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. 2024 Oct 20;128(43):9453–9461. doi: 10.1021/acs.jpca.4c06588

Rate Coefficient and Branching Ratio for the Formation of Criegee Intermediate Syn-/Anti-CH3CHOO from CH3CHI + O2 and the Self-Reaction of Syn-/Anti-CH3CHOO Determined with Simultaneous IR/UV Probes

Tang-Yu Kao †, Chen-An Chung †, Yuan-Pern Lee †,‡,*
PMCID: PMC11533191  PMID: 39427260

Abstract

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A flow reactor coupled with a light-emitting diode at 286 nm, an infrared quantum-cascade laser near 11 μm, and an ultraviolet laser at 335 nm was implemented to probe the precursor CH3CHI2, syn-CH3CHOO, and anti/syn-CH3CHOO, respectively, in the reaction of CH3CHI + O2. The branching between syn- and anti-CH3CHOO was determined to be ≈80:20 from two methods. The concentration temporal profiles of anti-CH3CHOO, derived on comparison of infrared and ultraviolet profiles, yielded the rate coefficient for the self-reaction of anti-CH3CHOO, kselfanti = (6 ± 2) × 10–10 cm3 molecule–1 s–1, ∼4 times the corresponding value, kselfsyn = (1.4 ± 0.3) × 10–10 cm3 molecule–1 s–1, for syn-CH3CHOO; the rate coefficient for the cross-reaction between syn-CH3CHOO and anti-CH3CHOO was estimated to be (2.1 ± 0.6) × 10–10 cm3 molecule–1 s–1. With determined concentrations of syn-CH3CHOO and self-reaction rate coefficients, the rate coefficient for the formation of CH3CHOO from CH3CHI + O2 was determined to be kform = (3.8 ± 0.7) × 10–12 cm3 molecule–1 s–1 at 298 K, ∼45% of previous reports.

1. Introduction

Carbonyl oxides, also known as Criegee intermediates, were produced from the ozonolysis of unsaturated organic compounds; they play critical roles in the production of OH under dim-light conditions1−6 and the formation of acids and secondary organic aerosols (SOA) in the atmosphere.2,3,7,8 In laboratory studies, small Criegee intermediates were produced via photolysis of diiodoalkanes in the presence of O2.9−11 This method made the direct probe of the carbonyl oxides using various spectral methods feasible, and consequently led to valuable insights into their reactivities, kinetics, and reaction mechanisms.3,11−17 For larger Criegee intermediates, various conformers might exist and the conformation-specific chemistry plays important roles in atmospheric chemistry.14

The two-carbon Criegee intermediates, acetaldehyde oxide (CH3CHOO), were generated in the atmosphere through the reactions of ozone with 2-alkenes. Taatjes et al. employed the reaction of CH3CHI + O2 to synthesize CH3CHOO and detect it with multiplex photoionization mass spectrometry (MPIMS).10 These authors were able to distinguish the two conformers, syn-CH3CHOO and anti-CH3CHOO, by their distinct ionization thresholds, ∼9.4 and 9.3 eV, respectively. According to quantum-chemical calculations, syn-CH3CHOO is more stable than anti-CH3CHOO by 11–16 kJ mol–1.18−20 A significant barrier of ∼160 kJ mol–1 prevents the interconversion between these conformers at ambient temperature.20,21

Sheps et al. reported broad and overlapping ultraviolet (UV) absorption bands of syn-CH3CHOO and anti-CH3CHOO, with peak positions near 323 and 360 nm, respectively.22 These spectra were derived by making use of the distinct reactivities of these two conformers toward H2O and SO2; anti-CH3CHOO has much greater reactivity than syn-CH3CHOO, so that it was depleted on adding excess H2O or SO2. Our group measured transient IR spectra of CH3CHOO with a step-scan Fourier-transform infrared (FTIR) spectrometer.23 Because the infrared (IR) spectra of syn-CH3CHOO and anti-CH3CHOO are similar and the practical spectral resolution was limited to ∼0.5 cm–1, most bands of syn- and anti-CH3CHOO were overlapped, so that they could only be distinguished through simulation of rotational contours according to high-level full-dimensional quantum-chemical calculations. To better resolute the detailed spectral information, our group later utilized a quantum-cascade laser coupled with a Herriott absorption cell to record the OO-stretching band of CH3CHOO in region 880–932 cm–1 with resolution 0.0015 cm–1; although spectral analysis was challenging, several lines solely due to syn-CH3CHOO were identified when CH3OH was added to the system to deplete anti-CH3CHOO via its rapid reaction.24 Liu et al. reported several absorption bands in region 5600–6100 cm–1 that are associated with overtone and combination bands of syn-CH3CHOO by detecting with laser-induced fluorescence the OH radicals that were produced upon IR activation of cold CH3CHOO;25,26 no band of anti-CH3CHOO could be identified with this method.

CH3CHOO serves as a prototype for understanding the conformation-specific reactivity.14,15,27 The syn-CH3CHOO, with the terminal O atom interacting with two H atoms of the methyl group, decomposes to OH radicals through the 1,4-hydrogen transfer, leading to a more rapid unimolecular decomposition than anti-CH3CHOO.16,25,26,28 Previous studies demonstrated that anti-CH3CHOO is significantly more reactive toward H2O and SO2 than syn-CH3CHOO.10,22,29−33 Additionally, syn- and anti-CH3CHOO show distinct differences in bimolecular rate coefficients in reactions with NO2,10,34 CH3OH,35−37 HC(O)OH,38 CH3C(O)OH,38 amino alcohol,39,40 and dimethylamine (DMA).41

To perform accurate conformation-specific kinetic analysis, it is crucial to monitor syn- and anti-CH3CHOO distinctively. Using the multiplex photoionization mass spectrometry (MPIMS), Taatjes et al. differentiated these two conformers by using ionization at 10.5 and 9.35 eV for the detection of syn- and anti-CH3CHOO respectively. According to these authors, at 10.5 eV, syn-CH3CHOO accounts for ∼80% of CH3CHOO.10 The UV absorption bands of syn- and anti-CH3CHOO, with maxima near 323 and 360 nm, respectively, are significantly overlapped.22 To probe the UV absorption of anti-CH3CHOO with a negligibly small contribution of syn-CH3CHOO at wavelength greater than 400 nm is possible,22 but the absorption of anti-CH3CHOO is small and IO might interfere in this region; below 360 nm, both syn- and anti-CH3CHOO have significant absorption. Consequently, previous kinetic measurements on reactions of syn-/anti-CH3CHOO with atmospheric species using UV absorption either monitored the rapid decay and ascribed it to anti-CH3CHOO or fitted the decay with two decay components and ascribed them to anti- and syn-CH3CHOO, respectively. Liu et al. reported that OH radicals were produced solely from the unimolecular decomposition of syn-CH3CHOO;42 they probed OH by laser-induced fluorescence to investigate the kinetics of syn-CH3CHOO + HCl. To the best of our knowledge, distinct probes of syn-CH3CHOO and anti-CH3CHOO simultaneously for kinetic studies have not been reported.

In this work, we set up a new IR/UV dual-probe multipass absorption system to determine the branching between syn- and anti-CH3CHOO and conduct kinetic measurements for the formation of CH3CHOO and the self-reaction of syn- and anti-CH3CHOO. This system combines the advantages of high-resolution IR absorption of a quantum-cascade lasers (QCL) near 11 μm, which can selectively detect syn-CH3CHOO, and UV absorption at 335 nm, which can detect both syn- and anti-CH3CHOO and quantify precisely their concentrations. Furthermore, absorption of light at 286 nm from a light-emitting diode (LED) quantifies the concentration of the CH3CHI2 precursor, hence the CH3CHI radical produced upon irradiation at 248 nm. With these probes, we were able to derive precisely the branching between syn- and anti-CH3CHOO, rate coefficients for the self-reactions of syn- and anti-CH3CHOO, and the rate coefficient for the formation of CH3CHOO from CH3CHI + O2.

2. Methods

The experimental setup is depicted in Figure 1. The flow reactor is equipped with a pair of Herriott mirrors (diameter 51 mm, radius of curvature 40 cm) at each end with a separation of 75.4 cm and an effective path length of ∼18.9 m; the central part (2.5 cm in diameter) of both mirrors was removed to allow passage of the photolysis beam at 248 nm from a KrF excimer laser (Coherent, Compex Pro 102F, max power ∼300 mJ pulse–1); CaF2 windows were installed at both ends of the reactor. syn- and anti-CH3CHOO were produced upon irradiation at 248 nm of a flowing mixture of CH3CHI2 and O2 at 5–9 Torr and 298 K. Typically, the photolysis laser was operated at 7.0 Hz with a beam size ∼23 × 15 mm2 and energy ∼140 mJ pulse–1 before entering the reactor.

Figure 1.

Figure 1

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UV/IR dual-probe experimental setup. We utilized external-cavity quantum-cascade lasers (EC-QCL) and a 335 nm laser coupled with a Herriott multipass absorption cell (effective path length ∼19 m) to record time-resolved IR spectra of syn-CH3CHOO and UV spectra of syn-/anti-CH3CHOO. Additional light-emitting diode at 286 nm quantifies the concentration of the precursor CH3CHI2. B.S.: beam splitter; M: mirror; F. M.: flip mirror; PA: photoacoustic cell; MCT: HgCdTe detector; DAQ: data acquisition system.

A Peltier-cooled continuous wave (cw) external-cavity quantum-cascade laser (QCL), operating in a mode-hop-free mode near 11 μm (Daylight Solutions, 41112-MHF, resolution ∼ 0.002 cm–1), was coupled to the Herriott absorption cell to record time-resolved IR absorption spectra in region 880–932 cm–1. Before entering the reactor, this output was split to have two additional beams to pass a germanium etalon (FSR = 0.025 cm–1) and a photoacoustic reference cell for wavelength calibration. An absorption path ∼12.4 m was estimated to be overlapped with the photolysis beam. After passing through the Herriot cell, the QCL beam was detected with a photovoltaic HgCdTe detector (Kolmar, KMPV13-1-J2, cooled to 77 K; typically, both ac-coupled and dc-coupled outputs were recorded with a data-acquisition board (Spectrum, M2p 5962-x4, 16 bit, up to 125 × 106 samples s–1). For taking a spectrum, the step size for scanning the wavenumber of the QCL was approximately 0.002 cm–1, with a probed duration of 2.1 s after wavelength tuning. During this probed duration, the photolysis UV laser was triggered 15 times at 7 Hz and the signal was averaged over 15 measurements at each wavelength; the sampling rate was typically set at 2–5 M samples s–1. For kinetic measurements, to avoid the shift of wavelengths, instead of setting the QCL at a specific wavenumber, we scanned the spectrum near 883 cm–1 and integrated the band in spectral regions 883.105–883.135 and 883.148–883.185 cm–1. A minimal concentration of syn-CH3CHOO is ∼5 × 1012 molecules cm–3 for practical kinetic measurements.

The system also incorporates two additional UV probes. A light-emitting diode (LED) at 286 ± 7 nm (with a nearly parallel beam of size ∼2.3 cm2 after passing through several lenses) was injected into the reactor via a dichroic mirror (Semrock, Di01-R266-25X36), which reflects the photolysis light at 248 nm and passes light at 286 nm. This probe beam was completely within the photolysis volume and was focused onto a Si detector (Thorlabs, PDA10A-EC, 200–1100 nm) via an elliptic mirror; the absorption length was estimated to be 87 cm. This LED probe determined the variation of [CH3CHI2] upon photolysis, hence the initial concentration of CH3CHI [CH3CHI]0, according to an absorption cross-section of CH3CHI2 at 286 nm (3.6 × 10–18 cm2).43

The second UV probe was a diode-pumped solid-state laser operating in a cw mode at 335 nm (CNI Laser, UV–F-335, 10 mW, ∼0.6 mm in diameter). To increase the absorption length, the probe beam was reflected once by an external mirror placed outside the Herriott cell and detected with a Si detector (Thorlabs, PDA10A-EC, 200–1100 nm); the effective absorption length inside the reactor was 174 cm. The absorption cross sections of syn- and anti-CH3CHOO at 335 nm were estimated to be 11.3 × 10–18 and 8.5 × 10–18 cm2 molecule–1, respectively, according to the figure of Sheps et al.22

In a typical experiment, we passed O2 through liquid CH3CHI2 before entering the reactor. A 51 cm absorption cell with light at 286 ± 7 nm from a LED and a Si-photodiode detector (Thorlabs, PDA10A-EC, 200–1100 nm) was installed upstream of the reactor to measure the partial pressure of CH3CHI2. The partial pressures of CH3CHI2, O2, and He in the reactor were evaluated from the flow rate of each gas steam, controlled by a mass flow controller (MKS). The concentration of CH3CHI2 in the reactor was also double-checked with the 286 nm LED to ensure consistency. CH3CHI2 (96–98%, Orgchem Tech.), O2 (99.99%, Chiah-Lung), and He (99.9995%, Chiah-Lung) were used as received.

3. Results and Discussion

3.1. Branching of CH3CHI + O2 → CH3CHOO

Following the report of Luo et al.,24 we probed the absorption band of syn-CH3CHOO in regions 883.105–883.135 and 883.148–883.185 cm–1. The UV light at 335 nm was absorbed by both syn- and anti-CH3CHOO.22 Hence, the temporal profiles of IR and UV absorption measured simultaneously are expected to differ in absorption of anti-CH3CHOO, as demonstrated in Figure 2. The UV (red) and IR (black) profiles normalized to the intensity maxima are shown in Figure 2a; the decay of the red trace is slightly more rapid than that of the black trace. Figure 2b compares the two traces with the slow decay (after ∼0.3 ms) matched; the slow decay component is presumably due to only syn-CH3CHOO. The UV absorption profile (red) contains an additional component, as compared with the IR profile (black, syn-CH3CHOO). This component corresponds to the UV absorption of anti-CH3CHOO. After conversion to concentrations, the temporal profiles of syn-CH3CHOO (black) and anti-CH3CHOO (blue) are shown in Figure 2c. This procedure demonstrates that we could probe both syn- and anti-CH3CHOO simultaneously by comparison of the UV and IR probes.

Figure 2.

Figure 2

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Derivation of temporal profiles of syn- and anti-CH3CHOO. IR (∼883.15 cm–1, black) and UV (335 nm, red) absorption temporal profiles are compared in (a), with maxima normalized, and (b), with the slow decay matched. IR absorption is due to only syn-CH3CHOO, whereas UV absorption is due to syn- and anti-CH3CHOO. (c) Comparison of temporal profiles of syn-CH3CHOO (black) and anti-CH3CHOO (blue), derived by subtracting the black curve from the red curve in (b) and conversion to concentration.

A detailed discussion on how we derived the concentrations of [CH3CHI]0 is presented in Section SA, Supporting Information. [CH3CHI]0 was determined from Δ[CH3CHI2], probed by the 286 nm LED (Figure S1). We employed two methods to measure the branching between syn- and anti-CH3CHOO. In Method A, we used the UV absorption at 335 nm and the LED absorption at 286 nm, and the reported σsyn and σanti at 335 nm to derive the branching ratio. The branching ratios for the three channels of CH3CHI + O2

3.1. 1a
3.1. 1b
3.1. 1c

are a: b: c, in which a + b + c = 1. Because O2 was in excess to react with all CH3CHI,

3.1. 2a
3.1. 2b

The absorbance at 335 nm, A3350, right after the reaction of CH3CHI with O2 is hence

3.1. 3

in which σsyn and σanti are absorption cross sections of syn-CH3CHOO and anti-CH3CHOO at 335 nm, and l (= 174 cm) is the absorption path length. We employed σsyn and σanti as ∼11.3 × 10–18 and 8.5 × 10–18 cm2 molecule–1, respectively, at 335 nm, according to the figure of Sheps et al.22 However, according to the figure in a recent paper by Lade et al., σsyn and σanti are ∼9.5 × 10–18 and 5.2 × 10–18 cm2 molecule–1, respectively, at 335 nm.44 The deviations in σsyn and σanti are ∼16 and ∼39%, respectively; the σanti/σsyn value at 335 nm is 0.75 from Sheps et al. and 0.55 from Lade et al. On the other hand, the σanti/σsyn value at 335 nm is 0.86 from Lin et al.31 We hence adapted the value 0.75 from Sheps et al. and consider a possible error of 27%.

Hence,

3.1. 4

At low pressure, a + b = 0.86 ± 0.11 (in 2 Torr of Helium) according to Howes et al.29 This value is expected to be similar for pressure below 10 Torr. Hence, eq 4 can be deduced to

3.1. 5
3.1. 6

Because the reaction of CH3CHI + O2 typically takes 3–7 μs to reach 90% completion, the [CH3CHOO]0, hence A3350, could not be determined directly upon irradiation at 248 nm. We assumed that the key loss of CH3CHOO is due to its self-reaction, so we plotted 1/A335 versus time and extrapolated the value to t = 0 to obtain A3350, as shown in Figure S2. In Table S1, we listed the experimental conditions and [CH3CHI]0, 1/A3350, and a derived using eq 6 (Method A) in each experiment. Satisfactorily consistent results of a values were found; the average value gives a = 0.69 ± 0.06, in which the error limit represents one standard deviation in fitting. The averaged a value implies b = 0.17 ± 0.06 and a/(a + b) = 0.80 ± 0.07; the branching ratio for syn-CH3CHOO:anti-CH3CHOO is hence a:b = (80 ± 7):(20 ± 7).

This ratio of [syn-CH3CHOO]0:[anti-CH3CHOO]0 can be cross-checked by comparison of temporal profiles of UV absorption at 335 nm and IR absorption in regions 883.105–883.135 and 883.148–883.185 cm–1 (Method B). By subtracting the scaled IR profile from the UV profile, shown in Figure 2b, we derived the UV absorption of anti-CH3CHOO; the UV absorption of syn-CH3CHOO was consequently derived. By taking into account of σsyn and σanti, we derived temporal profiles of [syn-CH3CHOO] and [anti-CH3CHOO], as shown in Figure 2c.

The branching ratio [syn-CH3CHOO]0/{[syn-CH3CHOO]0 + [anti-CH3CHOO]0} = a/(a + b) derived from Method B are also listed in Table S1 for comparison with those derived from Method A. The average value a/(a + b) = 0.74 ± 0.09, derived from Method B, gives a = 0.64 ± 0.08, consistent with the value a = 0.69 ± 0.06 determined from A3350 and Δ[CH3CHI2] (Method A); both methods employed the reported cross sections of syn- and anti-CH3CHOO.

Considering the error in the measurements of [CH3CHI]0 (8%), A3350 (3%), UV cross-section (∼16%) of syn-CH3CHOO, the error in σanti/σsyn (∼27%), the error in a + b (∼13% from 0.86 ± 0.11), and the fitting error of b (35%), we estimated the overall uncertainty in the branching of anti-CH3CHOO (b) to be ±50%. We hence report b = 0.17 ± 0.09. The branching between syn-CH3CHOO:anti-CH3CHOO is hence (80 ± 10):(20 ± 10). This ratio is between the ratio ∼70:30 (10 Torr of He at 293 K) reported by Sheps et al.22 and that (∼90:10) reported by Taatjes et al.;10 these two values are within our error limit.

3.2. Rate Coefficients for the Self-Reactions of Syn- and Anti-CH3CHOO

A preliminary analysis of the self-reactions of syn-CH3CHOO and anti-CH3CHOO,

3.2. 7
3.2. 8

by assuming no cross-reactions between syn-CH3CHOO and CH3CHOO,

3.2. 9

is presented in Section SB, Supporting Information; some representative plots of [syn-CH3CHOO]−1 vs t (time) and [anti-CH3CHOO]−1 vs t (time) are shown in Figure S3. A summary of experimental conditions and the fitted results are shown in Table S2; a statistical distribution of these measurements of kselfsyn and kselfanti appear in Figure S4. The average values of the model fit (Model A, listed in Table S3) gave kselfsyn = (1.5 ± 0.2) × 10–10 cm3 molecule–1 s–1 and kselfanti = (10.2 ± 1.5) × 10–10 cm3 molecule–1 s–1. These values are the upper limits of kselfsyn and kselfanti; kselfanti is expected to be decreased more significantly than kselfsyn after considering the cross-reaction because [anti-CH3CHOO]0 is much smaller than [syn-CH3CHOO]0.

With limited information, to derive accurately all three rate coefficients kselfsyn, kselfanti, and kselfcross from our model fitting is challenging; we hence used a range of kselfcross values to derive kselfsyn and kselfanti. When we added reaction 9 in the model listed in Table S3, and used kselfcross = 1.5 × 10–10 cm3 molecule–1 s–1, the value of kselfsyn derived without considering kselfcross, as the lower limit of kselfcross (Model B), the derived kselfsyn and kselfanti are listed in Table S4 and shown as open symbols in Figure 3. The average values are kselfsyn = (1.42 ± 0.12) × 10–10 cm3 molecule–1 s–1 and kselfanti = (7.4 ± 1.1) × 10–10 cm3 molecule–1 s–1. For the upper limit of kselfcross, we consider that the loss of anti-CH3CHOO was due to reaction with either syn-CH3CHOO or anti-CH3CHOO and assume that kselfcross is similar to kselfanti, so that the originally derived kselfanti without considering kselfcross should be reduced by a factor of 5 to yield kselfcross = 2.0 × 10–10 cm3 molecule–1 s–1 because [syn-CH3CHOO]0 is about 4 times [anti-CH3CHOO]0. Considering possible errors associated with the branching between syn-CH3CHOO and anti-CH3CHOO, we used kselfcross = 2.5 × 10–10 cm3 molecule–1 s–1 as an upper limit of kselfcross in the model (Model C). The average values of kself derived by Model C is kselfsyn = (1.36 ± 0.13) × 10–10 cm3 molecule–1 s–1 and kselfanti = (5.2 ± 1.0) × 10–10 cm3 molecule–1 s–1. When we used kselfcross ≥ 2.75 × 10–10 cm3 molecule–1 s–1 in the model fit, we could not obtain a satisfactory fit. Therefore, a reasonable value of kselfcross is estimated to be (1.5–2.7) × 10–10 cm3 molecule–1 s–1, that is kselfcross = (2.1 ± 0.6) × 10–10 cm3 molecule–1 s–1. We set kselfcross = 2.1 × 10–10 cm3 molecule–1 s–1 (Model D) and refit kselfsyn and kselfanti. The derived kselfsyn and kselfanti are also listed in Table S4 and shown as solid symbols in Figure 3. The average values are kselfsyn = (1.38 ± 0.13) × 10–10 cm3 molecule–1 s–1 and kselfanti = (6.2 ± 1.0) × 10–10 cm3 molecule–1 s–1; these are likely the best fits for the self-reactions.

Figure 3.

Figure 3

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Comparison of kselfsyn and kselfanti derived from Model B and Model D. (a) kselfsyn, with [syn-CH3CHOO]0 = (2.1–10.2) × 1013 molecules cm–3, PT = 5.0–30.0 Torr, and T = 298 K. (b) kselfanti, with [anti-CH3CHOO]0 = (0.8–3.8) × 1013 molecules cm–3, PT = 5.0–30.0 Torr, and T = 298 K. Model B: open symbols, kselfcross = 1.5 × 10–10 cm3 molecule–1 s–1; Model D: solid symbols, kselfcross = 2.1 × 10–10 cm3 molecule–1 s–1.

After considering possible errors, discussed in Section SB, Supporting Information, and assuming kselfcross = (2.1 ± 0.6) × 10–10 cm3 molecule–1 s–1 we report rate coefficients kselfsyn and kselfanti for the self-reactions of syn-CH3CHOO and anti-CH3CHOO to be kselfsyn = (1.4 ± 0.3) × 10–10 and kselfanti = (6 ± 2) × 10–10 cm3 molecule–1 s–1, respectively. The value of kselfsyn is similar to the value (1.6 ± 0.50.6) × 10–10 cm3 molecule–1 s–1 reported previously using QCL,24 with error limits overlapped. The value of kselfanti is new and is about 4 times that of kselfsyn.

If we consider the long-range dipole–dipole interaction, the capture rate coefficient derived by Georgievskii and Klippenstein45 is

3.2. 10

in which C is a constant, dCI is the dipole moment of CH3CHOO, and μ is the reduced mass. Using this equation and the dipole moment calculated with the B3LYP/cc-pVTZ method (dCIsyn = 4.69 D and dCIanti = 5.53 D), we obtained kcaptanti/kcaptsyn = 1.3, much smaller than our observation. Similarly, if the hard-sphere collision is considered with the effective radii rCIsyn = 2.8 Å and rCIanti = 3.4 Å calculated with the “volume” keyword in the Gaussian 16 program, kselfanti/kselfsyn = 1.5 was derived. The much greater kselfanti/kselfsyn might be related to the interaction of the terminal O atoms with the two hydrogen atoms in the methyl group of syn-CH3CHOO, and deserves further theoretical investigations.

3.3. Rate Coefficients for the Formation Reaction CH3CHI + O2

During the fitting of the self-reactions, we found that, when we used the literature values k = (8.0–8.6) × 10–12 cm3 molecule–1 s–1 for the formation of CH3CHOO from CH3CHI + O2 in the fitting,22,29 the rising part of the temporal profile could not be fitted well. We hence reinvestigated the formation reaction by using O2 with concentrations smaller than those used for self-reactions, so that the rising part could be characterized better. We integrated the absorbance of the band of syn-CH3CHOO in regions 883.105–883.135 and 883.148–883.185 cm–1 to obtain the temporal profile of syn-CH3CHOO; representative ones are shown in Figure 4.

Figure 4.

Figure 4

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Representative temporal profiles of syn-CH3CHOO at varied concentrations of O2 at 298 K. Signal was integrated over 883.105–883.135 and 883.148–883.185 cm–1. [CH3CHI2] = 28.6 mTorr, [O2] = (0.02–0.60) Torr, total pressure PT = 9.0 Torr; buffer gas: helium.

We fitted the rate coefficient for the formation of CH3CHOO from CH3CHI + O2 according to the model listed in Table S5 by using the branching ratio, the rate coefficients of the cross-reaction between syn-CH3CHOO and anti-CH3CHOO, and that of the self-reactions of syn-CH3CHOO and anti-CH3CHOO determined in this work. We made the following assumptions in the fitting. (1) The rate coefficients k1–k6 are unknown, so we used the same values as those corresponding to CH2OO. (2) the branching between CH3CHOO:CH3CHIOO from the reaction of CH3CHI + O2 (kform) was set to be 0.86:0.14 according to Howes et al.29 (3) The branching ratio of syn-CH3CHOO:anti-CH3CHOO from the reaction of CH3CHI + O2 was set to 0.80:0.20. (4) The first-order rate coefficient kI for the reaction CH3CHI + O2 was fitted.

For this formation reaction, our kinetic analysis was based on mostly the temporal profiles of the IR absorption that monitored only syn-CH3CHOO; the concentration of syn-CH3CHOO could be determined as discussed previously. A summary of experimental conditions and fitted first-order rate coefficient kI of 23 experiments with [CH3CHI]0 = (1.3–9.8) × 1013 molecules cm–3, [O2] = (0.7–30.5) × 1015 molecules cm–3, and total pressure PT = 5.0 and 9.0 Torr are listed in Table S6. Representative temporal profiles of syn-CH3CHOO in experimental set 4 with [CH3CHI]0 = (2.0–2.4) × 1013 molecules cm–3 and [O2] = (0.7–19.3) × 1015 molecules cm–3 (0.02–0.60 Torr) at 298 K are depicted as symbols in Figure 4; the fitted temporal evolution are presented with solid lines. The fitted kI values as a function of [O2] is shown in Figure 5. The bimolecular rate coefficient, kform, obtained from a linear least-squares fitting of Figure 5, is (3.75 ± 0.09) × 10–12 cm3 molecule–1 s–1; the error limit represents one standard deviation in fitting.

Figure 5.

Figure 5

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First-order rate coefficient kI as a function of [O2]. Rate coefficient kI was derived by kinetic model fitting. Total pressure 5–9 Torr, T = 298 K, [CH3CHI2]0 = (5.6–14.2) × 1014 molecules cm–3, [O2] = (0.07–3.05) × 1016 molecules cm–3. Linear fit yield the red line with a slope of kform = (3.75 ± 0.09) × 10–12 cm3 molecule–1 s–1 and an intercept 6690 ± 1300 s–1. Slope in the work of Sheps et al.22 is shown in the inset as a blue line covering the range of [O2] employed in their work.

For cross-checking, even though the quality and reliability are not as good as those from syn-CH3CHOO, we also analyzed several temporal profiles of anti-CH3CHOO, obtained on subtracting the scaled IR profile from the UV profile, as discussed previously. The results of fitted first-order rate coefficients kI in set 4 are also listed in Table S6; the averaged deviations from those fitted with temporal profiles of syn-CH3CHOO are (16 ± 11)% and kform = (4.4 ± 0.2) × 10–12 cm3 molecule–1 s–1 from these data, consistent with the results derived from the temporal profiles of syn-CH3CHOO.

Considering the error in estimates of concentrations of syn-/anti-CH3CHOO (20%) that transforms into an error of 11% in kform, the error in the self-reaction rate coefficient of syn-CH3CHOO (19%) that transforms into an error of 12% in kform, the error in the cross-reaction rate coefficient between syn-CH3CHOO and anti-CH3CHOO (29%) that transforms into an error of 8% in kform, and the fitting error of 3%, we estimated the overall uncertainty to be ±18%. Hence the rate coefficient kform for the rate coefficient of CH3CHI + O2 is reported to be (3.8 ± 0.7) × 10–12 cm3 molecule–1 s–1.

In Figure 4, results of Sheps et al.22 are also indicated as a blue line in the inset for comparison; the range of [O2] employed were ∼(0.02–0.14) × 1016 molecules cm–3. Howes et al.29 reported a similar rate coefficient with the range of [O2] in their experiments ∼(0.01–0.064) × 1016 molecules cm–3. Our work covers a much wider range of [O2], (0.07–3.05) × 1016 molecules cm–3. The rate coefficient obtained in this work is approximately 48% the values of (8.0 ± 0.8) × 10–12 cm3 molecule–1 s–1 reported by Sheps et al., who employed time-resolved broadband cavity-enhanced UV absorption of CH3CHOO for measurement; it is ∼44% the value of (8.6 ± 2.2) × 10–12 cm3 molecule–1 s–1 by Howes et al., who employed mass spectroscopy to probe iodine atom. It is unclear why such a large discrepancy exists, but because our work covers a much wider range of [O2] and also provides accurate measurements of [syn-CH3CHOO] and the rate coefficient of self-reactions and the cross-reaction, we expect that our results to be more reliable. Our reported rate coefficient is only about 2.5 times the corresponding rate coefficient for the formation of CH2OO from CH2I + O2, ∼1.5 × 10–12 cm3 molecule–1 s–1;46−50 this ratio appears to be more reasonable if we consider the size difference of CH3CHI (2.6 Å) versus CH2I (2.2 Å) and the larger dipole moment of dCICH3CHI = 1.39 D versus dCICH2I = 0.67 D.

4. Conclusions

By utilizing an IR/UV dual-probe system with a quantum-cascade laser near 11 μm to probe syn-CH3CHOO and a 335 nm solid-state laser to probe syn-/anti-CH3CHOO, and an additional light-emitting diode at 286 nm to quantify the concentration variation of the precursor CH3CHI2, we were able to obtain concentration profiles of syn-CH3CHOO and anti-CH3CHOO and characterize the branching between syn-CH3CHOO and anti-CH3CHOO to be (80 ± 10): (20 ± 10) from CH3CHI + O2, larger than the previous result of ∼70:30 reported by Sheps et al.,22 but smaller than the ratio 90:10 reported by Taatjes et al.;10 these literature values are within our error limit. By model-fitting the temporal profiles of syn-CH3CHOO and anti-CH3CHOO, we estimated the rate coefficient of the cross-reaction between syn-CH3CHOO and anti-CH3CHOO to be kselfcross = (2.1 ± 0.6) × 10–10 cm3 molecule–1 s–1 and determined the rate coefficient for the self-reaction of syn-CH3CHOO to be kselfsyn = (1.4 ± 0.3) × 10–10 cm3 molecule–1 s–1, consistent with our previous report of (1.6 ± 0.50.6) × 10–10 cm3 molecule–1 s–1.24 The determination of the rate coefficient for the self-reaction of anti-CH3CHOO, kselfanti = (6 ± 2) × 10–10 cm3 molecule–1 s–1, is new; it is more than 4 times of kselfsyn, likely because the dipole moment of anti-CH3CHOO is greater than syn-CH3CHOO and the OO moiety of anti-CH3CHOO is free as compared with that of syn-CH3CHOO. With the branching and the rate coefficients of cross-reaction and self-reactions determined, we investigated the formation rate coefficient of CH3CHOO from CH3CHI + O2 and determined kform = (3.8 ± 0.7) × 10–12 cm3 molecule–1 s–1 at 5–9 Torr and 298 K, which is approximately 45% of the two previously reported values. It is unclear why such a discrepancy exists, but because our work covers a much wider range of [O2] and also provides accurate measurements of [syn-CH3CHOO] and the cross-reaction and self-reaction rates of CH3CHOO, we expect our results to be more reliable. A smaller rate coefficient for the formation of CH3CHOO might affect the kinetic analysis of reactions of CH3CHOO with other atmospheric species.

This work demonstrates the advantages of combining both UV and high-resolution IR absorption for kinetic studies of conformation-specific Criegee intermediates. In the case of CH3CHOO, the IR absorption provides direct measurements of syn-CH3CHOO and the UV absorption provides temporal profiles of syn-/anti-CH3CHOO, from which temporal profiles with accurate concentration measurements of anti-CH3CHOO could be derived, which are critical for precise measurements of rate coefficient. This method is suitable for direct investigations on reactions of syn-CH3CHOO and anti-CH3CHOO with atmospheric species.

Acknowledgments

This work was supported by Ministry of Science and Technology, Taiwan (grants MOST112-2639-M-A49-001-ASP and MOST112-2634-F-009-026), and the Center for Emergent Functional Matter Science of National Yang Ming Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. We thank the National Center for High-Performance Computing (NCHC) of National Applied Research Laboratories (NARLabs) in Taiwan for providing computational and storage resources.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.4c06588.

  • Concentration measurements of [CH3CHI]0; preliminary kinetic analysis and error analysis of the self-reactions of syn- and anti-CH3CHOO; experimental conditions and measured parameters to derive the branching ratio a; experimental conditions and derived rate coefficients for the self-reactions of syn-CH3CHOO (kselfsyn) and anti-CH3CHOO (kselfanti) with various models; kinetic model for fitting the rate coefficients of self-reactions of syn-/anti- CH3CHOO; kinetic model for fitting the rate coefficient of the reaction CH3CHI + O2; experimental conditions and the fitted first-order rate coefficient (kI) of CH3CHI + O2 using various models; the intensity of the 286 nm light before and after photolysis of CH3CHI2 at 248 nm; representative plot of [A335]−1 versus reaction period at 298 K; estimation of kselfsyn and kselfanti from the plot of [CH3CHOO]−1 vs reaction period; and comparison of kselfsyn and kselfanti derived from the second-order fit and the model fit (PDF)

The authors declare no competing financial interest.

Supplementary Material

jp4c06588_si_001.pdf (640.3KB, pdf)

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