Predicting Gaseous Reaction Rates of Short Chain Chlorinated

Nov 5, 2014 - Article Views: 593 Times. Received 7 January 2014. Date accepted 5 November 2014. Published online 5 November 2014. Published in print 2...
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Predicting Gaseous Reaction Rates of Short Chain Chlorinated Paraffins with #OH: Overcoming the Difficulty in Experimental Determination Chao Li, Hong-bin Xie, Jingwen Chen, Xianhai Yang, Yifei Zhang, and Xianliang Qiao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es504339r • Publication Date (Web): 05 Nov 2014 Downloaded from http://pubs.acs.org on November 10, 2014

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Predicting Gaseous Reaction Rates of Short Chain Chlorinated Paraffins with ⋅OH:

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Overcoming the Difficulty in Experimental Determination

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Chao Li†, Hong-Bin Xie†, Jingwen Chen†∗, Xianhai Yang†, Yifei Zhang‡, Xianliang Qiao†

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Environmental Science and Technology, Dalian University of Technology, Dalian 116024,

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China.

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‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024,

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China.

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of

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Table of Contents

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ABSTRACT

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Short chain chlorinated paraffins (SCCPs) are under evaluation for inclusion in the Stockholm

13

Convention on persistent organic pollutants. However, information on their reaction rate

14

constants with gaseous ⋅OH (kOH) is unavailable, limiting the evaluation of their persistence in

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the atmosphere. Experimental determination of kOH is confined by the unavailability of

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authentic chemical standards for some SCCP congeners. In this study, we evaluated and

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selected density functional theory (DFT) methods to predict kOH of SCCPs, by comparing the

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experimental kOH values of 6 polychlorinated alkanes (PCAs) with those calculated by the

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different theoretical methods. We found that the M06-2X/6-311+G(3df,2pd)//B3LYP/6-311 ∗

Corresponding author phone/fax: +86-411-84706269; e-mail: [email protected]. 1

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+G(d,p) method is time-effective and can be used to predict kOH of PCAs. Moreover, based on

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the calculated kOH of 9 SCCPs and available experimental kOH values of 22 PCAs with low

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carbon chain, a quantitative structure-activity relationship (QSAR) model was developed. The

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molecular structural characteristics determining the ⋅OH reaction rate were discussed. logkOH

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was found to negatively correlate with the percentage of chlorine substitutions (Cl%). The

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DFT calculation method and the QSAR model are important alternatives to the conventional

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experimental determination of kOH for SCCPs, and are prospective in predicting their

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persistence in the atmosphere.

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INTRODUCTION

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Chlorinated paraffins (CPs) are commercially produced polychlorinated alkanes (PCAs)

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having carbon chain lengths from C10 to C30 and chlorine content from 30% to 70% by

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mass.1-3 Among them, short chain CPs (SCCPs) with carbon chain lengths ranging from C10 to

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C13 have received much attention due to their widespread occurrence and persistence in the

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environment,4-15 their potential

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bioaccumulate8 and high toxicity.16,17 SCCPs show long-term toxicity to algae, aquatic

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invertebrates and fish at concentrations as low as 19.6, 8.9 and 3.1 µg/L, respectively.18 There

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is also sufficient evidence for carcinogenicity to humans from SCCPs of C12 carbon chain

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length group with an average chlorine content of 60%.17 The vapor pressures of SCCPs are in

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the range of other chlorinated organics with the same molecular weight range such as

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polychlorinated biphenyls and toxaphenes, suggesting that they have the potential to undergo

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long-range atmospheric transport through the grasshopper effect.4 SCCPs have been detected

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in environmental media including air,10,11,13 water19 and sediment;9,12 in humans20 and

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organisms like fish,14 birds,15 marine mammals;21 and even in remote areas like Canadian

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Arctic lakes, the Hazen lake and the European Arctic.5,8,15,21 As SCCPs represent a potential

for

long-distance

transport,12,15

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their

tendency

to

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“new” category of persistent organic pollutants (POPs), several countries (e.g., the United

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States and Canada) and the European Union have imposed regulations on the use of SCCPs.22

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Additionally, SCCPs are now under evaluation for inclusion in the Stockholm Convention on

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POPs.

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Despite the efforts to characterize the extent of SCCP contamination in the environment,

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little is known about their atmospheric degradation kinetics that governs the fate and

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persistence of SCCPs. Among the various atmospheric reactions, the reactions initiated by

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hydroxyl radicals (⋅OH) produced by atmospheric photochemical reactions, are of paramount

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importance. Thus, the ⋅OH reaction rate constant (kOH) is a key parameter for the fate

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assessment of pollutants in the troposphere. However, there are no kOH values available for

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any SCCPs.

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To date, several direct and indirect experimental methods have been developed to measure

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kOH values.23-26 However, the experimental determination of kOH for SCCPs is difficult as the

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quantities and kinds of authentic chemical standards for some SCCPs, which are necessary for

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the determination, are very limited. Moreover, there are a large number of isomers for

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SCCPs,22 which could further increase the workloads and difficulties encountered in the

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experimental determination. Another solution is to predict kOH based on quantitative

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structure-activity relationship (QSAR) models, on which the Organization for Economic

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Co-operation and Development (OECD) has issued guidelines on model development and

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validation.27 Nevertheless, the utility of QSAR models is constrained by the applicability

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domain of the models.28,29 The currently available QSAR models on kOH do not cover SCCPs.

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For example, two latest QSAR models on kOH prediction cover only a limited number of

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PCAs with chain lengths ranging from C1 to C6.30,31 Thus, seeking a new solution for

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predicting kOH of SCCPs is necessary.

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In recent years, the increasing computational power has enabled quantum chemical 3

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calculations for molecules or systems comprising up to a few hundred atoms.32 Some studies

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in recent years showed that the density functional theory (DFT) can accurately predict kinetics,

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pathways and products for the reactions of chemicals with ⋅OH.33--35 For example, Zhou et al.

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employed DFT to calculate the reaction of 4,4'-dibromodiphenyl ether with ⋅OH, and found

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that the predicted products and the overall rate constants (kOH) at 298 K agreed well with the

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experimental results.33 Ci et al. employed DFT to compute the hydrogen abstraction reactions

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of CF3CH2CHO with ⋅OH, also found that the calculated kOH values were consistent with the

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experimental data.35 Furthermore, the theoretical approaches have the advantage that they can

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provide details involving reaction sites and favorable reaction pathways.33-36 As far as we

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know, there are no previous quantum chemical calculations on reactions of SCCPs with ⋅OH.

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It was the purpose of this study to find a solution for kOH prediction of SCCPs. By

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employing 6 molecules (CH3CCl3, CH2ClCHCl2, CH2ClCHClCH3, CH3(CHCl)2CH3,

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CH2Cl(CH2)3CH3 and CH2Cl(CH2)4CH3) with available experimental kOH values as test cases,

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we compared the prediction accuracy of several DFT methods. The expensive ab initio

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second-order Moeller-Plesset (MP2)37 method, which consistently describes all types of

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correlation energy including intermolecular correlation and intramolecular correlation terms,

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and therefore is reliable for geometry optimization,38-40 was also adopted as a reference. Based

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on the established method, kOH values for 9 SCCPs with carbon chain ranging from C10 to C13

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(Figure 1), were calculated. A QSAR model for predicting kOH of more SCCPs was developed

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based on the DFT calculated and available experimental kOH values for PCAs. To the best of

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our knowledge, this is the first study concerning kOH of SCCPs. The DFT calculation method

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and the QSAR model can be employed to predict kOH of other SCCPs.

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Figure 1. Atom numbers of nine SCCPs (Dark gray: C; Light gray: H; Green: Cl).

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COMPUTATIONAL DETAILS

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Global Minimum Search. As the molecular structures of PCAs consist of sp3-hybridized

97

C-X (X = H, Cl) bonds, they have a range of conformations. Thus, the global minimum search

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is needed to identify the optimal structures of the reactants under study, and the optimal

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structures were then included in further structure calculations followed by dynamic

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calculations of kOH.

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The calculations were carried out using TURBOMOLE.41 Ab initio molecular dynamics

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(AIMD) was used to generate reasonable gas-phase geometries by employing the BLYP

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functional42 along with a triple-ξ valence polarized basis set (TZVP) within the 5

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resolution-of-the-identity (RI) approximation.41 In order to sample enough conformations in a

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short time and increase to some extent the possibility for getting the real global minimum, the

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temperature was set to 700 K with the premise that the molecules would not dissociate in the

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AIMD calculation (detailed in the SI). A time step of 0.96752 fs was used, and the total length

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of each AIMD simulation was 20000 time steps. We selected the conformations from the

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AIMD run as the starting point for geometry optimization at the B3LYP/6-311+G(d,p) level

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using the Gaussian 09 program suite.43 The minimum with the lowest energy was identified as

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the global minimum used to investigate its reaction with ⋅OH.

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Electronic Structure Calculations. A gradual scheme was used to screen reliable

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theoretical methods for the geometry optimization and frequency analysis of the studied

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systems. For all the systems, the single point energy calculation was performed at the

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M06-2X/6-311+G(3df,2pd) level, since the M06-2X functional gives the best performance

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(mean signed errors of -0.51 kcal/mol) without increasing the computational time for the

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hydrogen-transfer barrier height calculation.44 As CH3CCl3 has a C3v symmetry that can

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reduce the computational time, its reaction with ⋅OH was taken as an initial test to screen the

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different computational methods (the functional BHandHLYP,42,45 MPW1K,46 B3LYP,42,47

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TPSSh,48 B97-149 and ab initio MP237 in conjunction with the 6-311+G(d,p) basis set) by

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comparing the calculated zero-point corrected reaction barriers (Ea). Subsequently, the

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selected time-effective and reliable methods (B3LYP, TPSSh and B97-1 functionals) were

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evaluated with CH2ClCHCl2 and CH2ClCHClCH3 as test cases, followed by using

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CH3(CHCl)2CH3, CH2Cl(CH2)3CH3 and CH2Cl(CH2)4CH3 for further method validation.

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Finally, an optimum method (B3LYP/6-311+G(d,p)) was selected for the SCCPs. All the

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stationary points including reactants, products, pre-complexes and product-like complexes

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have real frequencies, whereas the transition states (TSs) have only one imaginary frequency.

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The minimum energy paths (MEP) were obtained by the intrinsic reaction coordinate (IRC) 6

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theory for each reaction channel.50

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Dual-level Direct Dynamic Calculation. By means of the POLYRATE 2010-A

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program,51 the rate constant for each reaction channel (ki) was calculated using the improved

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canonical variational transition-state theory (ICVT) with small-curvature tunneling (SCT)

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correction51 over the temperature range 200-500 K. The frequencies of the selected points

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along the MEP were calculated at the B3LYP/6-311+G(d,p) level to obtain the Cartesian

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coordinates, gradient and Hessian matrix; and the single-point energy of these points was

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calculated by the M06-2X/6-311+G(3df,2pd) method. kOH was calculated by summing up the

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ki values of different channels.

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QSAR Modeling. The DFT calculated kOH values (T = 298 K) of 9 SCCPs, together with

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those of 22 low carbon chain PCAs (C1~C6) with available experimental kOH values,31 were

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employed for QSAR modeling. According to previous studies,30,31,52-55 the following quantum

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chemical descriptors were selected to construct the model, including the atomic Mulliken

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charges [the most negative net C-atom charge (QC-), the most positive net H-atom charge and

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the most negative net Cl-atom charge], polarizability (α), the energy of the highest occupied

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molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO),

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chemical potential (µ = (ELUMO + EHOMO)/2)56, and absolute hardness (η = (ELUMO -

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EHOMO)/2)56. These descriptors were calculated from the geometries optimized by the

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B3LYP/6-311+G(d,p) method using the Gaussian 09 program suite. Additionally, two

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descriptors that describe the chlorine substitution characteristics [the number of chlorine

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atoms (nCl) and percentage of chlorine substitutions (Cl%)], 3D-MoRSE (3D-molecular

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representation of structure based on electron diffraction) and geometrical descriptors were

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also considered, for which the DRAGON software was employed for the calculation.57

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For the modeling, the data were randomly split into a training set and a validation set

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with a ratio of 4:1. Stepwise multiple linear regression (MLR) analysis was employed to 7

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construct the QSAR model. The applicability domain of the QSAR model was assessed by the

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Euclidean distance based method, and was conducted using the AMBIT Discovery (version

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0.04).58

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RESULTS AND DISCUSSION

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Evaluation of the Different Calculation Methods. The reaction of PCAs with ·OH

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proceeds via H-atom abstraction, as the abstraction of Cl atoms has much higher Ea and is

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endothermic.59 For the reaction CH3CCl3+·OH, we considered only one H-abstraction process

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from -CH3, as the three H atoms in -CH3 are equivalent (Figure S1). The Ea values calculated

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from the different methods, together with the experimental values are listed in Table S1. We

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found that the Ea values from the geometries optimized by the TPSSh, B3LYP and B97-1

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functionals are close to those calculated by the more-reliable MP2/6-311+G(d,p) method, and

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are closer to the experimental values25,60-63 than the other functionals under investigation.

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Thus, the TPSSh, B3LYP and B97-1 functionals were employed for geometry optimization

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and frequency analysis of the other two tested PCAs.

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For CH2ClCHCl2 and CH2ClCHClCH3, the possible reaction pathways for the ·OH

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reaction are described in Figure S1. The potential energy surface profiles for the 3 tested

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PCAs above are shown in Figure S2. As H-abstraction by ·OH can occur from -CHCl2 and

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CH2Cl-, CH2ClCHCl2 has three TSs (TS1b, TS2b and TS3b). Similarly, CH2ClCHClCH3 has six

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TSs (TS1c TS2c, TS3c, TS4c, TS5c and TS6c).

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The calculated and experimental25,26, 60-67 kOH values for the 3 PCAs are shown in Figures

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2-4. Several experimental studies reported the kOH values of CH3CCl3,25,60-66 which makes it

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possible to estimate the deviation of the experimental determinations. At 298 K and at 95%

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confidence level, the deviation of experimental kOH values of CH3CCl3 is within a factor of

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0.4-2.8 (detailed in the SI). As for CH2ClCHCl2, only two studies reported the kOH values,25,26 8

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and the biggest ratio between them is 1.7 (T = 295 K).

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Figure 2. Calculated and experimental kOH values for CH3CCl3 over different temperature

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ranges (a), and the enlarged view of (a) at T = 200-330 K is shown in (b).

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Figure 3. Calculated and experimental kOH values for CH2ClCHCl2 over different temperature

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ranges (a), and the enlarged view of (a) at T = 200-330 K is shown in (b).

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Figure 4. Calculated and experimental kOH values for CH2ClCHClCH3 over different

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temperature ranges (a), and the enlarged view of (a) at T = 200-320 K is shown in (b).

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By comparing the calculated with the available experimental values (Figures 2-4), it can 9

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be concluded that the TPSSh functional overestimates the kOH values of CH3CCl3 in the range

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T = 200-720 K except for the experimental kOH value of Chang et al.,61 and underestimates kOH

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values of CH2ClCHClCH3 in the range T = 200-372 K. Thus, the TPSSh functional was

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excluded for its inability for accurately predicting kOH of the PCAs.

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There are no significant differences between the kOH values calculated from the B97-1 and

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B3LYP functionals in the low temperature range (200-330 K). However, in the range of T >

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340 K, the kOH values calculated from the B3LYP functional are generally more consistent

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with the experimental values than those calculated with the B97-1 functional. The kOH data in

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the high temperature range are important in understanding the incineration process of

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PCAs.25,65 Thus, the B3LYP method is more universal than the B97-1 functional for

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predicting kOH in both the low and high temperature ranges. Furthermore, Wang et al.

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calculated the kOH of CH3CH2CH2Cl and CH3CHClCH3, and also concluded that the

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B3LYP/6-311G(d,p) method is reliable for geometries optimization.68 As the studied system

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involves radicals and Cl atoms, inclusion of a diffuse function to the basis set should increase

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the

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M06-2X/6-311+G(3df,2pd)//B3LYP/6-311+G(d,p) method for predicting kOH values of

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SCCPs.

accuracy

of

kOH

prediction.

Thus,

we

selected

the

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The selected method was further verified with the 3 PCAs with C4-C6 carbon chain

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lengths, and the results are listed in Table S2-S5. The deviations between the calculated kOH

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values and the corresponding experimental values are within a factor of 0.6-1.7, 0.4-0.7,

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0.6-0.7,

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CH3(CHCl)2CH3, CH2Cl(CH2)3CH3 and CH2Cl(CH2)4CH3, respectively, which are generally

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acceptable.69,70 Thus, the deviation of the DFT calculated kOH values of the PCAs is estimated

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to be within a factor of 0.4-1.7.

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0.5,

0.4-0.6

and

0.5-0.6

for

CH3CCl3,

CH2ClCHCl2,

CH2ClCHClCH3,

Reactions of the SCCPs with ⋅OH. For convenience, all the atoms of the 9 SCCPs are 10

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numbered (Figure 1). Due to the C1 symmetry of the SCCPs, all the channels of H-abstraction

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for the SCCPs were considered. Similar to the 3 PCAs with low carbon chains, all the

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H-abstraction channels proceed via pre-complexes, TSs, product-like complexes and products

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(Figure S3). The calculated activation free energy (∆G) and Ea for each channel are listed in

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Tables S6-S14.

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By comparing the Ea values of the different H-abstraction channels, we found the channels

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with six-membered ring TSs formed by H-bonds either between H and O or between H and Cl

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atoms, tend to have low Ea and ∆G values. Generally, the conformations with cis-H-C-C-Cl

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sub-structural units facilitate the formation of the six-membered ring TSs. Thus, chlorine

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substitutional positions and steric structures of SCCPs have great effects on the reaction rates

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with ⋅OH. Beyond that, there seem no regularities for the H-abstraction channels. Thus, it is

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necessary to take every H atom into account when calculating the kOH values.

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The DFT calculated kOH values for the 9 SCCPs over the temperature range 200-500 K are

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shown in Figure 5 and Table S15. In general, the kOH values display a negative temperature

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dependence in the low temperature range. The reason can be ascribed to the negative Ea of the

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reactions (Tables S6-S14). In principle, for reaction pathways with negative Ea, the reaction

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rates at low temperatures are mainly controlled by the kinetics, and at high temperatures are

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mainly affected by the thermodynamics.36 The similar temperature dependence of kOH was

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also reported in the reaction of other compounds with ⋅OH.33,35,36,71

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Figure 5. Plot of the DFT calculated kOH values for the 9 SCCPs versus temperature 11

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Driven by curiosity, we compared the DFT calculated kOH values at 298 K with those

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predicted by QSAR models, although the training sets of the QSAR models did not cover

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SCCPs. The QSAR predicted kOH values are all lower than the DFT calculated values. The

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deviations between the DFT calculated kOH values and those predicted by the QSAR models

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of Li et al.,31 Wang et al.,30 Gramatic et al.53 and Roy et al.,55 are within a factor of 3.0-6.8,

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2.8-22.5, 1.2-15.9 and 40.5-397.4, respectively. Thus, it is necessary to develop a new QSAR

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model that covers SCCPs in the applicability domain.

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QSAR Modeling. The following optimum QSAR model for logkOH prediction (T = 298 K) was obtained: logkOH = 5.057SPH + 1.167QC- -13.991η - 12.186

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The compounds and the descriptor values involved in the QSAR model are listed in Table S16.

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For this model, there are 25 PCAs (7 SCCPs) included in the training set. The variable inflation

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factors (VIF) for the predictor variables are all