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Experimental and Theoretical Study of Reactions of OH Radical with Hexenols: An Evaluation of the Relative Importance of H-abstraction Reaction Channel Yan-Bo Gai, Xiao-Xiao Lin, Qiao Ma, Chang-Jin Hu, Xuejun Gu, Weixiong Zhao, Bo Fang, Wei-Jun Zhang, Bo Long, and Zheng-wen Long Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01682 • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 18, 2015
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Experimental and Theoretical Study of Reactions of OH Radical
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with Hexenols: An Evaluation of the Relative Importance of
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H-abstraction Reaction Channel
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Yanbo Gai,1,2 Xiaoxiao Lin,1,2 Qiao Ma,1,2 Changjin Hu,1,2 Xuejun Gu,1,2 Weixiong
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Zhao,1,2 Bo Fang,1,2 Weijun Zhang,1,2,3,* Bo Long,4 Zhengwen Long5
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1
of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, Anhui,
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China
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2
3
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College of Information Engineering, Guizhou Minzu University, Guiyang 550025, China
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School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei, 230026, Anhui, China
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Laboratory of Atmospheric Physico-Chemistry, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, 230031, Anhui, China
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Key Laboratory of Atmospheric Composition and Optical Radiation, Anhui Institute
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Department of Physics, Guizhou University, Guiyang, 550025, China
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* Corresponding author：
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Phone: +86-551-6559 1551. Fax: +86-551-6559 1560.
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E-mail:
[email protected] 22 23
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ABSTRACT
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C6 hexenols are one of the most significant groups of volatile organic
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compounds with biogenic emissions. The lack of corresponding kinetic parameters
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and product information of their oxidation reactions will result in incomplete
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atmospheric chemical mechanisms and models. In this paper, experimental and
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theoretical studies are reported for the reactions of OH radicals with a series of C6
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hexenols, (Z)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (Z)-4-hexen-1-ol, (E)-2-hexen-1-ol,
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(E)-3-hexen-1-ol, and (E)-4-hexen-1-ol, at 298 K and 1.01×105 Pa. The corresponding
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rate constants were 8.53±1.36, 10.1±1.6, 7.86±1.30, 8.08±1.33, 9.10±1.50 and
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7.14±1.20 (in units of 10-11 cm3 molecule-1 s-1), respectively, measured by gas
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chromatography with a flame ionization detector (GC-FID), using a relative technique.
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Theoretical calculations concerning the OH-addition and H-abstraction reaction
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channels were also performed for these reactions to further understand the reaction
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mechanism and the relative importance of the H-abstraction reaction. By contrast to
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previously reported results, the H-abstraction channel is a non-negligible reaction
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channel for reactions of OH radicals with these hexenols. The rate constants of the
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H-abstraction channel are comparable with those for the OH-addition channel and
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contribute > 20% for most of the studied alcohols, even > 50% for (E)-3-hexen-1-ol.
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Thus, H-abstraction channels may have an important role in the reactions of these
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alcohols with OH radicals and must be considered in certain atmospheric chemical
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mechanisms and models.
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1. INTRODUCTION
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Biogenic volatile organic compounds (BVOCs) are involved in plant growth,
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reproduction, protection, defense, and other processes. Emissions of BVOCs were
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reported to share approximately 90% of the world’s total hydrocarbon emissions,
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about 1150 Tg C every year worldwide.1-3 The oxidation of BVOCs in the atmosphere
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can affect the formation of ozone and secondary organic aerosols (SOA), thus causing
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significant effects on atmospheric reactions and the climate.2-4 These effects may
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become increasingly significant because environment and climate changes are
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increasing the emissions of BVOCs through increased O3 concentrations and global
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warming.5 Therefore, understanding the atmospheric abundance of BVOCs and their
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atmospheric chemistry, such as reaction rates and degradation processes, is important
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for accurate model simulation and for prevention and control purposes.
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C6 hexenols, including their (Z)- and (E)-geometry isomers, were known as one
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of the most important groups of green leaf volatiles. Because of their antibacterial
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properties, C6 hexenols can be emitted by a wide number of plants in response to
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changes in the ambient environment.6,7 Many studies have reported the emission of
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these C6 hexenols. For example, emissions of (Z)-3-hexen-1-ol with a series of other
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C6 alcohols have been observed when plants were exposed to stress, such as in high
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ozone
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(Z)-2-hexen-1-ol as well as (Z)-3-hexen-1-ol will be significantly enhanced when
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grasses and clovers are freshly mown and increased markedly with temperature and
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with the intensity of solar radiation.10 (E)-3-hexen-1-ol is frequently reported as one
concentrations,
pathogen
attacks,
and
wounding.8,9
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of the main volatile fractions emitted from fresh and dry leaves such as those from
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olive trees11 and other ecosystems. 12 Several C6 hexenols, such as (Z)-4-hexen-1-ol,
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(E)-4-hexen-1-ol and (Z)-3-hexen-1-ol, have also been identified as important volatile
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constituents of select fruits and freshly distilled samples of French spirits.13 In most
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cases, the emission of these C6 alcohols are often accompanied with each other.
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After their emission into the atmosphere, unsaturated VOCs are degraded mainly
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by reactions with tropospheric oxidants, OH radical being the most dominant
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oxidant,3 which proceeds in principle through OH-addition channel and H-abstraction
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channel. However, for most unsaturated VOCs previously reported, OH radical
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reactions were considered to proceed completely by OH electrophilic addition, with
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H-atom abstraction of minor importance, which was largely ignored.14-17 According to
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this understanding, for unsaturated alcohols, the reactivity of the C=C double bond
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would be reduced because of the electron-withdrawing OH group. This process would
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lower the reaction reactivity of the addition pathway, decreasing the rate constants of
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the unsaturated alcohols below those for their corresponding alkenes.18 However, the
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available experimental results are not consistent with this inference. Several groups
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have studied select C6 hexenols with OH radicals using absolute or relative methods,
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19-23
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alkenes.24,25 In addition, the estimated rate constants for (Z)-3-hexen-1-ol,
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(E)-2-hexen-1-ol and (E)-3-hexen-1-ol using structure-activity relations (SARs) based
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on the basic alkene structure were far more smaller than their experimental rate
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constants.18,22
but the rate constants measured were larger than those of their corresponding
Furthermore,
when
comparing
the
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(E)-/(Z)-geometry isomers towards the OH radical, differences were obtained between
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the experimental and the SAR results. That is, (Z)-geometry compounds reacted faster
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with OH radicals than (E)-geometry in the experimental results, conflicting with the
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SAR results.22,23
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The possible reason for these disagreements above may result from the H-atom
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abstraction reaction pathway, neglected in the SAR estimation.19,21,23 For these
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unsaturated hexenols, the introduction of the –OH group may have an activating
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effect on the H-abstraction pathway and thus cause the H-abstraction pathway to be
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notable in OH reactions. However, no previous experimental or theoretical study has
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investigated the H-abstraction channel for the title reactions.
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Thus, to better understand the kinetics of the title reactions and to further assess
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the relative importance of the H-abstraction pathway, reactions of the OH radical with
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a series of C6 hexenols, (Z)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (Z)-4-hexen-1-ol,
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(E)-2-hexen-1-ol, (E)-3-hexen-1-ol, and (E)-4-hexen-1-ol, were studied using both
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experimental and theoretical methods in the present work. A series of experiments
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were conducted in a smog chamber using relative method to determine the accurate
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overall rate constants. Theoretical calculation was also performed to study both the
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OH-addition channel and the H-abstraction channel and their contribution to the total
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rate constants. Based on the results, the relative importance of the H-atom abstraction
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channel in the title reactions and their tropospheric implications were evaluated.
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2. EXPERIMENTAL 5
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2.1 Experimental Method
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The rate constants were measured using the relative rate method.26,27
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2-methyl-3-butene-2-ol and cyclohexane were chosen as the reference compounds.
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The reference compounds have been selected by considering the following: (1) the
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compounds must be easily followed by the GC-FID used in this work through the
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reaction time, and their rate constants with OH radicals must be well characterized in
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the literature; (2) the compounds must represent different classes of VOCs to increase
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the accuracy of the measurement; and (3) 2-methyl-3-butene-2-ol and cyclohexane
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were previously demonstrated as reference compounds for reactions of OH radical
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with some unsaturated VOCs. 27, 28 Thus, the rate constants (khex) could be determined
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by measuring the simultaneous disappearance of the hexenol and the reference
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compound, as shown in previous works26,27:
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[hexenol]0 k hex [reference]0 = ln ln [ hexenol ] k [ reference ] t t ref
(1)
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Experiments were performed in a 150 L Teflon bag, which was similar with that
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previously introduced.26,27,29 The bag was surrounded by six ultraviolet lamps,
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providing a maximum output at 254 nm (Philips TUV G13 36 W). Clean air was used
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as the bath gas generated by a pure air generator (AADCO 737-series). The reactants
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are liquids under ambient conditions and were flushed into the Teflon bag using
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purified air through a 3-way glass tube, warming when necessary. In a typical
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experiment, two or four lamps were used. OH radicals were produced by the
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photolysis of H2O2 at 254 nm. Each photolysis step varied from 5 to 10 minutes, and
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8-12 photolysis steps were used. Concentrations of the hexenols and the reference 6
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compounds were monitored by GC-FID (GC7820A, Agilent Technologies). A
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capillary column (HP-5, 30 m, 0.25 µm film, 0.320 mm i.d., Agilent Technologies)
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was used. The GC oven parameters used were as follows: 30 °C for 3 min then 40 °C
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min-1 to 100 °C and held for 5 min. Experiments were performed at 298 ± 1 K and
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1.01×105 Pa. Detailed information of the reagents used and their stated purities is
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provided in the Supporting Information (SI).
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2.2 Experimental Results
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The relative rate method is used providing that the hexenols and the reference
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compounds react only with the OH radicals. Several experiments were conducted to
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confirm this assumption before measuring the rate constants. Mixtures of the hexenols,
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the reference compounds and the OH precursor (H2O2) were introduced into the
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chamber. GC/FID measurements revealed that a
