethanol abundance with respect to CH3OH, with values ranging between 0.03 and 0.06. Conclusions. We find that methoxymethanol is formed by co-deposition of CO and H2CO with H atoms through the recombination of CH2OH and CH3O radicals. In both the experimental and modeling studies, it is found that the efficiency of this channel alone is not sufficient to explain the observed abundance of methoxymethanol with respect to methanol. The rate of a proposed alternative channel, the direct hydrogenation of methyl formate, is found to be even less efficient. These results suggest that our knowledge of the reaction network is incomplete or involving alternative solid-state or gas-phase formation mechanisms.

Methanol formation chemistry with revised reactions scheme

The aim of the presented work is to analyze the impact of experimentally evaluated reactions of hydrogen abstraction on surfaces of interstellar grains on the chemical evolution of methanol and its precursors on grains and in the gas phase under conditions of a cold dark cloud and during the collapse of a translucent cloud into a dark cloud. Analysis of simulation results shows that those reactions are highly efficient destruction channels for HCO and H2CO on grain surfaces, and significantly impact the abundances of almost all molecules participating in the formation of CH3OH. Next, in models with those reactions, maximum abundances of methanol in gas and on grain surfaces decrease by more than 2–3 orders of magnitude in comparison to models without surface abstraction reactions of hydrogen. Finally, we study the impact of binding energies of CH2OH and CH3O radicals on methanol chemistry.

Insights into the high-temperature oxidation of methylcyclohexane

Reactive molecular dynamics simulations were performed under different conditions in order to investigate in-detail the chemical events associated with high-temperature oxidation of methylcyclohexane (MCH). The corresponding kinetic behaviors of the major intermediates and products were systematically analyzed at the atomistic level. Thus the overall reaction scheme of MCH oxidation was established from the initial step to the final products. It was observed that the oxidation of MCH was mainly initiated by two kinds of reactions, including unimolecular decomposition and H abstraction, with the former being more important. In agreement with the available experimental results, C2H4, CH2O, CO, CO2 and H2O were found to be the major products during the oxidation process. The results revealed that CH3O2, CH3O and C3H5O radicals were the precursors for CH2O production, which was the key intermediate to generate CO. Additionally, C2H3O also had closed relationship with the formation of CO. For a better description of the combustion behavior, small oxides related to intermolecular reactions should be considered in the oxidation of MCH mechanisms. The temperature and density had a positive effect on the oxidation of MCH; it was also found that an increase of the equivalence ratio had a negligible effect on the MCH oxidation.

Assigning the low lying vibronic states of CH3O and CD3O.

The assignment of lines in vibrational spectra in strongly mixing systems is considered. Several low lying vibrational states of the ground electronic X∼2E state of the CH3O and CD3O radicals are assigned. Jahn-Teller, spin-orbit, and Fermi couplings mix the normal mode states. The mixing complicates the assignment of the infrared spectra using a zero-order normal mode representation. Alternative zero-order representations, which include specific Jahn-Teller couplings, are explored. These representations allow for definitive assignments. In many instances it is possible to plot the wavefunctions on which the assignments are based. The plots, which are shown in the adiabatic representation, allow one to visualize the effects of various higher order couplings. The plots also enable one to visualize the conical seam and its effect on the wavefunctions. The first and the second order Jahn-Teller couplings in the rocking motion dominate the spectral features in CH3O, while first order and modulated first order couplings dominate the spectral features in CD3O. The methods described here are general and can be applied to other Jahn-Teller systems.

Photodissociation of the CH3O and CH3S radical molecules: an ab initio electronic structure study.

The electronic states and the spin-orbit couplings between them involved in the photodissociation process of the radical molecules CH3X, CH3X → CH3 + X (X = O, S), taking place after the Ã(2A1) ← X[combining tilde](2E) transition, have been investigated using highly correlated ab initio techniques. A two-dimensional representation of both the potential-energy surfaces (PESs) and the couplings is generated. This description includes the C-X dissociative mode and the CH3 umbrella mode. Spin-orbit effects are found to play a relevant role in the shape of the excited state potential-energy surfaces, particularly in the CH3S case where the spin-orbit couplings are more than twice more intense than in CH3O. The potential surfaces and couplings reported here for the present set of electronic states allow for the first complete description of the above photodissociation process. The different photodissociation mechanisms are analyzed and discussed in light of the results obtained.

The origin of gas-phase HCO and CH3O radicals in prestellar cores

The recent unexpected detection of terrestrial complex organic molecules in the cold (~ 10 K) gas has cast doubts on the commonly accepted formation mechanisms of these species. Standard gas-phase mechanisms are inefficient and tend to underproduce these molecules, and many of the key reactions involved are unconstrained. Grain-surface mechanisms, which were presented as a viable alternative, suffer from the fact that they rely on grain surface diffusion of heavy radicals, which is not possible thermally at very low temperatures. One of the simplest terrestrial complex organic molecules, methanol is believed to form on cold grain surfaces following from successive H atom additions on CO. Unlike heavier species, H atoms are very mobile on grain surfaces even at 10 K. Intermediate species involved in grain surface methanol formation by CO hydrogenation are the radicals HCO and CH3O, as well as the stable species formaldehyde H2CO. These radicals are thought to be precursors of complex organic molecules on grain surfaces. We present new observations of the HCO and CH3O radicals in a sample of prestellar cores and carry out an analysis of the abundances of the species HCO, H2CO, CH3O, and CH3OH, which represent the various stages of grain-surface hydrogenation of CO to CH3OH. The abundance ratios between the various intermediate species in the hydrogenation reaction of CO on grains are similar in all sources of our sample, HCO:H2CO:CH3O:CH3OH ~ 10:100:1:100. We argue that these ratios may not be representative of the primordial abundances on the grains but, rather, suggest that the radicals HCO and CH3O are gas-phase products of the precursors H2CO and CH3OH, respectively. Gas-phase pathways are considered and simple estimates of HCO and CH3O abundances are compared to the observations. Critical reaction rate constants, branching ratios, and intermediate species are finally identified.

Ab Initio Chemical Kinetics for the CH3 + O((3)P) Reaction and Related Isomerization-Decomposition of CH3O and CH2OH Radicals.

The kinetics and mechanism of the CH3 + O reaction and related isomerization-decomposition of CH3O and CH2OH radicals have been studied by ab initio molecular orbital theory based on the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVTZ, CCSD/aug-cc-pVDZ, and G2M//B3LYP/6-311+G(3df,2p) levels of theory. The predicted potential energy surface of the CH3 + O reaction shows that the CHO + H2 products can be directly generated from CH3O by the TS3 → LM1 → TS7 → LM2 → TS4 path, in which both LM1 and LM2 are very loose and TS7 is roaming-like. The result for the CH2O + H reaction shows that there are three low-energy barrier processes including CH2O + H → CHO + H2 via H-abstraction and CH2O + H → CH2OH and CH2O + H → CH3O by addition reactions. The predicted enthalpies of formation of the CH2OH and CH3O radicals at 0 K are in good agreement with available experimental data. Furthermore, the rate constants for the forward and some key reverse reactions have been predicted at 200-3000 K under various pressures. Based on the new reaction pathway for CH3 + O, the rate constants for the CH2O + H and CHO + H2 reactions were predicted with the microcanonical variational transition-state/Rice-Ramsperger-Kassel-Marcus (VTST/RRKM) theory. The predicted total and individual product branching ratios (i.e., CO versus CH2O) are in good agreement with experimental data. The rate constant for the hydrogen abstraction reaction of CH2O + H has been calculated by the canonical variational transition-state theory with quantum tunneling and small-curvature corrections to be k(CH2O + H → CHO + H2) = 2.28 × 10(-19) T(2.65) exp(-766.5/T) cm(3) molecule(-1) s(-1) for the 200-3000 K temperature range. The rate constants for the addition giving CH3O and CH2OH and the decomposition of the two radicals have been calculated by the microcanonical RRKM theory with the time-dependent master equation solution of the multiple quantum well system in the 200-3000 K temperature range at 1 Torr to 100 atm. The predicted rate constants are in good agreement with most of the available data.

Infrared absorption of CH3O and CD3O radicals isolated in solid para-H2

Irradiation of a p-H2 matrix containing methyl nitrite (CH3ONO) at 3.2K with laser light at 355nm yielded vibrational fundamental absorption lines at 2773.1/2804.5 (1+ mixed with background states), 2866.5/2887.5?/2967.0? (1− mixed with 4+ and 22−), 1365.7 (2+), 1411.3 (2−), 1041.8 (3+), 2901.3?/2917.4/2982.2 (4− mixed with background states), 2940.5 (4+), 1346.8 (5+), 1427.2 (5−), 1519.9 (5+), 1523.0 (5−), 689.3/694.4 (6+), 945.9/951.7 (6−), 1224.6/1228.9 (6+), and 1233.7/1239.1 (6−)cm−1 that are assigned to the methoxy (CH3O) radical; the numbers in parentheses indicate vibrational normal modes, the superscript + and − indicate the spin-orbit components 2E3/2 and 2E1/2, respectively, and the “?” marks indicate tentative assignments. Experiments with CD3ONO yielded lines of CD3O at 2012.4? (1−), 1005.9 (3+), 1037.3 (3−), 2168.8 (4+), 2174.9 (4+), 2202.8 (4−), 2212.3 (4−), 974.3 (5+), 1185.2 (5+), 1224.8 (5−), 529.6/531.3 (6+), 728.3/731.3 (6−), 912.4 (6+), and 923.2 (6−)cm−1. Many additional lines associated with combination or overtone modes were also observed. The assignments are based on the photolytic behavior and comparisons of observed vibrational wavenumbers and IR intensities with those predicted with a fitted CCSD(T)/cc-pVTZ force field; deviations between experiments and fitted theoretical results are within 2% with an average value (0.4±0.4)%. These newly observed spectra show vibrational wavenumbers of CH3O consistent with most vibrational energies derived from stimulated-emission-pumping experiments on gaseous CH3O and those of CD3O are consistent with some lines derived from laser-induced fluorescence. Intensities of these lines of CH3O diminished with a rate coefficient (7.6±0.3)×10−3s−1 after irradiation, likely due to the tunneling reaction of CH3O with p-H2 to form CH2OH.

Photodissociation of CH3OCl to CH3O + Cl at 248 nm

This study examines the 248 nm photodissociation of methyl hypochlorite (CH3OCl), a molecule that serves as an atmospheric chlorine reservoir. The data show that the primary photodissociation channel is cleavage of the O−Cl bond to produce Cl atoms and CH3O radicals, a result consistent with the direct dissociation mechanism found by other computational and experimental studies of alkyl hypohalites. Photofragment translational spectroscopy with a crossed laser-molecular beam apparatus, coupled with tunable VUV photoionization detection, identified the momentum-matched products at m/e = 35 (Cl+ from Cl atoms) and m/e = 29 (the CHO+ daughter ion of CH3O). Products were formed with a narrow range of recoil kinetic energies, peaking at 48 kcal/mol in the center-of-mass reference frame, with a full-width half-maximum of 4 ± 2 kcal/mol. This kinetic energy distribution shows that the CH3O products are formed with a very narrow range of internal energies, and a simple model shows that to conserve angular momentu...

Theoretical investigation of the structures and properties of fluoromethyl peroxyl radicals

AbstractThe equilibrium geometries, harmonic vibrational frequencies, charge distributions, spin density distributions, dipole moments, electron affinities (EAs), and CO bond dissociation energies (BDEs) of HO, CH3O, CH2FO, CHF2O, and CF3O peroxyl radicals have been calculated using ab initio molecular orbital theory and density functional theory (DFT) at the B3LYP level. The CH bond dissociation energies of the parent fluoromethanes have been calculated using the same levels of theory. Both the MP2(full) and B3LYP methods, using the 6‐31G(d,p) basis set, are found to be capable of accurately predicting the geometries of peroxyl radicals. Electron correlation accounts for ∼25% of the CH BDE of fluoromethanes and for ∼50% of the CO BDE of the corresponding peroxyl radicals. The B3LYP/6‐31G(d,p) method is found to be comparable to high ab initio levels in predicting CO BDEs of studied peroxyl radicals and CH BDEs of the parent alkanes. The progressive fluorine substitution of hydrogen atoms in methyl peroxyl radicals results in shortening of the CO bond, lengthening of the OO bond, an increase (decrease) of the spin density on the terminal (inner) oxygen, a decrease in the dipole moments, and an increase in electron affinities. Both CO BDEs and EAs of peroxyl radicals (RO) correlate well with Taft σ* substituent constants for the R group in peroxyl radicals. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Formation of Methyl Nitrite and Methyl Nitrate during Plasma Treatment of Diesel Exhaust

FTIR spectroscopy was used to identify CH3ONO and CH3ONO2 as products of the nonthermal plasma treatment of simulated diesel exhaust. This is the first observation of CH3ONO formation in such systems. The yield of CH3ONO relativeto CH3ONO2 scaled linearly with the average [NO]/ [NO2] ratio in the system. A plot of [CH3ONO]/[CH3ONO2] versus [NO]/[NO2] gives a slope of 1.81 +/- 0.30. This result is indistinguishable from the literature value of the rate constant ratio k(CH3O + NO)/k(CH3O + NO2) = (2.6 x 10(-11))/ (1.5 x 10(-11)) = 1.73 +/- 0.37. The experimental observations suggest that reactions of CH3O radicals with NO and NO2 are the sources of CH3ONO and CH3ONO2 in such systems. The linear relationship between the yields of CH3ONO and CH3ONO2 provides a means of estimating the yield of these compounds during nonthermal plasma treatment of diesel exhaust.

CH3O Yield in the CH3+ O3Reaction Using the LP/LIF Technique at Room Temperature

CH3O(X2E) radicals have been detected by laser-induced fluorescence (LIF) for the first time as a product of the reaction between CH3 and O3. CH3 radicals were generated by excimer-laser photolysis of CH3Br at 193 nm in the presence of O3 at room temperature and 100 Torr of He. The rate constant for reaction 1a (CH3 + O3 → CH3O + O2) has been determined by monitoring the CH3O build-up as a function of reaction time and k1a was found to be (9.68 ± 1.10) × 10-14 cm3 molecule-1 s-1. The rate coefficient of the global reaction 1 (CH3 + O3 → products) has been determined by the numerical analysis of the CH3O temporal profiles, yielding a value of k1 = (2.2 ± 0.3) × 10-12 cm3 molecule-1 s-1. Reaction of CH3 with NO2 has also been studied under the same experimental conditions as reaction 1 and has been used to calibrate the CH3O LIF signal. On the other hand, the rate constant k2 for reaction 2 (CH3O + O3 → products) has been directly determined (k2(T = 298 K) = (2.53 ± 0.75) × 10-14 cm3 molecule-1 s-1) using t...

Rate Constants for the Reactions of CH3O with Cyclohexane, Cyclohexene, and 1,4-Cyclohexadiene: Variable Temperature Experiments and Theoretical Comparison of Addition and H-Abstraction Channels

First kinetic measurements for CH3O reactions have been obtained for three cyclohydrocarbons using the discharge flow reactor combined with the laser induced fluorescence technique to detect CH3O radicals over the pressure and temperature ranges 1−7 Torr of helium and 300−513 K. Measurements have been performed for the cyclohydrocarbons c-C6H12 (k1), c-C6H10 (k2), and 1,4-c-C6H8 (k3). In addition to the experimental work, we have performed ab initio molecular orbital computations to get an insight into the mechanism of the three reactions, using PMP2, HF-DFT (B3LYP), and CASPT2 methods. The rate constant k1 has been calculated using the Transition State Theory. Measured rate constants are pressure independent in our experimental range. Arrhenius expressions are (ki in cm3 s-1) k1 = 8.8 × 10-12 exp[−(24.5 ± 3.0) kJ mol-1/RT] (403−513 K), k2 = (3.1 ± 0.8) × 10-12 exp[−(15.3 ± 0.8) kJ mol-1/RT] (300−503 K), and k3 = 1.9 × 10-12 exp[−(7.6 ± 1.9) kJ mol-1/RT] (300−513 K). A good agreement between the experimental and theoretical rate constants k1 has been found. The comparison between the computed and the experimentally determined barrier heights serves as an endorsement of the increasing reactivity in the series from cyclohexane system to the 1,4-cyclohexadiene system.

An ab initio and density functional theory study of the structure and vibrational frequencies of the CH2FO[rad] and CHF2O[rad] radicals

Atmospheric degradation of fluorinated ether compounds, which are being considered as replacements for the current generation of freon substitutes, and is expected to produce the CH2FO and CHF2O radicals, and the latter radical is believed to be formed in the course of flame suppression by halons. In the present study, the molecular structures of the two radicals are determined using one density functional and two correlated ab initio methods. Structures were determined using Becke's three-parameter exchange functional with non-local correlation from the Lee, Yang, and Parr functional (the B3LYP functional) and second order unrestricted Moller–Plesset (UMP2) theory. Basis sets ranging from 631G(d) to 6311G(3df,2pd) were employed. The quadratic configuration interaction method with singles, doubles, and a perturbative treatment of the triples excitations (QCISD(T)) was employed at a more restricted range of basis sets. The C–O bond length is somewhat sensitive to the level of theory employed but somewhat less dependent on the basis set employed. Comparison is made to structural features of the CH3O and CF3O radicals. Vibrational frequencies were determined at B3LYP for basis sets up to 6-311G(2df,2p).

Rate constants for the reactions ofCH3O with CH2O, CH3CHO and i-C4H10

The rate constants for the reactions of CH3O radicals with 3 reactants have been measured by two different techniques: laser photolysis (LP) and fast flow reactor (FF), both coupled with a detection of CH3O radicals by laser induced fluorescence. The reaction with formaldehyde CH2O has been measured in the temperature range 295 - 450 K. Both sets of results (LP and FF) are in excellent agreement and lead to a pressure independent rate constant of k1=(1.1±0.3)×10-12exp (9.6±1.0 kJ mol-1/RT) cm3 s-1. The reaction with acetaldehyde CH3CHO has been measured between 286 and 493 K. Both sets of experiments are again in excellent agreement and lead to a pressure independent rate constant of k2=(5.7±1.3)×10-13exp (-5.2±0.7 kJ mol-1/RT) cm3 s-1. No reaction could be detected with isobutane i-C4H10 at temperature up to 356 K, leading to an upper limit for k3≤2.5×10-15 cm3 s-1.

Determination of Rate Constants and Product Branching Ratios for the Reactions of CH3O2 and CH3O with Cl Atoms at Room Temperature

AbstractReactions of CH3O2 and of CH3O radicals with Cl atoms have been investigated at 300 K using a discharge flow system with LIF and mass‐spectrometric detection. Rate constants k1 = (1.15±0.3)·10−10 and k2 = (1.00±0.2)·10−10, in units cm3 molecule−1 s−1 have been obtained observing pseudo‐first‐order decays of CH3O2 and CH3O radicals, respectively. The branching ratio k1a/k1 = 0.51 ±0.09 has been found from the determination of OCl yields. Formation of the Criegee intermediate CH2O2 in reaction (1b) was inferred from CO2 generation. A lower limit of 0.8±0.5 was derived for the CO2 yield from unimolecular CH2O2 decomposition.Possible reaction pathways for steps (1a) and (1b) are suggested on the basis of ab initio model calculations. The calculations include the estimation of structures and energies of intermediate CH3O2Cl collision complexes and the decomposition of CH3OOCl to yield products.