Kinetics of the Gas-Phase Reactions of syn- and anti-CH3CHOO Criegee Intermediate Conformers with SO2 as a Function of Temperature and Pressure

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/O2/N2/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 ∼298 K. Results indicate a rate coefficient of (4.80 ± 0.46) × 10–11 cm3 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) × 10–10 cm3 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.


■ INTRODUCTION
The chemistry of Criegee intermediates (R 2 COO) exerts potential impacts on air quality and climate through their involvement in atmospheric oxidation processes, and there has been considerable interest in the potential production of sulfate aerosols resulting from the reactions of Criegee intermediates with sulfur dioxide (SO 2 ) in the gas phase.Production of Criegee intermediates in the atmosphere occurs following the oxidation of unsaturated volatile organic compounds (VOCs) by ozone (O 3 ), with the Criegee intermediate initially produced with high internal energy. 1 Collisional stabilization of the nascent Criegee intermediate occurs in competition with unimolecular decomposition, leading to the production of stabilized Criegee intermediates (SCIs), which can participate in a range of processes, including reactions with SO 2 . 1,2or the reaction of the simplest SCI, CH 2 OO, with SO 2 , there is now general consensus regarding the kinetics at room temperature, with a current IUPAC recommendation of (3.7 −0.40 +0.45 ) × 10 −11 cm 3 s −1 at 298 K. 3 The kinetics of CH 2 OO + SO 2 have been demonstrated to be independent of pressure under typical atmospheric conditions, 4,5 with a negative temperature dependence 6 and reaction products dominated by formaldehyde (HCHO) 4,7 and sulfur trioxide (SO 3 ). 8,9−15 In the case of CH 2 OO + SO 2 , there is negligible stabilization of the SOZ under atmospheric conditions, 11,12 and contributions from two stereochemical pathways lead to the production of HCHO + SO 3 via submerged barriers. 12For reactions of other SCIs with SO 2 , there is greater potential for stabilization of the SOZ, 11 leading to the potential for pressure-dependent kinetics and product yields. 11−19 Measurements in a boreal forest in Finland led to the suggestion that reactions of SCIs with SO 2 may have been responsible for up to 50% of the H 2 SO 4 observed in the gas phase, 16,17 while observations in a rural location in Germany have indicated that SCI + SO 2 reactions could be responsible for up to 80% of H 2 SO 4 produced at night. 17Similarly, field experiments in Texas, United States, have suggested that nighttime production of H 2 SO 4 is dominated by SCI + SO 2 reactions, with potentially important contributions in the afternoon, 18 and agreement between observations and model predictions for sulfate aerosol over the Southeast of the US has shown improvement when SCI + SO 2 chemistry is included in the model. 20The role of SCI + SO 2 chemistry in the atmosphere has also been investigated using observations made on Corsica, in the Mediterranean, where it was found that SCI + SO 2 reactions could be responsible for 10% of the observed H 2 SO 4 production during the day and 40% at night. 19Potential impacts of SCI reactions with SO 2 have also been reported in vehicle exhausts 21 and power plant plumes. 226][17][18][19][20]23 The SCI CH 3 CHOO exists in two conformers, syn-CH 3 CHOO and anti-CH 3 CHOO, which are separated by a significant barrier to interconversion (∼160 kJ mol −1 ) 24 and thus behave as distinct species under ambient conditions. 25,26he first direct measurements of the reaction kinetics of CH 3 CHOO conformers, made using laser flash photolysis of CH 3 CHI 2 in the presence of excess O 2 coupled with tunable synchrotron photoionization mass spectrometry (PIMS), demonstrated rapid reactions with SO 2 (R1 and R2). 25 + syn CH CHOO SO products 3 2 (R1) The PIMS experiments were performed at 298 K and a total pressure of 4 Torr in He, giving k 1 = (2.4 ± 0.3) × 10 −11 cm 3 s −1 and k 2 = (6.7 ± 1.0) × 10 −11 cm 3 s −1 .Formation of SO 3 was observed, with a rate that suggested direct production from reactions of CH 3 CHOO conformers with SO 2 .Subsequent experiments using the PIMS technique at a fixed ionization energy gave a value for k 1 of (1.7 ± 0.3) × 10 −11 cm 3 s −1 at 295 K and total pressures between 1 and 2.5 Torr in N 2 , with measurements indicating production of acetaldehyde (CH 3 CHO) from R1 at a yield of (0.86 ± 0.11) at 2 Torr. 5 The reaction of syn-CH 3 CHOO with SO 2 has also been investigated by monitoring the kinetics of OH radical production from the decomposition of syn-CH 3 CHOO occurring in competition with R1, giving k 1 = (2.5 ± 0.2) × 10 −11 cm 3 s −1 at 298 K and 10 Torr in Ar. 27 Experiments using laser flash photolysis of CH 3 CHI 2 /O 2 mixtures with broadband UV absorption spectroscopy have also indicated that R1 and R2 are rapid. 26,28A rate coefficient of (2.0 ± 0.3) × 10 −11 cm 3 s −1 at 295 K and pressures between 7.5 and 500 Torr of N 2 was reported from experiments in which the conformer-specific contributions to the total absorbance were not resolved. 28However, the result is expected to be dominated by syn-CH 3 CHOO on the basis of results from the earlier PIMS experiments, 25 which indicated that syn-CH 3 CHOO represents 90% of the total CH 3 CHOO produced using the photolytic method.Conformer-specific measurements using broadband UV absorption spectroscopy have been achieved in experiments performed at 293 K and a total pressure of 10 Torr in He, giving k 1 = (2.9 ± 0.3) × 10 −11 cm 3 s −1 and k 2 = (2.2 ± 0.2) × 10 −10 cm 3 s −1 . 26The conformer-specific UV experiments indicated that syn-CH 3 CHOO is the dominant conformer produced, 26 in agreement with the earlier PIMS experiments, 25 although a lower yield of 70% was reported, which may result from the different experimental conditions or uncertainties in the UV absorption cross-sections, particularly for anti-CH 3 CHOO. 26here are discrepancies in the literature for values of k 1 and k 2 , but studies so far have all taken place at room temperature over a relatively narrow range of pressures (Table 1).Significant conformer dependence is shown for the reactivity of CH 3 CHOO with SO 2 , 25,26 with studies also showing distinct conformer-dependent reactivity for reactions of asymmetric CIs with H 2 O 25,26,29 and acids, 30 as well as differences in their decomposition rates. 31CH 3 CHOO is the simplest Criegee intermediate that exists as two conformers and can therefore be used as a prototype to characterize the reactions of the larger CIs, which requires rate coefficients to be well established across a range of conditions.In this work, we report the kinetics of R1 and R2 at temperatures between 242 and 353 K and pressures between 10 and 600 Torr determined using time-resolved broadband UV absorption spectroscopy.

■ EXPERIMENTAL SECTION
The kinetics of R1 and R2 were studied as a function of temperature and pressure using laser flash photolysis of CH 3 CHI 2 /O 2 /N 2 /SO 2 mixtures coupled with time-resolved broadband UV absorption spectroscopy.The experimental apparatus has been described in detail in previous work, 6,31 and only a brief description is given here.
A dilute mixture of a known concentration of SO 2 (Sigma-Aldrich, 99.9%) was prepared manometrically in N 2 (BOC, 99.998%) and stored in a glass bulb before mixing in a gas manifold with N 2 (BOC, 99.998%) and O 2 (BOC, 99.5%) at known flow rates controlled by calibrated mass flow controllers (MKS Instruments).A known fraction of the total gas flow,  The reaction cell was 100 cm in length and 3 cm in diameter and sealed with fused silica windows at each end.The temperature of the cell was controlled by flowing liquid from a recirculating thermostatting unit (Huber Unistat 360) through the jacket surrounding the cell and calibrated by measuring the temperature of a flow of N 2 , under conditions identical to those used in kinetics experiments, at 5 cm increments along the length of the cell using a K-type thermocouple. 6,31Pressure in the cell was controlled by a rotary pump (EM2, Edwards) by throttling the exit to the reaction cell and measured by a capacitance manometer (MKS Instruments).The total flow rate through the cell was set to an equivalent of 1200 standard cm 3 min -1 (sccm) at 50 Torr and adjusted with pressure to maintain a constant residence time in the cell of ∼2.6 s.
An excimer laser (KrF, Lambda-Physik CompEx 210) with output at λ = 248 nm and typical fluence of 30−40 mJ cm −2 was aligned along the length of the reaction cell using a dichroic turning mirror (Edmund Optics) and used to initiate production of syn-and anti-CH 3 CHOO in the cell via reactions R3 and R4a.
A delay generator (SRS DG535) was used to control the timing of the laser, which was operated with a pulse repetition frequency of 0.33 Hz to ensure that the gas mixture in the cell was replaced between each laser pulse.
Absorbing species in the cell were monitored by UV/visible radiation provided by a laser-driven light source (LDLS, Energetiq EQ-99X), which provided ∼10 mW cm −2 of light with a near constant radiance from 200 to 800 nm.The LDLS output was collimated by an off-axis parabolic mirror (ThorLabs) and aligned through the reaction cell in a multipass arrangement consisting of ten mirrors (Knight Optical), each of 12 mm diameter, resulting in an effective path length of (595 ± 53) cm for the experiments described in this work, which was determined using the method in our earlier work. 6Light exiting the cell was passed through a sharp cut-on filter (248 nm RazorEdge ultrasteep long-pass edge filter) to reduce the impact of scattered 248 nm light and focused into a fiber optic via a fiber launcher (Elliot Scientific).Light exiting the fiber optic was directed through a 25 μm slit onto a diffraction grating with 600 grooves/mm and imaged onto a thermoelectrically cooled charge-coupled device (CCD) detector (FER-SCI-1024BRX, Princeton Instruments).Photocharge generated on the CCD was shifted from an illuminated region to a storage region shielded from incoming radiation at set time intervals throughout the reaction, with the experimental setup used in this work giving a spectral resolution of ∼1 nm and a temporal resolution between 70 and 100 μs.Intensity data were typically recorded for 500 photolysis shots and transferred to a PC for analysis.

■ RESULTS
Absorbance spectra were determined from measured intensity data and related to the concentration of each species present using the Beer−Lambert law (eq 1) i k j j j j j y where A λ,t is the total absorbance at wavelength λ and time t, I λ,0 is the average pre-photolysis light intensity at wavelength λ, I λ,t is the post-photolysis light intensity at wavelength λ and time t, σ i,λ is absorption cross-section of species i at wavelength λ, c i,t is the concentration of species i at time t, and l is the effective path length, which has a value of (595 ± 53) cm for experiments reported in this work.
Figure 1 shows the typical absorbance measured following photolysis, which contains contributions from CH 3 CHI 2 , synand anti-CH 3 CHOO, and IO radicals produced by secondary chemistry within the system.Reference spectra for CH 3 CHI 2 , 32 syn-and anti-CH 3 CHOO, 26 and IO 33 were fit to the observed absorbance at each time point to determine the concentration of each species throughout the reaction.While absolute concentrations are reported here, it should be noted that uncertainties in the effective path length and absorption cross-sections do not contribute to uncertainties in measured kinetics for the pseudo-first-order conditions employed in this work.
Figure 2 shows the concentration−time profiles for syn-and anti-CH 3 CHOO in the presence of SO 2 , which were each fit according to a first-order kinetic loss (eq 2) convoluted with a Gaussian instrument response function (IRF) to describe the

The Journal of Physical Chemistry A
shifting of photocharge on the CCD detector (see the Supporting Information for further details).
where C t is the concentration of syn-or anti-CH  31 with potential additional contributions from reactions with iodine atoms, IO, or Criegee−Criegee chemistry as well as diffusion out of the probe region.
Rate coefficients k 1 and k 2 were determined from the dependence of k 1 ′ and k 2 ′ on [SO 2 ], respectively, with the typical results shown in Figure 3. Potential impacts of secondorder losses for the CH 3 CHOO conformer through reactions such as CH 3 CHOO + CH 3 CHOO or CH 3 CHOO + I were also investigated by fitting concentration−time profiles to mixed first-and second-order kinetic losses convoluted with the IRF.Results for k 1 and k 2 obtained from the mixed-order fits were within 5% of those obtained from the first-order fits.Further details are given in the Supporting Information.All results reported here were obtained from first-order fits (eq 2).
Figure 4 shows the results for k 1 , which are summarized in Table 2.At 298 K, the results demonstrate an increase in k 1 from (3.02 ± 0.32) × 10 −11 cm 3 s −1 at 10 Torr to (4.66 ± 0.52) × 10 −11 cm 3 s −1 at 600 Torr, where the uncertainties represent a combination of the statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of SO 2 , with results at other

The Journal of Physical Chemistry A
temperatures also showing significant pressure dependence and overall negative temperature dependence.Equations 3−6, which describe a chemical activation mechanism with a nonzero rate coefficient at zero pressure, 34 were fit globally to results obtained in this work for k 1 over all temperatures and pressures to provide a parametrization for use in atmospheric models.
where k int represents the rate coefficient at zero pressure, k 0 is the low-pressure limiting rate coefficient, and k ∞ is the highpressure limiting rate coefficient, and these are given by eqs 4−6  4. While kinetics reported by Smith et al. 28 at 295 K over the pressure range of 7.5−500 Torr are in broad agreement with low pressure values for k 1 reported in this work and in previous work, Smith et al. were unable to distinguish between the syn-and anti-conformers, so the rate coefficient reported will contain contributions from the reactivity of both syn-CH 3 CHOO and anti-CH 3 CHOO.
Figure 5 and Table 2 summarize the results obtained in this work for k 2 .In contrast to the results for k 1 , no significant dependence of k 2 on the temperature or pressure was observed.At 298 K, results gave a mean value for k 2 of (1.15 ± 0.16) × 10 −10 cm 3 s −1 between 10 and 600 Torr, with results over all temperatures and pressures giving a mean value of (1.18 ± 0.21) × 10 −10 cm 3 s −1 .The effect of pressure on k 2 at each temperature is shown in Figure S3 in the Supporting Information.
The kinetics of R2 have been reported in two previous studies 25,26 at room temperature.Taatjes et al. 25 performed experiments at 4 Torr using the PIMS technique and reported a value for k 2 of (6.7 ± 1.0) × 10 −11 cm 3 s −1 , while Sheps et al. 26 performed experiments at 10 Torr using cavity-enhanced UV absorption spectroscopy and reported a value for k 2 of (2.2 ± 0.2) × 10 −10 cm 3 s −1 .Differences between the studies reflect the challenges associated with measuring such rapid kinetics, with the lack of dependence of k 2 on temperature and pressure observed in this work potentially indicating that the kinetics for R2 are controlled by collision-limited or capture-limited kinetics.The difference in behavior between the syn-and anti-conformers is potentially influenced by lower steric hindrance for the anti-conformer, coupled with the higher ground state energy for anti-CH 3 CHOO by ∼15 kJ mol −124 compared to syn-CH 3 CHOO and a higher dipole moment for anti-CH 3 CHOO than syn-CH 3 CHOO (5.53 D compared to 4.69 D, calculated at the B3LYP/AVTZ level of theory 35 ).
Figure 6 compares the experimental results for k 2 with estimated values using a collision model (eq 7) and a capture model (eq 8).
where r CI and SOd 2 are the effective radii of anti-CH 3 CHOO 36 and SO 2, 37 respectively, k B is the Boltzmann constant, T is the temperature, and μ is the reduced mass.The effective radius for anti-CH 3 CHOO was assumed to be the same as that reported in the literature for syn-CH 3 CHOO. 36gure 4. Effects of pressure on k 1 at (a) 298 K and (b) all temperatures studied in this work.Solid lines show the fits to k 1 using eqs 3−6 (which were performed globally using all data for k 1 measured in this work).−27 Error bars represent a combination of the statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of SO 2 .

The Journal of Physical Chemistry
where C is a constant (4.08 for the case of isotropic capture) 35,38 and D CI and D SOd 2 are the dipole moments of anti-CH 3 CHOO 35 and SO 2 , 39 respectively.The experimental results for k 2 obtained in this work are lower than the estimated rate coefficients using either the collision model or the capture model, with experimental values a factor of ∼2 lower than those calculated from collision theory and a factor of ∼6 lower than those calculated from capture theory.However, the calculated values do offer some insight into the kinetics and suggest that R2 is close to the collision limit.
The reaction between syn-CH 3 CHOO and SO 2 has been investigated using theoretical approaches, which indicate a barrierless reaction with a 98% yield of acetaldehyde (CH 3 CHO) + SO 3 at 298 K and 200 Torr of He and a rate coefficient for CH 3 CHO + SO 3 production of 4.49 × 10 −11 cm 3 s −1 at 298 K. 13 However, the possible impacts of pressure were not fully discussed, and the reaction of anti-CH 3 CHOO + SO 2 was not considered.The calculations 13 predicted a positive temperature dependence for reactions of CH 2 OO, syn-CH 3 CHOO, and (CH 3 ) 2 COO with SO 2 , despite the reactions being barrierless, and this is in contrast to the Uncertainties represent a combination of the 1σ statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of SO 2 .

The Journal of Physical Chemistry A
experimental results for syn-CH 3 CHOO + SO 2 obtained in this work, our previous experiments for CH 2 OO + SO 2 , 6 and experimental results for (CH 3 ) 2 COO + SO 2 40 (see the Supporting Information for further details).Where potential impacts of pressure have been considered in detail in theoretical studies of SCI + SO 2 reactions, there is an agreement with the lack of observed pressure dependence in the kinetics for CH 2 OO + SO 2 under atmospheric conditions, 3,4,11,12 but there are differences in the predicted pressure dependence of the reaction between (CH 3 ) 2 COO and SO 2 . 11,12Vereecken et al. suggested that >80% of the SOZ formed by (CH 3 ) 2 COO + SO 2 undergoes prompt decomposition to acetone (CH 3 C(O)CH 3 ) and SO 3 at 298 K and a pressure of 4 Torr, while >97% of the SOZ collisionally stabilizes at 298 K and 760 Torr, with the difference compared to CH 2 OO + SO 2 attributed to the greater number of degrees of freedom in the SOZ formed via (CH 3 ) 2 COO + SO 2 , which would also be relevant to the comparison between the SOZ formed via CH 2 OO + SO 2 and those from reactions of CH 3 CHOO conformers with SO 2 .However, Kuwata et al. calculated a different potential energy surface for the reaction between (CH 3 ) 2 COO and SO 2 compared to that reported by Vereecken et al., and thus a different mechanism for the reaction, with calculations predicting no significant collisional stabilization of the SOZ at 298 K and pressures below 10 4 Torr and SO 3 yields greater than 96% at 298 K and pressures from 1 to 760 Torr.Experimental measurements of the kinetics for (CH 3 ) 2 COO + SO 2 have indicated significant pressure dependence and negative temperature dependence under atmospheric conditions, 40−42 similar to the observations in this work for the reaction between syn-CH 3 CHOO and SO 2 .Differences between theoretical approaches and between experiments and theory indicate that the application of theory to the prediction of SCI kinetics remains a challenge.

■ ATMOSPHERIC IMPLICATIONS
−45 Figure 7 compares the pseudo-first-order losses for CH 3 CHOO conformers through unimolecular decomposition and reactions with SO 2 and water vapor for a range of SO 2 and water vapor concentrations as a function of temperature at 760 Torr.Rate coefficients for unimolecular decomposition (k dec ) were taken from our recent work, 31 and those for reactions with SO 2 (k SOd 2 ) were taken from those determined in this work.Rate coefficients for reactions with water vapor (k H2O and k (Hd 2 O)d 2 ) were based on the upper limit for syn-CH 3 CHOO + H 2 O reported by Sheps et al. at 298 K, 26 which forms the basis of the current IUPAC recommendation, 3 and temperaturedependent measurements for anti-CH 3 CHOO + H 2 O and anti-CH 3 CHOO + (H 2 O) 2 reported by Lin et al. 45 Water dimer concentrations were calculated from the monomer concentration using equilibrium constants reported by Ruscic et al. 46 There are no current reports of rate coefficients or upper limits for a possible reaction of syn-CH 3 CHOO with water dimers, and it should be noted that current IUPAC recommendations 3 for anti-CH 3 CHOO reactions with water vapor do not extend beyond 298 K, owing to uncertainties in temperature-dependent measurements, which will impact the analysis shown in Figure 7.For anti-CH 3 CHOO, results show that the reaction with water vapor will dominate under all conditions relevant to the troposphere, but chamber studies employing high SO 2 concentrations and low humidity will need to consider the impact of R2.For syn-CH 3 CHOO, the reaction with SO 2 will be competitive with other losses in the atmosphere in areas with high SO 2 concentrations and low humidity, particularly at low temperatures, contributing to the atmospheric oxidation of SO 2 .The pressure dependence of k 1 indicates that there may be significant collisional stabilization of the SOZ produced in the reaction between syn-CH 3 CHOO and SO 2 , potentially limiting the production of SO 3 and subsequently H 2 SO 4 .However, the fate of the SOZ is uncertain, and even if there is significant stabilization of the SOZ, it may still contribute to the atmospheric production of H 2 SO 4 through subsequent chemistry.A more detailed assessment of the atmospheric impacts of CI reactions with  The Journal of Physical Chemistry A SO 2 would benefit from further experimental investigation of the nature and yields of the products, particularly as a function of pressure.

■ CONCLUSIONS
The kinetics of syn-and anti-CH 3 CHOO reactions with SO 2 have been investigated in the temperature range from 242 to 353 K at pressures between 10 and 600 Torr using laser flash photolysis of CH 3 CHI 2 /O 2 /N 2 /SO 2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy.
Results for syn-CH 3 CHOO + SO 2 show that the kinetics are pressure-dependent, with a negative dependence on temperature.The kinetics can be parametrized by a model that indicates a role for chemical activation, which gives a rate coefficient of k 1 = (4.80 ± 0.46) × 10 −11 cm 3 s −1 at 298 K and 760 Torr.The observed pressure dependence reconciles apparent discrepancies in previous measurements of syn-CH 3 CHOO + SO 2 kinetics performed at ∼298 K but at different pressures.
Kinetics of the reaction between anti-CH 3 CHOO and SO 2 display no significant dependence on temperature or pressure over the ranges investigated.Results give a mean value for k 2 of (1.15 ± 0.16) × 10 −10 cm 3 s −1 at 298 K and (1.18 ± 0.21) × 10 −10 cm 3 s −1 over all conditions studied in this work.
Comparisons with unimolecular decomposition kinetics of syn-and anti-CH 3 CHOO and reactions with water vapor under typical atmospheric conditions indicate that the reaction with SO 2 will play an enhanced role in the removal of the syn-CH 3 CHOO in areas of low humidity and at low temperatures and the removal of anti-CH 3 CHOO is dominated by its reaction with water vapor under all conditions relevant to the troposphere.

Figure 5 .
Figure 5. Effects of pressure on k 2 at 298 K.The solid line shows the mean value for k 2 at 298 K ((1.15 ± 0.16) × 10 −10 cm 3 s −1 ).Previous results reported for k 2 are also shown.Error bars represent a combination of the statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of SO 2 .

Figure 6 .
Figure 6.Mean values for k 2 determined at each temperature.The solid line represents that the mean value for k 2 over all conditions investigated in this work is (1.18 ± 0.21) × 10 −10 cm 3 s −1 .Previous results reported for k 2 and rate coefficients calculated using collision theory (k col , red dashed line) and capture theory (k capt , blue dashed line) are also shown.Error bars represent a combination of the statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of SO 2 .
Description of the instrument response function, comparison of first-and mixed-order analysis, effects of pressure on k 2 across the studied temperature range, comparison to theoretical calculations, and summary of experimental data (PDF) ■ AUTHOR INFORMATION Corresponding Author Daniel Stone − School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.; orcid.org/0000-0001-5610-0463;Email: d.stone@leeds.ac.uk

Figure 7 .
Figure 7. Pseudo-first-order losses of (a) syn-CH 3 CHOO and (b) anti-CH 3 CHOO through reaction with SO 2 (orange), reaction with water vapor (blue), and unimolecular decomposition (pink) at a total pressure of 760 Torr.Pseudo-first-order losses were calculated as described in the main text.

Table 1 .
Summary of Literature Results for k 1 and k 2 controlled by a needle valve, was then passed through a bubbler containing liquid CH 3 CHI 2 (SynHet, 90%) held at a constant temperature in a water bath before being recombined with the rest of the gas flow and passed into a jacketed Pyrex reaction cell.Experiments were performed under pseudo-firstorder conditions, with the concentrations of SO 2 in large excess over initial CH 3 CHOO concentrations.Concentrations were varied in the range [CH 3 a LFP = laser flash photolysis, PIMS = photoionization mass spectrometry, UV abs = ultraviolet absorption.The Journal of Physical Chemistry A 3CHOO at time t, C 0 is the initial concentration of the Criegee intermediate conformer, and k′ is the rate coefficient describing the sum of first-order losses of the CH 3 CHOO conformer and is given by k′ = k x + k 1 [SO 2 ] for syn-CH 3CHOO and k′ = k x + k 2 [SO 2 ] for anti-CH 3 CHOO, where k x represents losses of syn-or anti-CH 3 CHOO via any reaction or process other than the reaction with SO 2 .Unimolecular decomposition and bimolecular reactions with the CH 3 CHI 2 precursor contribute significantly to k x for both syn-and anti-CH 3 CHOO,