Temperature-Dependent Rate Coefficient for the Reaction of CH3SH

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Article

Temperature-Dependent Rate Coefficient for the Reaction of CHSH with the Simplest Criegee Intermediate 3

Yu-Lin Li, Yen-Hsiu Lin, Cangtao Yin, Kaito Takahashi, Che-Yu Chiang, Yuan-Pin Chang, and Jim Jr-Min Lin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12553 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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The Journal of Physical Chemistry

Temperature-dependent Rate Coefficient for the Reaction of CH3SH with the Simplest Criegee intermediate Yu-Lin Li, 1,2 Yen-Hsiu Lin, 1,2 Cangtao Yin,1 Kaito Takahashi,1* Che-Yu Chiang,3 Yuan-Pin Chang, 3* Jim Jr-Min Lin1,2 1. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan 2. Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan 3. Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan

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ABSTRACT

The kinetics of the reaction of the simplest Criegee intermediate CH2OO with CH3SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17) × 10–12 cm3 s–1. The results exhibit a weak negative temperature dependence: the activation energy Ea (k = Ae–Ea/RT) is –1.83 ± 0.05 kcal mol–1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6 × 10–12 cm3 s–1 at 298 K; Ea = –2.80 kcal mol–1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH2OO with CH3SH are compared to the reactions of CH2OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicate that this reaction has a negligible influence to atmospheric CH2OO or CH3SH.


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INTRODUCTION Recently, the investigations of Criegee intermediates (CIs) have gained great interest, because of their potential roles for clarifying unsolved issues in atmospheric chemistry.1–12 CIs, which were first postulated in 1949 by Rudolf Criegee,13 are highly reactive carbonyl oxides created during ozonolysis, i.e., reactions of ozone with unsaturated hydrocarbons. The CIs produced from ozonolysis may have excess internal energy and subsequently subject to isomerization or unimolecular decomposition which could be responsible for the efficient production of OH radicals from ozonolysis.14–16 Under atmospheric conditions, these energetic CIs may be collisionally stabilized besides those unimolecular processes. However, CIs could also be initially formed with less internal energy, and their longer lifetimes allow them to be thermally relaxed prior to reaction.10,17 These thermalized CIs, so called stabilized CI, will either undergo thermal decomposition which could produce OH radicals or react with other atmospheric species, such as SO2, NO2, organic and inorganic acids, various volatile organic compounds (VOCs) and water vapors.8,11 Those reactions involving CIs can produce acids (such as H2SO4), radicals or low-volatility products which can be the key components for forming secondary organic aerosols.18–21 The present work focuses on the simplest Criegee intermediate CH2OO because of its importance as a prototypical CI for elucidating the relationships between the structures and reactivities of CIs. There are many reports on the rate coefficients of reactions involving CH2OO and atmospheric


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trace gases, and, therefore, we can exploit the substituent effects on the co-reactants. In addition, being the smallest CI, accurate theoretical calculations are feasible for CH2OO, and the comparison between experiment and theory for reactions of this CI can lead to benchmarking the theoretical descriptions of other reactions of CI that cannot easily be measured, such as reactions of large CIs or reactions of CIs with other radicals. One of the motivations to study the reactions of CIs with CH3SH is to elucidate further the element dependence in CI chemistry, such as comparing the reactivities of CH3OH and CH3SH with CIs. As CH3OH or CH3SH could form a hydrogen-bonded prereactive complex with CI via its XH (X= O or S) moiety, this heteroatom substitution may significantly affect the nature of the subsequent reaction, such as temperature dependence, rate coefficient, energy barrier ... etc. A few recent studies have compared the reactions of CH2OO with H2S and H2O, and found that the CH2OO + H2S reaction is 2−3 orders of magnitude faster than the CH2OO + H2O reaction, and such a trend can be rationalized by the hydrogen donating ability or the bond length of the XH bond.22,23 We expect a similar trend may also appear in larger reactants, such as alcohols. Several laboratory measurements have studied the reactions of CIs with alcohols, as they are an important class of VOCs in the atmosphere. McGillen et al.24 investigated the kinetics of the reaction of CH2OO with methanol using cavity ring-down spectroscopy, and they obtained the rate coefficients from 254 K to 319 K at total pressure 10 Torr. Their results show that this reaction exhibits a weak negative temperature dependence: (1.98 ± 0.03) ×


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10–13 cm3 s–1 at 254 K, (1.04 ± 0.02) × 10–13 cm3 s–1 at 293 K to (0.77 ± 0.01) × 10–13 cm3 s–1 at 319 K. Tadayon et al.25 studied the kinetics of the same reaction by means of broadband UV transient absorption spectroscopy, and they obtained the rate coefficient of (1.4 ± 0.4) × 10–13 cm3 s–1 at 295 K and a total pressure of 80−100 Torr. The present study of the CH2OO + CH3SH reaction can also help us to further understand the influence of CIs on the sulfur chemistry in the atmosphere. Besides sulfur gas emissions from volcanic regions and geothermal fields, the biological reduction of sulfur compounds is one of the major natural sources of atmospheric sulfur,26 and CH3SH is one of the simplest reduced sulfur compounds. The concentration of atmospheric CH3SH in the regions containing anthropogenic sources, such as pulp and paper plants and gas and petroleum producing activities, could range from few ppbv to several tens of ppbv.27,28 The atmospheric CH3SH could be further oxidized by the reaction with OH or NO3,29–31 which may result in the abstraction of the hydrogen atom from CH3SH and produce H2O or HNO3.31,32 Recently, Chang et al. have utilized a high-resolution mid-IR quantum cascade laser (QCL) spectrometer with a spectral resolution of 0.004 cm–1 to study the spectrum of the ν4 fundamental band of CH2OO in region 1273−1290 cm1 and to measure the absorption cross section of this band.33,34 They have also utilized such QCL spectrometer to study the kinetics of CH2OO reaction with ozone.35 Very recently, Luo


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et al.36,37 employed an external-cavity QCL and a Herriott cell to study the spectra of CH2OO and syn-CH3CHOO in region 880−932 cm1 at resolution 0.0015 cm–1. These works demonstrate that the high-resolution IR spectroscopy has numerous advantages for kinetics studies, such as higher selectivity due to rovibrationally-resolved IR spectra (thus, fewer byproduct interferences), and high sensitivity due to the narrow laser line width. In the present work, we report the measured bimolecular rate coefficients for the CH2OO + CH3SH reaction from 278 to 349 K and at total pressure from 10 to 300 Torr, utilizing a direct detection of CH2OO with the QCL transient absorption spectrometer. Complementary quantum chemistry calculations and master equation modeling were also performed to obtain the calculated bimolecular rate coefficients at different temperatures and pressures. Finally, we discuss the reaction mechanism and the potential impact of this reaction to atmospheric chemistry.

EXPERIMENTAL AND THEORETICAL METHODS Experimental

Methods.

The

experimental

apparatus

has

been

described

elsewhere,33,35 and thus it is briefly described here. The vapor from liquid CH2I2 (heated to 304 K) was mixed with O2, N2, and CH3SH in Teflon tubes, and their flow rates were controlled by mass flow controllers (Brooks, 5850E). The concentration of CH2I2 was


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monitored in an absorption cell (75 cm in length) with a 296 nm LED and a UV spectrometer (Ocean Optics, Flame). The concentration of CH3SH (Matheson) was also monitored in another absorption cell (exp. #1~25: 90 cm in length, others: 40 cm in length) with a D2 lamp (Hamamatsu L12515) and a UV spectrometer (Ibsen photonic, FHW-380). The flowing gas mixture was preheated or precooled in a ~1 m copper tubing before entering a reaction cell which was immersed in a temperature-controlled water tank. The reaction cell is a 75 cm-long, 19 mm-inner-diameter glass tube with two BaF2 windows, which were purged with nitrogen, reducing the effective path length to 65 cm. A 352 nm pulsed excimer laser (XeCl, Lambda Physik, LPX-210i, repetition rate: 2 Hz) photolyzed CH2I2 in the reaction cell and produced CH2I, which subsequently reacted with oxygen to form CH2OO within a few microseconds.38,39 Similar to the case of CH2OO reaction with ozone,35 we choose this photolysis wavelength to minimize the effect of CH3SH photolysis, which could generate H atom fragments and other unwanted byproducts, such as HO2.30 The IR absorption of CH2OO was probed by a pulsed IR QCL (Alpes Lasers SA, 60 μs pulse period and 40% duty cycle), which was driven by an intermittent CW driver (Alpes Lasers), powered by a DC-power supply (HAMEG Instruments, HMP2020). The laser temperature was controlled by a temperature controller (Alpes Lasers SA, TC-3). The laser frequency of each laser pulse was down-chirped between 1286.4 to 1285.2 cm–1, which was set by the DC-power supply voltage (9 V) and the laser temperature (285 K). The UV photolysis and IR probe


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lasers were synchronized by a digital delay generator (Stanford Research Systems, DG535). Note that the repetition rate of the IR probe laser (~16.7 kHz) is much larger than of that of the UV photolysis laser (2 Hz), and the pulse period of the IR probe laser (60 μs) is much smaller than the lifetime of CH2OO (several ms). Thus, the IR probe laser operated in such quasi-CW mode can probe a full time profile of [CH2OO] created by each UV photolysis laser pulse, facilitating single shot measurements. The output beam of QCL was projected by a lens (f = 25.4 mm) into the reaction cell, which was in between a BaF2 right angle prism (length of leg = 7 mm) and a concave mirror (R = 1016 mm) (See Figure S1 in Supporting Information for a detailed experimental schematic). The IR probe beam propagated through the reactor cell up to six times, achieving an optical path of 4.0 ± 0.2 m (See Supporting Information for the details of the multi-pass scheme). The remaining IR probe beam exited from the reactor cell was detected by a HgCdTe (MCT) detector (Kolmar technologies, KMPV11-1-J2). The absolute IR laser wavelength was determined by measuring a reference gas spectrum (3 Torr N2O) and an etalon signal (Ge etalon 3 inch in length, FSR = 0.0163 cm– 1).

The IR signals from the DC outputs of all MCT detectors were acquired by a digital

oscilloscope (Lecroy, HDO4034, 12 bits vertical resolution). For each measurement, we averaged the data for 120 photolysis laser shots. Theoretical Methods. The stationary points for the singlet ground electronic states for CH2OO, CH3SH, and the prereactive van der Waals (VDW) complex CH2OO−CH3SH


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were optimized by Becke’s three-parameter Lee−Yang−Parr hybrid functional (B3LYP)40,41 with the 6-311+G(2d,2p)42,43 basis sets. We determined the transition-state (TS) structures for the H-abstraction reaction channel that forms CH2(SCH3)OOH. For this reaction channel, we obtained two reaction paths which are attributed to the two different directions of the methyl group compared to the OO bond at the TS. To quantify the minima (transition state) geometries, we performed frequency calculations and confirmed that there were zero (one) imaginary frequencies at the B3LYP method. For the transition states, we performed intrinsic reaction path calculations to confirm the reactants and products for the reaction using the B3LYP method. All B3LYP calculations were performed using the Gaussian09 program.44 Using these B3LYP geometries, we refined the electronic energies by complete basis set (CBS) extrapolation45 using Dunning’s augmented core-valance aug-cc-pCVXZ (X = D, T, Q)46,47 basis sets with QCISD(T) correlating both core and valance electrons.48 The Hartree−Fock energy was extrapolated using the equation 𝐸𝐶𝐵𝑆 + Ae ―BX whereas the correlation energy was extrapolated using the equation 𝐸𝐶𝐵𝑆 +Ae ― (𝑋 ― 1) + Be ― (𝑋 ― 1) , 2

where X is the cardinal number of the basis set and ECBS, A, and B are optimization parameters. All QCISD(T) calculations were performed using the MOLPRO program.49 The Cartesian geometries optimized by the B3LYP method are given in Table S3. We have evaluated the effect of using geometries optimized by different quantum chemistry methods and larger basis set in Supporting Information. We have shown that


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this geometrical effect will results in an error of about 0.2 kcal mol-1 in the present case. While questions remain in the error caused by using a single reference method, our previous studies for the reaction with water has shown that multireference methods can give TS energies which are ~1 kcal mol-1 higher compared to the single reference methods.50 The bimolecular reaction rate coefficients were obtained using canonical transition state theory. As done by Buras et al., we calculate a direct passage through the H-abstraction TS ignoring the effect of the prereactive complex.51 Furthermore, we also estimated the pressure dependence of the room temperature rate coefficients using the Master equation method as done by Jalan et al. for the reaction between CIs and aldehydes.52 We used the THERMO and MULTIWELL programs in the MultiWell suite53,54 to calculate the temperature and pressure dependence of k(T,P). Here we used the harmonic-oscillator rigidrotor approach for the calculation of the partition functions. Furthermore, as given in Supporting Information, we have checked that considering the methyl group as a hindered internal rotor rather than a harmonic oscillator will cause a 10% increase in the rate coefficient. We estimated that the largest error in the rate coefficient come from the quantum chemistry energies, ~1.2 kcal mol-1, which will result in an uncertainty of a factor of 7 at 298 K.50,55,56


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RESULTS AND DISCUSSION Reaction Rate Coefficients. Figure 1 shows the transient IR absorption spectra of CH2OO measured at several concentrations of the reactant CH3SH. The transient absorption means the change in the IR absorbance with respect to that before the UV photolysis pulse. Figure 1 shows the results at a photolysis-probe delay of 1.2 ms. The spectral range corresponds to the Q branch of the ν4 fundamental band of CH2OO.33 As described in previous works,35,33 each peak between 1285.7 cm1 and 1285.9 cm1 correspond to the congested transitions from the same K levels (from K = 3 to 8), and the broad peak at 1285.61 cm1 is the perturbed transitions of higher K levels.33 Some oscillating noise in the spectra can mainly be attributed to the etalon effect of the IR laser beam. Figure 1 shows that the spectral intensity decreases when increasing the concentration of CH3SH, indicating the reaction of CH2OO with CH3SH. The decay trends of these K-resolved bands are similar, indicating no rotational state dependence.

4.0 3.5 3.0

Abs. / 103

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[CH3SH] / 1014 cm3 < 0.03 13.5 26.5

8

7

2.5 2.0

5

1.5 1.0

6

4 K=3

0.5 0.0 1286.0

1285.9

1285.8

1285.7

1285.6

Wavenumber / cm1


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Figure 1. Transient IR absorption spectra of CH2OO at 30 Torr total pressure. The peaks correspond to the Q branch of the ν4 fundamental band of CH2OO, and the K number is assigned to each resolved sub-band. The photolysis-probe delay time is 1.2 ms in this example. Two dashed vertical lines indicate the integration region for data analysis.

2.0 [CH3SH] / 1014 cm3

Temperature-Dependent Rate Coefficient for the Reaction of CH3SH

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