Temperature-Dependent Rate Coefficients for the Reaction of CH2OO

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Article

Temperature-Dependent Rate Coefficients for the Reaction of CHOO with Hydrogen Sulfide 2

Mica C. Smith, Wen Chao, Manoj Kumar, Joseph S. Francisco, Kaito Takahashi, and Jim Jr-Min Lin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12303 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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The Journal of Physical Chemistry

Temperature-Dependent Rate Coefficients for the Reaction of CH2OO with Hydrogen Sulfide

Mica C. Smith1, Wen Chao1,2, Manoj Kumar,3 Joseph S. Francisco,3 Kaito Takahashi1*, Jim Jr-Min Lin1,2*

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

2. Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

3. Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE, 68588, USA

* Corresponding authors. +886-2-2366-8237, [email protected] (Kaito Takahashi); +886-2-2366-8258, [email protected] (Jim Lin).

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Temperature-Dependent Rate Coefficients for the Reaction of CH2OO with Hydrogen Sulfide

Abstract

The reaction of the simplest Criegee intermediate CH2OO with hydrogen sulfide was measured with transient UV absorption spectroscopy in a temperature-controlled flow reactor, and bimolecular rate coefficients were obtained from 278 to 318 K and from 100 to 500 Torr. The average rate coefficient at 298 K and 100 Torr was (1.7±0.2)×1013 cm3 s1. The reaction was found to be independent of pressure and exhibited a weak negative temperature dependence. Ab initio quantum chemistry calculations of the temperature-dependent reaction rate coefficient at the QCISD(T)/CBS level are in reasonable agreement with the experiment. The reaction of CH2OO with H2S is 2-3 orders of magnitude faster than the reaction with H2O monomer. While rates of CH2OO scavenging by water vapor under atmospheric conditions are primarily controlled by the reaction with water dimer, the H2S loss pathway will be dominated by the reaction with monomer. The agreement between experiment and theory for the CH2OO + H2S reaction lends credence to theoretical descriptions of other Criegee intermediate reactions that cannot easily be probed experimentally.

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Introduction

The reaction between alkenes and ozone in the atmosphere (alkene ozonolysis) produces Criegee intermediates (R2COO), highly reactive radicals that are known to play an important role in atmospheric oxidation chemistry and possibly in aerosol formation.1,2 Once they are produced, Criegee intermediates (CIs) may undergo a number of possible loss processes such as thermal decomposition,3–7 UV photolysis,8– 11

or reaction with other atmospheric species, most notably water and SO2.12–14 The

thermal decomposition of CIs is thought to be a major non-photolytic source of OH radicals,15,16 also affecting the oxidation capacity of the atmosphere. Bimolecular reactions involving CIs can produce aerosol precursors including H2SO4 and lowvolatility organic compounds.17–19 Thus, it is important to investigate the properties and kinetics of CIs to better understand their atmospheric impact.

The high reactivity of CIs and slow rates of alkene ozonolysis reactions lead to low steady state concentrations of CIs, preventing direct detection and time-resolved kinetics measurements. Therefore, many recent studies of CIs have utilized or adapted the laboratory preparation scheme CH2I2 + h → CH2I + I; CH2I + O2 → CH2OO + I introduced by Welz et al.,12 rather than the alkene + ozone reaction. This scheme generates high concentrations of “cold” CIs with low internal energies that can be detected spectroscopically prior to their unimolecular decomposition. The concentrations of CIs prepared in this manner are high enough to enable time-resolved kinetics measurements, which have yielded reaction rate coefficients quite different from the values estimated using the complicated ozonolysis reaction.12

Welz et al.12 found that the simplest CI, CH2OO, reacts quickly with SO2, with a rate coefficient of 4×1011 cm3 s-1. Given a typical atmospheric SO2 concentration of 50 ppb, 3



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this reaction rate corresponds to an effective loss rate of ~50 s1 for CH2OO due to reaction with SO2.13 Later, a number of groups measured the rate of CH2OO reaction with water, which is several orders of magnitude more abundant than SO2 in the atmosphere. The results were inconsistent, with rate constants ranging from 1012 to 1017 cm3 s1.12,20–22 Chao et al.13 resolved the discrepancy by directly measuring the transient absorption of CH2OO and showing that the reactivity of CH2OO with water is primarily due to reaction with water dimer, with a rate coefficient of ~7×10-12 cm3 s-1; this reaction is only relevant at high water concentrations (higher than the experimental conditions of several previous studies by other groups).

The reactivity of CIs has been found to depend quite sensitively on their molecular structure and geometry. For example, syn-conformer CIs such as syn-CH3CHOO and (CH3)2COO react much more slowly with water vapor than anti-conformer CIs (e.g., CH2OO and anti-CH3CHOO).23–25 Because many different types of alkenes are emitted to the atmosphere, a wide variety of CI structures can form, all of which may have differing reactivity. Hence, it is important to achieve a fundamental understanding of the mechanisms of atmospherically relevant CI reactions and predict how they may change with CI structure. Both theory and experiment play a crucial role in elucidating the key features of these reaction mechanisms. The water reaction is central to determining the atmospheric fate of CIs, and the relative significance of the water monomer and water dimer reactions for different CIs is controlled by the nature of the hydrogen bonding in the transition states and pre-reactive complexes.24-27 It stands to reason that understanding the formation of hydrogen-bonded intermediate complexes in bimolecular reactions involving CIs is important to predict their fate in the atmosphere.

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Other kinetics studies of CIs have focused on hydrogen-containing reactants. Most notably, the self-reaction of CH2OO is extremely fast, on the order of 1011 to 1010 cm3 s1,28–31 which is attributed to a dimer structure that forms between the two CH2OO molecules. HCl and HNO3 were both shown to react rapidly with CH2OO as well, and quantum chemistry calculations revealed that both reactions are nearly barrierless.32 Similarly, the rates of CI reactions with carboxylic acids are close to the collision limit, indicating the absence of a reaction barrier,33–35 whereas the reactions of CIs with aldehydes and ketones are somewhat slower (1011 to 1013 cm3 s1).36,37

H2S is a significant part of the atmospheric sulfur cycle, especially in volcanic regions and geothermal fields. Typical H2S concentrations in these regions are about 500 ppb, and in some cases ppm levels have been reported.38,39 H2S also poses an important human health concern, as high levels can be emitted from oil and gas producing regions which are often close to residential areas and may cause an increase in the air toxicity. The lifetime of H2S produced in volcanic areas is a few days, as it is slowly oxidized to SO2 in a multistep mechanism involving several intermediates such as SH and HSO.40 Accordingly, H2S is a factor in the formation of H2SO4, which contributes to aerosol nucleation and acid rain.

Since H2S differs from H2O only by the period of the central atom, some interesting insights into the nature of CI hydrogen bonded complexes may be gained by studying the reaction of CH2OO with H2S. For example, how does the temperature dependence of the reaction rate coefficient compare with the distinct properties found for the CH2OO reaction with water (which exhibits a strong negative temperature dependence for the water dimer and a weak positive temperature dependence for the water monomer)? The dimerization energy of H2S is 3 or more times lower than that of 5



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H2O,41–45 such that the dimer reaction with CH2OO is not expected to be important. Does the monomer reaction then play a more significant role than in the case of water vapor? Recent calculations of CI bimolecular reactions by Kumar and Francisco46 indicate that the reaction barrier is substantially lowered by heteroatom substitution of the CI reaction partner (e.g., when going from H2O to H2S). Hence, a faster rate may be expected for the reaction of CH2OO with H2S relative to the reaction with monomer H2O.

Here, we report the temperature and pressure dependence of bimolecular rate coefficients for the reaction between CH2OO and H2S measured by direct detection of CH2OO with UV transient absorption spectroscopy, and the high pressure limit bimolecular rate coefficients calculated by quantum chemistry methods.

Experimental and Theoretical Methods

1. Experimental Methods

The rate of CH2OO loss in the presence of H2S was measured by UV transient absorption of CH2OO formed in a 75 cm long glass flow reactor. The absorption apparatus has been described previously6,13,14 and is briefly introduced here. A flow of precursor gas is formed by mixing N2 and O2 in Teflon tubes with a stream of N2 passing through a container of liquid CH2I2 heated to ~303 K. The precursor mixture flows into another absorption cell, where the absorption of CH2I2 is monitored throughout the experiment with a 290 nm LED (UVTOP285TO39BL) and a UV spectrometer (Ocean Optics, Flame). Downstream of the absorption cell, the precursor gas is diluted with a mixture of N2 and the reactant H2S (Matheson Gas Products); the reactant gas is also monitored upstream of this mixing point in a 1 cm cell by a UV spectrometer (Ocean 6


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Optics, Maya2000 Pro) with D2 lamp as probe light (Ocean Optics D-2000 or Hamamatsu L12515). Flow rates of all gases are controlled with mass flow controllers (Brooks, 5850E or 5800E). A summary of experimental conditions is given in Table S1.

The gas mixture containing H2S/CH2I2/N2/O2 then enters a temperature-controlled water tank surrounding the reactor, where it is preheated (or precooled) in ~1 m copper tubing before entering the inlet in the center of the reactor. The reactor is equipped with a pressure gauge (Inficon CDG), three wells spaced equally along the reactor containing resistance temperature detectors, pump outlets on each end, and a small stream of purge N2 between the pump outlets and the absorption windows. UV absorption in the reactor is probed with a laser-driven plasma light source (Energetiq, EQ-99) which passes 8 times through the reactor and is then directed to a balanced photodiode detector (Thorlabs, PDB450A) through a 335-345 nm bandpass filter, along with a reference beam from the EQ-99 source. Subtracting the reference from the absorption signal eliminates oscillations in the plasma source and improves the signal-to-noise ratio.

CH2OO is produced in the reactor via photolysis of CH2I2 by a 248 nm pulsed KrF excimer laser which is aligned collinearly with the probe light using two 257 nm long pass optical filters. The photolysis laser initiates the CH2OO formation chemistry, summarized in reactions 1 and 2 below. Formation of CH2OO occurs within a few μs after the laser pulse, and the subsequent decay of CH2OO depends on a number of loss processes including unimolecular decomposition, wall loss, self-reaction, and the bimolecular reaction with H2S. The differential transient absorption trace over 40 ms (relative to the absorption signal before the laser pulse) is averaged over 120 laser shots and background-corrected by subtracting the absorption signal without CH2I2 in the reactor, which is an average of background traces collected before and after the CH2OO 7



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measurements. The laser pulse rate is set at 1.04 Hz and the gas refresh time is adjusted to

Temperature-Dependent Rate Coefficients for the Reaction of CH2OO

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