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A Kinetic Study of the Gas-Phase O( 1 D) + CH3OH and O( 1 D) + CH3CN Reactions. Low Temperature Rate Constants and Atomic Hydrogen Product Yields

Atomic oxygen in its first excited singlet state, O(1 D), is an important species in the photochemistry of several planetary atmospheres and has been predicted to be a potentially important reactive species on interstellar ices. Here, we report the results of a kinetic study of the reactions of O(1 D) with methanol, CH3OH, and acetonitrile, CH3CN, over the 50-296 K temperature range. A continuous supersonic flow reactor was used to attain these low temperatures coupled with pulsed laser photolysis and pulsed laser induced fluorescence to generate and monitor O(1 D) atoms respectively. Secondary experiments examining the atomic hydrogen product channels of these reactions were also performed, through laser induced fluorescence measurements of H(2 S) atom formation. On the kinetics side, the rate constants for these reactions were seen to be large (>2 x 10-10 cm 3 s-1) and consistent with barrierless reactions, although they display contrasting dependences as a function of temperature. On the product formation side, both reactions are seen to yield non-negligible quantities of atomic hydrogen. For the O(1 D) + CH3OH reaction, the derived yields are in good agreement with the conclusions of previous experimental and theoretical work. For the O(1 D) + CH3CN reaction, whose H-atom formation channels had not previously been investigated, electronic structure calculations of several new product formation channels were performed to explain the observed H-atom yields. These calculations demonstrate the barrierless and exothermic nature of the relevant exit channels, confirming that atomic hydrogen is also an important product of the O(1 D) + CH3CN reaction.


Introduction
The reactions of electronically excited atomic oxygen, O( 1 D), have been extensively studied over the years due to their importance in Earth's atmospheric chemistry. In particular, ozone photolysis at UV wavelengths in the stratosphere produces high yields of O( 1 D) atoms that react rapidly with trace gases producing reactive radicals such as OH and H which play important roles in catalytic ozone destruction cycles in the stratosphere. Methanol, CH3OH, is the second most abundant organic molecule present in the atmosphere after CH4 and is thought to be emitted mostly during the growth and decay of plants in addition to anthropogenic sources such as solvent use, automobile emissions and industrial processes. 1 Acetonitrile, CH3CN, is also present in the atmosphere as a result of biomass burning and automobile emissions. In contrast to CH3OH, which has a short atmospheric lifetime of only a few days, 1 CH3CN has a relatively long lifetime of more than a year due to its low inherent reactivity, 2 allowing trace amounts of CH3CN to be transported to the stratosphere. Both produce COMs such as CH3OH, C2H5OH, CH3CHO and CH2(O)CH2. Astrochemical models are now beginning to incorporate such processes, 9,10 where O( 1 D) atoms are generated by the photodissociation/radiolysis of oxygen bearing molecules already present in the ices. As CH3OH typically accounts for 5 % of the ice composition (with respect to the major constituent H2O) 11 the reaction between O( 1 D) and CH3OH could represent an important source of COMs in interstellar chemistry. Although CH3CN has only been detected in the gas-phase ISM so far, 12,13,14 it is considered to be present on icy interstellar dust grains where it could contribute to the formation of nitrogen bearing COMs. 15,16 Although we investigate these processes in the gas-phase and not on ice surfaces, gas-phase studies provide insights into their surface reactivity (if the rate constants are slow, this could indicate the presence of a barrier to product formation) and the associated theoretical calculations and experimental product studies elucidate the intermediate species formed which are the likely primary products in the equivalent surface reaction.
Mechanistically at room temperature and below, O( 1 D) atoms react with organic molecules predominantly by insertion into C-H and O-H bonds, forming long-lived intermediates such as CH3OH during the O( 1 D) + CH4 reaction and H2O2 during the O( 1 D) + H2O reaction before dissociating. 17,18,19,20,21 When CH3OH is employed as the coreagent species, as a bifunctional molecule two possibilities present themselves for the insertion process. Here, O( 1 D) can insert into either one of the C-H bonds of the CH3 moiety or into the O-H bond itself, leading to the possible formation of two distinct intermediate species, HOCH2OH following C-H insertion or CH3OOH formation following O-H insertion. 22,23,24 While OH radicals are observed as the major product species, studies performed by selectively substituting H-atoms for D-atoms 22 To evaluate the importance of the O( 1 D) + CH3OH and O( 1 D) + CH3CN reactions in atmospheric and interstellar chemistry, an experimental kinetic study has been performed at room temperature and below using a supersonic flow reactor coupled with pulsed laser photolysis and laser-induced fluorescence for the production and detection of O( 1 D) atoms.
In addition, measurements of the temperature dependent H-atom product yields for the O( 1 D) + CH3OH reaction and the H-atom yield for the O( 1 D) + CH3CN reaction at room temperature have also been performed to elucidate the preferred product channels of these reactions. In addition to the experimental work, electronic structure calculations of the exit channels of the O( 1 D) + CH3CN reaction have also been undertaken. These calculations are complementary to those of a previous theoretical study of this reaction, 25 and have been performed to allow us to explain the experimental findings.
The paper is structured as follows: Section 2 describes the experimental methodology used to study these reactions, while Section 3 presents the experimental results and discusses them in the context of earlier work where available. Section 3 also provides a brief description of the supplementary theoretical calculations. The conclusions are presented in Section 4.

Experimental Methods
The experiments reported here were made using a supersonic flow type reactor (also known as CRESU -cinétique de réaction en écoulement supersonique uniforme or reaction kinetics in a uniform supersonic flow). A detailed description of this apparatus was provided in earlier publications. 26,27 The main feature of this system, the Laval nozzle, allows supersonic flows of a specified carrier gas to be generated in the reactor under vacuum. These flows, which are created by isentropic expansion through the nozzle possess uniform density, velocity and temperature profiles, providing suitable environments to study the kinetics of fast reactions at low temperature. The original apparatus as described has been modified over the years, to incorporate a tunable narrow-band VUV source for detection purposes. In this way, it has been possible to study the low temperature reactivity of several atomic radicals such as Ar, the present experiments were restricted to using argon type nozzles as the rate constants for O( 1 D) quenching by Ar are small enough (6-7 ´ 10 -13 cm 3 s -1 ) to allow kinetics measurements to be performed. 30,35 Indeed, N2 is known to rapidly quench O( 1 D) atoms at room temperature and below. 30,36 Three different nozzles were employed here, providing supersonic flows with characteristic temperatures of 127 K, 75 K and 50 K with Ar as the carrier gas. Other relevant properties for flows generated by these Laval nozzles are reported in Table 1 of Nuñez-Reyes and Hickson. 37 To increase the range of the measurements to higher temperatures, the Laval nozzle was removed from the system, thereby allowing experiments to be performed at room temperature (296 K).
As both of the coreagent molecules used here are liquids at room temperature, it was necessary to employ a reliable method to introduce a known quantity of their vapour into the supersonic flow. During these experiments, methanol vapour was added to the gas flow upstream of the Laval nozzle via a controlled evaporation mixing system (CEM). Here, liquid methanol was contained in a 1 litre stainless steel reservoir maintained at a positive pressure of 2 bar and at room temperature. This reservoir was connected to an evaporation device heated to 373 K via a liquid flow meter which allowed precise control of liquid methanol flows between 0.2 and 1.7 g hr -1 . Approximately 700 sccm (standard cubic centimetres per minute) of the Ar carrier gas flow was diverted into the evaporation device through a calibrated massflow controller, allowing the vaporized methanol to be carried into the reactor. The gas-phase methanol concentration was determined spectroscopically, by connecting the output of the CEM to a pressure controlled 10 cm long quartz cell at room temperature before entering the reactor. Methanol vapour concentrations were determined by absorption using the 185 nm line of a mercury pen-ray lamp and a solar blind channel photomultiplier tube (CPM) operating in photon counting mode. To ensure that only radiation from this specific mercury line was detected, an interference filter with a 10 nm FWHM transmission around 185 nm was placed in front of the CPM. To test for eventual fluctuations of the lamp and/or methanol vapour flows, the transmitted intensity was measured before, during and after each set of experiments at a specified methanol flow allowing the attenuated (I) and non-attenuated (I0) intensities to be determined. The pressure within the cell was generally maintained at a value around 420 Torr, ensuring an adequate level of methanol absorption. The room temperature absorption cross-section of methanol was taken to be 6.05 ´ 10 -19 cm 2 , 38 allowing its concentration to be calculated by the Beer-Lambert law. Methanol condensation between the absorption cell and the reactor was prevented by connecting the cell exit port to the reactor with a heated hose maintained at 353 K. Further condensation losses upstream of the Laval nozzle were neglected as the methanol vapour was diluted by at least a factor of five on entering this region, through mixing with the main Ar flow. CH3OH concentrations as high as 5.0 ´ 10 14 cm -3 were used during experiments at 296 K, while lower concentration ranges (£ 2.0 ´ 10 14 cm -3 ) were used below room temperature.
For the CH3CN experiments, acetonitrile vapour was introduced into the flow using a bubbler type method. Here, a small fraction of the Ar carrier gas was flowed into a bubbler containing CH3CN at room temperature. At the exit of the bubbler, the CH3CN vapour laden Ar was passed into a cold trap, held at 17 °C and a known pressure, before passing into the Laval nozzle reservoir through the same heated hose described above. It was not possible to determine the gas-phase CH3CN concentration spectroscopically during these experiments, as its absorption cross-section is negligible at 185 nm. Despite this, the bubbler arrangement described above ensured that CH3CN was maintained at its saturated vapour pressure value at 17 °C, 39 allowing the gas-phase CH3CN concentration in the supersonic flow to be calculated precisely.
In common with our earlier studies of the reactions of ground state atomic carbon with are unreactive with CH3OH and CH3CN at the temperatures of the present study. O3 was generated by the UV irradiation of molecular oxygen in a quartz cell upstream of the Laval nozzle reservoir using a mercury lamp. O2 molecules are photodissociated at these UV wavelengths, producing O( 3 P) atoms which undergo a termolecular association reaction with O2 to form O3. In order to increase the O3 production yield, the cell pressure was maintained close to 760 Torr by a valve at the exit. The output of the O3 generation cell was connected to an inlet port of the Laval nozzle reservoir while the valve and the connectors upstream of the O3 cell were made from plastic to limit O3 dissociation. O2 concentrations in the range (3-7) ´ 10 13 cm -3 were used during the present study. If we assume a 1% efficiency for O3 production In order to decrease the detection of scattered residual UV and VUV probe radiation by the detector, the tripling cell was not attached directly to the reactor. Instead, a 75 cm sidearm containing circular diaphragms was placed in between the two. In this way, most of the divergent UV radiation was blocked while the VUV probe beam was allowed to propagate into the reactor. Nevertheless, as this sidearm was open to the reactor it was always filled with residual gases. Attenuation of the VUV beam by absorption was reduced by continuously flushing the sidearm with Ar or N2. In this configuration, the VUV probe beam crossed the cold supersonic flow at 90 degrees, exciting unreacted O( 1 D) atoms (or product H( 2 S) atoms). The resonant emission was collected by a LiF lens at right angles to both the flow and the probe laser, focusing it onto a solar blind photomultiplier tube (PMT). To avoid damage of the lens and the PMT window by reactive gases, both of these were isolated from the reactor by a LiF window. This region was evacuated to prevent further detection losses through atmospheric absorption of the VUV radiation. The output signal was integrated over time by a boxcar system (a gate of 20-40 ns was typically used) and averaged over 30 laser shots for each time point. The delay between the photolysis and probe lasers was controlled by a digital delay generator, which also served to synchronize the acquisition electronics. At least 100 different time points were recorded to establish each decay curve, with at least 15 time points recorded with the probe laser firing before the photolysis laser to determine the baseline level. The Ar carrier gas (99.999%) and O2 precursor gas (99.999%) used to establish the supersonic flow during these experiments were regulated by calibrated mass-flow controllers, while flush gases N2 (99.999%) and Ar were flowed into the sidearm through a needle valve. All gases were flowed directly from cylinders without further purification while CH3OH and CH3CN solvents were degassed prior to use.

Rate Constants
The   During these experiments, all decay profiles were analysed using a function of the form where I(t) and I0 are the time dependent and initial LIF intensities respectively, k1st is the decay constant (the pseudo-first-order decay rate) and t is the time.  value, no attempt was made to study the C( 3 P) + CH3CN reaction below 75 K.
The derived second-order rate constants for both these reactions are summarized in Table 1 with other relevant information and are displayed as a function of temperature in Figure 4, alongside previous work.

H-atom Product Yields
In previous work on the reactions of C( 1 D) and C( 3 P) atoms with hydrogen bearing molecules, Here, X = CH3OH, CH3CN or H2 and !( ! #)%4 .and !( ! #)%56 are the rate constants for reaction with X and quenching by Ar respectively. The expression for IHmax can be written where  ± 0.03 at 296 K and 127 K respectively after the numerical correction is applied.
In the case of the O( 1 D) + CH3CN reaction, H-atom yield measurements were only performed at 296 K. Here, no correction was required to account for CH3CN photolysis at 266 nm, due to its negligibly small absorption cross section at this wavelength, although a maximum correction of 5 % was applied to account for VUV absorption by CH3CN. These experiments determined an average absolute H-atom yield of 0.183 ± 0.03 which is corrected to 0.177 ± 0.03. The corrected product branching ratios are summarized in Table 2.   25 In contrast, channel (4) is significantly less exothermic than channel (3) and those described by Sun et al. 25 and might therefore be less important than these pathways. Channel (5)

Conclusions
This paper presents the results of an experimental kinetic study of the reactions between atomic oxygen in its first excited state and the coreagents methanol and acetonitrile. A continuous supersonic flow reactor was used to perform the measurements, allowing these processes to be investigated at a range of temperatures between 50 and 296 K. O( 1 D) was generated by pulsed laser photolysis of O3 at 266 nm, while pulsed laser induced fluorescence was used to detect these atoms at 115.2 nm. The measured rate constants were seen to be large for both reactions with values greater than 2 ´ 10 -10 cm 3 s -1 at all temperatures. The O( 1 D) + CH3OH and O( 1 D) + CH3CN reactions are seen to display contrasting temperature dependences however. While rate constants for the former process were seen to increase with temperature, the measured rate constants for the latter process are mostly independent of temperature considering the overall experimental uncertainties. In addition to the kinetic study, the product channels were also investigated by comparing H-atom yields for both