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Clusters, Radicals, and Ions; Environmental Chemistry

Identification and Self-Reaction Kinetics of Criegee Intermediates syn-CHCHOO and CHOO via HighResolution Infrared Spectra with a Quantum-Cascade Laser 3

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Pei-Ling Luo, Yasuki Endo, and Yuan-Pern Lee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01824 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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The Journal of Physical Chemistry Letters

Identification and Self-reaction Kinetics of Criegee Intermediates syn-CH3CHOO and CH2OO via Highresolution Infrared Spectra with a Quantum-cascade Laser Pei-Ling Luo,*, 1 Yasuki Endo, 1 and Yuan-Pern Lee*,1,2,3 1

Department of Applied Chemistry and Institute of Molecular Science National Chiao Tung

University, Hsinchu 30010, Taiwan. 2

Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu

30010, Taiwan. 3

Institute of Atomic and Molecular Sciences Academia Sinica, Taipei 10617, Taiwan.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The Criegee intermediates, carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play important roles in atmospheric chemistry. The two conformers of CH3CHOO exhibit distinct reactivity toward several atmospheric species, but a distinct conformer-specific probe is challenging because ultraviolet and infrared absorption bands of syn- and antiCH3CHOO overlap at low-resolution. Employing a quantum-cascade laser and a Herriott cell, we recorded the O−O stretching bands of CH2OO and syn-CH3CHOO in region 880−932 cm−1 at resolution 0.0015 cm−1. In addition to completely resolved vibration-rotational lines of CH2OO extending over 50 cm−1, some spectral lines associated with hot bands were identified. Spectral lines solely due to syn-CH3CHOO were also identified. Probing these lines, we determined the −10 rate coefficient for the self-reaction of syn-CH3CHOO to be kself = (1.6 . cm3 . )×10

molecule−1 s−1, about twice that of CH2OO.

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The Criegee intermediates, carbonyl oxides proposed by Criegee1 in 1949 as key intermediates in the ozonolysis of alkenes, play important roles in many aspects of atmospheric chemistry.2,3,4 The recent applications of new schemes of production, using photolysis of a mixture of diiodoalkanes and O2 to facilitate reactions of iodoalkyl radicals with O2 to generate various Criegee intermediates,5,6,7 enable a direct detection of Criegee intermediates with various spectral techniques and have stimulated active research on these important species.8,9,10 Our understanding of critical atmospheric reactions involving Criegee intermediates is becoming clarified because of the direct probing of Criegee intermediates in kinetic experiments.11,12,13,14 Utilizing this reaction scheme, researchers typically employed either photoionization mass spectrometry5,6 or ultraviolet (UV) absorption15,16,17 to probe Criegee intermediates for kinetic studies. Photoionization mass spectrometry utilizes tunable ultraviolet (VUV) light for ionization to distinguish various isomers. The simplest Criegee intermediate CH2OO was readily characterized in the mass spectrum because its ionization threshold is smaller than those of other isomers HCOOH, dioxirane, and methylene bisoxy H2C(O)O.5 In the case of CH3CHOO, the photoionization threshold of syn-CH3CHOO is, however, slightly greater than those of antiCH3CHOO and vinyl hydroperoxide H2C=C(H)OOH,6 making kinetic measurements by specifically probing syn-CH3CHOO challenging. The intense UV absorption of Criegee intermediates is suitable for a kinetic probe, but, because UV absorption features of various species are typically broad and located in a similar region, probing Criegee intermediates with UV absorption suffers from interference by absorption of its isomers or other species. For example, the UV absorption spectrum of syn-CH3CHOO severely overlaps that of antiCH3CHOO.15,18,19 It is hence challenging to employ UV absorption to probe solely synCH3CHOO for its kinetic studies. Of the three reported kinetic investigations of syn-CH3CHOO,


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the authors using a photoionization probe6,20 assumed that syn-CH3CHOO is dominant (90 %) in the system; those using the UV absorption15 for periods > 2 ms assumed that the more reactive anti-CH3CHOO, estimated to be 30 % upon production from CH3CHI + O2, diminished completely after that period. Infrared (IR) spectra are expected to show fingerprints of chemical species that provide their unique identification, but typically spectral detection with high-resolution is required to achieve suitable ratios of signal to noise at concentrations appropriate for kinetic investigations. This condition of small concentration is particularly critical for kinetic studies of Criegee intermediates because the rate coefficients of their self-reactions are ~10−10 cm3 molecule−1 s−1.12 To minimize the interference from the self-reaction, the initial concentration of the Criegee intermediate should hence be less than 1013 molecule cm−3. We previously reported transient IR spectra of CH2OO,21,22 CH3CHOO,23 and (CH3)2COO 24

at resolutions 0.25−1.0 cm−1 with a step-scan Fourier-transform infrared (FTIR) spectrometer.

For the simplest Criegee intermediate CH2OO, the six features observed at 1434.1, 1285.9, 1234.2, 1213.3, 909.3, and 847.4 cm−1 provided a definitive identification of this intermediate.21,22 The spectrum at resolution 1 cm−1 shows, however, only rotational contours, whereas that at 0.25 cm−1 begins to show some details, but the individual rotational lines remain unresolved at resolution 0.25 cm−1, as shown in Figures 1 (a) and 1(b) for the band for the O−O stretching mode. For CH3CHOO and (CH3)2COO, the spectra recorded at resolution 0.5 cm−1 are broad, partly because of their much smaller rotational parameters and partly because of contributions of hot bands involving the excited low-energy modes of internal rotation.23,24 Furthermore, all IR bands of the two conformers, syn- and anti-CH3CHOO, nearly overlap with each other. It is hence desirable to develop a method to record high-resolution IR spectra of


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Criegee intermediates that can provide a conformer-specific probe with adequate sensitivity to facilitate accurate kinetic measurements. Although a spectrum of CH2OO in region 1273−1290 cm−1 recorded with a pulsed quantum cascade laser at resolution 0.004 cm−1 was recently reported,25 it covers only 17 cm−1 and has a temporal resolution of only 50 µs, unsuitable for kinetic studies of Criegee intermediates other than CH2OO. We employed a continuous wave (cw) quantum-cascade laser (QCL) system coupled with a Herriott multi-pass absorption cell with an effective path length ~13 m to record time-resolved IR spectra of Criegee intermediates CH2OO and syn-CH3CHOO in region 880−932 cm−1 upon irradiation at 248 nm of a gaseous mixture of their diiodo-precursors and O2 at low pressure ( 0.5 ms when the thermal equilibrium was attained, as shown in (a). At the initial stage, the ground vibrational state derived some population from quenching of the upper states, so that its apparent rise time appeared to be slightly slower than that of the hot band. These observations enable us to identify many lines in the B1-band region to be associated with hot bands. The presence of internally excited CH2OO might have some effects on the kinetic measurements, especially at low pressure. The increased resolution improves our detectivity. As shown in Figure S1, with a concentration [CH2OO]0 = 1.22×1012 molecule cm−3, its absorbance is ~1.63×10−3 for the line at 896.876 cm−1 and the signal to noise ratio (SNR) is 109, yielding a minimum detectable [CH2OO]0 ~1.1×1010 molecule cm−3, about 200 times improved from using the step-scan FTIR instrument.21 An application of this technique to detect the next member of the Criegee intermediates, CH3CHOO, is more challenging because the two conformers syn- and anti-CH3CHOO have severely overlapped infrared absorption bands, as reported previously.23 Taking advantage of the distinct reactivity between syn- and anti-CH3CHOO, we were, however, able to identify absorption lines solely due to syn-CH3CHOO. The best reactant to distinguish syn- and antiCH3CHOO is methanol, CH3OH. Reaction of syn-CH3CHOO with CH3OH is slow, with a theoretical rate coefficient k = 9.9×10−17 cm3 molecule−1 s−1, whereas reaction of anti-CH3CHOO with CH3OH is rapid, with k = 9.5×10−12 cm3 molecule−1 s−1.26


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Figure 3. Partial absorption spectra of CH3CHOO at resolution 0.0015 cm−1 recorded 12.5 µs after irradiation of (a) 5.26 Torr of CH3CHI2/O2 (1/219), (b) 5.41 Torr of CH3CHI2/CH3OH/O2 (1/6/219), (c) 5.59 Torr of CH3CHI2/CH3OH/O2 (1/14/219). All traces were recorded at 298 K.

Figure 3(a) shows a spectrum in region 883.00−883.26 cm−1 recorded at 12.5 µs after irradiation at 248 nm of a flowing mixture of CH3CHI2/O2 (1/219) at 5.26 Torr. Figures 3(b) and 3(c) show similar spectra recorded with 144 and 334 mTorr of CH3OH added to the system; the original spectrum in Figure 3(a) is also presented in grey for comparison. The intensities of some lines clearly decreased when CH3OH was added, whereas several lines, with some representative lines indicated with pink arrows, remained nearly unaltered. The temporal profiles also assist in the identification of varied species of which absorption lines are in the same region. For example, some C2H4 was produced in the reaction system CH3CHI + O2, as shown in Figure S2. The initial decay and subsequent rise of the temporal profile of line at 883.237 cm−1 (marked P2) clearly indicates that this line contains contributions from both CH3CHOO and C2H4. After inspection of their temporal profiles, we attributed these unaltered lines indicated with pink arrows in Figure 3(a) to absorption due to only syn-CH3CHOO. We were unable to locate definitively lines solely attributable to anti-CH3CHOO in the available spectral region. According to a previous analysis of their IR spectra at resolution 4


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cm−1, absorption lines of anti-CH3CHOO that are not overlapped with those of syn-CH3CHOO lie in a region below 840 cm−1,23 beyond the range of our QCL laser. As shown in Figure S4, with a concentration [CH3CHOO]0 ≈ 4.3×1013 molecule cm−3, the SNR of the temporal profile of the line at 883.149 cm−1 is ~45, yielding a minimum detectable [CH3CHOO]0 ≈ 1×1012 molecule cm−3. The detectivity of CH3CHOO is not as good as that of CH2OO, partly because the intensity is distributed among a lot more lines and also between two isomers, and partly because several other products are absorbing in this region to interfere the detectivity. For kinetic measurements, the IR probe with a QCL laser has several advantages over the step-scan FTIR instrument. To acquire temporal profile of a single line requires only < 30 s, whereas the FTIR system simultaneously acquires lines in a particular spectral range, which requires at least 1 h for spectra with resolution better than 0.25 cm−1. The improved resolution not only enhances the detectivity but also enables a conformer-specific probe for syn-CH3CHOO. The path length is increased from 3.2 m to 13 m; the UV photolysis beam path overlaps well with the IR probe beam in the QCL system, in contrast with our FTIR system in which the photolytic and probe beams are almost perpendicular to each other, hence making accurate measurements of the concentrations of the radicals difficult. We performed some kinetic measurements to demonstrate the advantages of this new QCL system. The self-reaction of CH2OO is reported to be rapid, with k = (6−8)×10−11 cm3 molecule−1 s−1.16,17,27 In these experiments, we used an absorption cell (51 cm) and a LED (285 nm) to deduce the concentration of CH2I2. The initial concentration of CH2I was estimated from [CH2I2] and the fractional yield of photodissociation of CH2I2. We probed several rotational lines of CH2OO and derived a rate coefficient k = (9.2±2.0)×10−11 cm3 molecule−1 s−1 in 16 experiments


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with [CH2OO]0 in range (0.51−4.69)×1013 molecule cm−3, P = 4.5−9.4 Torr, and T = 298 K using an established kinetic model,15 summarized in Table S1. A detailed description of these measurements and an error analysis are presented in Supporting Information, Table S2, and Figures S5 and S6. This result is consistent with previous reports, supporting the feasibility of this new method in kinetic studies.

Figure 4. Representative plot of [syn-CH3CHOO]−1 vs. reaction period at 298 K. (a) 5.16 Torr of CH3CHI2/O2 (1/460), (b) 5.25 Torr of CH3CHI2/O2 (1/219), (c) 9.75 Torr of CH3CHI2/O2 (1/136).

No investigation on the self-reaction of CH3CHOO has been reported. We monitored several lines of syn-CH3CHOO as a function of reaction period t, and present some representative plots of [syn-CH3CHOO]−1 vs. t (Time) in Figure 4. The initial conentration of CH3CHI after photolysis, [CH3CHI]0, was estimated from absorption cross section of CH3CHI2 at 248 nm (3.52×10−18 cm2 molecule−1),28 the laser fluence, and [CH3CHI2]0. We adopted the fraction of yield 0.9 for CH3CHOO from the reaction of CH3CHI + O2 after photolysis and a branching ratio [syn-CH3CHOO]0 : [anti-CH3CHOO]0 = 7 : 3, according to UV absorption experiments,15 to derive [syn-CH3CHOO]0. The slopes in Figure 4 yield 2kself, the rate coefficient of the self-reaction of syn-CH3CHOO. A detailed description of our kinetic analysis of the selfreaction of syn-CH3CHOO is presented in the Supporting Information, Table S3, and Figures


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S7. To understand the contribution of the cross reaction between syn-CH3CHOO and antiCH3CHOO, we compared the temporal profiles for the line of CH3CHOO at 883.168 cm−1 with and without added CH3OH, as shown in Figure S3. These two temporal profiles are similar, implying that either anti-CH3CHOO was produced in negligible amount or the cross reaction is unimportant, perhaps because the rate coefficient for the self-reaction of anti-CH3CHOO is much larger than that of the cross reaction. From our experimental data, the estimated error of the rate coefficient for the self-reaction of syn-CH3CHOO due to neglecting the cross reaction of synCH3CHOO with anti-CH3CHOO is less than 5 %. The average of 17 experiments with [syn-CH3CHOO]0 = (0.91−5.82)×1013 molecule cm−3, P = 3.1−9.8 Torr, and T = 298 K yields kself = (1.55±0.04)×10−10 cm3 molecule−1 s−1; the uncertainty limits represent one standard deviation of the fit. This value serves as an upper limit because secondary reactions such as CH3CHOO + I were not taken into account because its rate coefficient is unknown. When this reaction was included in the model using a rate coefficient the same as that of CH2OO + I, the derived rate coefficient decreased by less than 14 %, as described in Supporting Information. The largest error in kself comes from the error ( %) in estimating

[syn-CH3CHOO]0, because the branching between syn- and anti-CH3CHOO and the fraction of decomposition of the internally excited CH3CHOO have not been well characterized. A rate −10 coefficient for self-reaction of syn-CH3CHOO is hence reported to be kself = (1.6. cm3 . )×10

molecule−1 s−1. This rate coefficient is about twice that of the self-reaction of CH2OO, with their error limits overlap slightly, but is smaller than the corresponding value, kself = (6.01.1)×10−10 cm3 molecule−1 s−1, for the self-reaction of (CH3)2COO.29 Kinetic investigations of the reactions involving CH3CHOO should hence take into account the interference due to the rapid selfreactions of CH3CHOO. With this conformer-specific method and by taking the self-reaction


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into account, kinetic investigations of reactions of syn-CH3CHOO with atmospheric species will be determined with less uncertainty.

EXPERIMENTAL METHODS High-resolution transient absorption spectra of CH2OO and CH3CHOO were recorded with a cw tunable quantum-cascade laser (QCL) system coupled with a multi-pass Herriott cell. The Herriott mirrors are separated by 73 cm; the central part (2.5 cm in diameter) of the mirror was removed to allow photolysis of the sample. CaF2 windows were installed at both ends of the cell to allow passage of the photolysis beam at 248 nm from a KrF excimer laser (Coherent, Compex Pro 50F). The QCL (Daylight Solutions, 41112-MHF, in which MHF indicates mode-hop free) has a beam size ~5 mm and an average power 40 mW within the tuning range 880−932 cm−1 (11.36−10.73 µm). The QCL emits a vertically linearly polarized TEM00 beam with narrow linewidth (FWHM < 5 MHz). This output was split into three beams to pass a germanium etalon (FSR = 0.025 cm−1), a photoacoustic reference cell, and the Herriott cell; the former two were used for wavelength calibration. The beam directed into the cell was multiply reflected to make 21 passes through the cell, resulting in total path length 16.2 m. A path ~13 m was estimated to be overlapped with the photolysis beam. After passing through the cell, the QCL beam was detected with a HgCdTe (MCT) detector with both dc- and ac-coupled outputs, which were recorded with a 14-bit data-acquisition board (National Instruments, PCI-6132) with sampling rate 2.5 M Samples s−1. The IR spectra were recorded on scanning the wavenumber of the QCL with step size ~0.0015 cm−1. The signals were typically averaged over 16 laser shots at each scan step. For the measurement of a time-resolved


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spectrum in the full range 880−932 cm−1, ~34667 scan steps were completed in ~ 7 h. For kinetic measurements, the acquisition of the temporal profile of a single line typically requires

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