Predicting Gaseous Reaction Rates of Short Chain Chlorinated

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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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Environmental Science & Technology

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

2

Overcoming the Difficulty in Experimental Determination

3

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

4

†

5

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

6

China.

7

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

8

China.

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

9

Table of Contents

10 11

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

15

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

16

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

17

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

18

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

19

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

ACS Paragon Plus Environment


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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

22

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

26

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

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persistence in the atmosphere.

28 29

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

32

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

33

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

34

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

2


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



70

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.

4


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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

98

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

99

structures were then included in further structure calculations followed by dynamic

100

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

103

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



104

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

107

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

110

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

114

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

116

(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

120

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

121

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

132

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

135

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

139

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

141

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

142

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 -

146

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

150

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

153

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

156

0.04).58

157 158

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

165

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

166

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

167

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

168

and frequency analysis of the other two tested PCAs.

169

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

170

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).

174

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

182

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

183 184

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

185

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

186 187

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).

189 190

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

192

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

193

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

194

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

195

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

196

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

197

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

198

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

200

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

201

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

203

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

204

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

205

the

206

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

207

SCCPs.

accuracy

of

kOH

prediction.

Thus,

we

selected

the

208

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

209

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

213

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

214

to be within a factor of 0.4-1.7.

215

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

219

(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

227

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

228

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

229

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

230

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

231

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

233

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

234

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

235 236

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



237 238

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

239

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

240

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

241

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

242

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,

243

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

244

model that covers SCCPs in the applicability domain.

245 246 247

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

248

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

249

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

250

factors (VIF) for the predictor variables are all

Predicting Gaseous Reaction Rates of Short Chain Chlorinated

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