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Theoretical Investigations on the Mechanism and Kinetics of OH Radical Initiated Reactions of Monochloroacetic Acid Ravichandran Bhuvaneswari, Lakshmanan Sandhiya, and Kittusamy Senthilkumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b03760 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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The Journal of Physical Chemistry

Theoretical Investigations on the Mechanism and Kinetics of OH Radical Initiated Reactions of Monochloroacetic Acid R. Bhuvaneswari, L. Sandhiya and K. Senthilkumar* Department of Physics, Bharathiar University, Coimbatore, India-641 046. *Corresponding author: Fax No: +91-422-2422387, E-Mail: [email protected] ABSTRACT The oxidation mechanism of monochloroacetic acid (CH2ClCOOH) by OH radical has been systematically investigated employing quantum mechanical methods coupled with kinetic calculation using canonical variational transition state theory. Three distinct transition states were identified for the titled reaction, two corresponding to the hydrogen atom abstraction and one corresponding to the chlorine atom abstraction. The rate constant of the titled reactions are computed over the temperature range of 278-350 K and the branching ratios calculated for the hydrogen atom abstraction from –C(O)OH site and –CH2Cl site is 25% and 75%, respectively at 298K. The computed branching ratio indicates that the kinetically favorable reaction is the hydrogen atom abstraction from –CH2Cl site resulting in the formation of CHClC(O)OH radical, which further undergo secondary reaction with O2 and other atmospheric species. The calculated overall rate constant for the hydrogen atom abstraction reactions are in consistent with the reported experimental rate constant. The atmospheric lifetime of CH2ClCOOH is found to be around 18 days.

Keywords: Monochloroacetic acid, atmospheric reactions, OH radical, kinetics, atmospheric lifetime

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INTRODUCTION Haloacetic acids (HAAs) are ubiquitous environmental contaminants and originate from both natural and anthropogenic sources. HAAs are toxic to living organisms and their increase in the concentration presents a threat to the biosphere. HAAs are recognized as potentially carcinogenic and are atmospheric oxidation products of airborne chlorinated C2hydrocarbons.1,2 Presently, the annual global production of the C2-halohydrocarbon is over two million tons.3,4 Chlorofluorohydrocarbons are suggested to be the substitutes for long lived chlorofluorocarbons and are oxidized to various haloacetic acids.5 Monochloroacetic acid (MCAA) is one among the HAA emitted into the atmosphere as a chemical byproduct of chlorination and chloramination of drinking water. The industrial processes where chlorine is used at high concentration, such as paper industry results in the release of MCAA. In addition to industrial production, other anthropogenic source of MCAA is photochemical degradation of certain volatile organochlorines (VOCl).6 Monochloroacetic acid is one of the widely distributed environmental pollutants, found in the atmosphere and in precipitation,7,8 and is partially responsible for a change in global climate. Amongst all of the oxidizing species present in the atmosphere, OH radicals are abundantly present in the troposphere and responsible for the removal of pollutants from the earth’s atmosphere. Hence, OH radical is often termed as an atmospheric cleaning agent. The major degradation of MCAA in the atmosphere is its reaction with photo chemically produced hydroxyl radical. Although a large number of investigations have been done on the reactions of OH radicals with HAA,9 only one experimental study is available for the reactions of MCAA with OH radical in aqueous phase.10 The experimental kinetic study for this reaction was performed using pulse radiolysis by Buxton et al.,10 and the rate constant was measured as 7.14x10-14 cm3molecule-1s-1. Literature survey reveals that there is no detailed experimental and theoretical study performed on reactions of MCAA with OH radical in the gas phase. However, to the best of our knowledge this is the first detailed theoretical study performed on the mechanism and kinetics of the oxidation of MCAA by OH radical in gas phase. As shown in Scheme 1, the primary reaction proceeds through hydrogen atom abstraction from -C(O)OH site, from –CH2Cl (α-carbon ) site and chlorine abstraction from –CH2Cl (α-carbon) site.

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Scheme 1: Initial steps involved in the reaction of MCAA with OH radical

The objective of the present work is to figure out the dominant pathways corresponding to OH initiated oxidation of MCAA using quantum chemical methods. The contribution of individual reaction channels to the overall fate of MCAA is studied by calculating the rate constant and branching ratio for each reaction using transition state theory. From the thermochemical and kinetic data obtained in this work, the radical intermediate formed in the most promising reaction path is identified and is subjected to undergo reaction with the molecular oxygen producing peroxy radical intermediate, the formed peroxy radical would react with NO and HO2 producing alkoxy radical intermediate and a stable product. Hence, in the present study the OH initiated oxidation of MCAA and its subsequent oxidation reactions are explored in detail. COMPUTATIONAL DETAILS The geometry optimization of the species involved in the reactions R1, R2 and R3 were performed using the M06-2X11 and MPW1K12 methods using 6-311+G(d,p) basis set. Previous studies have shown that DFT calculations with the M06-2X functional provides reliable results for thermochemistry and kinetics.13-15 The MPW1K method is a competitive method in predicting the energies and geometries of the stationary points along the minimum energy path and provides promising accuracy for the calculations of reaction energies and kinetics.16 Hence, these methods were employed in the present study. All minima along the potential energy 3



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surface of the studied reactions were identified with zero imaginary frequency and each transition state was identified with one imaginary frequency. Intrinsic reaction coordinate (IRC)17,18 calculations were performed to verify that the transition state connects the required reactant and product at all the above mentioned level of theories. To ascertain the accuracy of the DFT results we have also performed the single point energy calculations at CCSD(T) level with aug-cc-pVDZ basis set based on the geometries optimized at M06-2X/6-311+G(d,p) level of theory. CCSD(T) method is the most popular ab initio method in use today for the application to atmospheric chemistry problems. CCSD(T) was found to be highly accurate and the errors in the relative energy is within 1 kcal/mol.19 The enthalpy of the reaction and Gibbs free energy values were calculated by including thermodynamic corrections to the energy at 298.15 K and 1 atm pressure. The thermochemistry of the reactions are also calculated using highly accurate CBSQB320 and G3B3 methods. Gaussian 0921 program was employed to perform all the electronic structure calculations. The rate constant for the hydrogen abstraction reactions of MCAA with OH radical was calculated using canonical variational transition state theory.22-24 Small curvature tunneling method was used to include the quantum tunneling effect.23,24 The canonical variational transition state (CVT) rate constant at temperature T is given by

k CVT (T ) = min s k c

GT

The generalized transition state (GTS) rate constant k c

kc

GT

(1)

(T , s ) GT

(T , s ) at the dividing surface s is

σk BT Qc GT (T , s) −βV (T , s) = e βh φc R (T )

(2)

MEP ( s )

where, σ is the symmetry factor accounting for the possibility of more than one symmetry R

related reaction path, kB is the Boltzmann constant and h is the Planck’s constant, φc (T ) is the reactants classical partition function per unit volume,

V MEP ( s )

is the classical potential energy at the

GT

point s on the minimum energy path, Qc (T , s) is the classical partition function for the generalized transition state at the dividing surface, s. The quantity ( β h )-1 is called the universal 4


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transition state frequency factor. As k CVT neglects tunneling at low temperature, to account for the dynamical quantum effects of reaction coordinate tunneling, a multiplicative transmission coefficient k (T ) is used in equation 1 as

kc

CVT

(T ) = k (T )k CVT (T )

(3)

The transmission coefficient k (T ) corresponding to tunneling is evaluated by small curvature approximation to the vibrational adiabatic potential energy surface. The equilibrium constant ( Kc , in concentration units) for the reactions in equilibrium is evaluated using the standard formula,25

Kc = K p (R′T )

(4)

RT ln K p = −∆GT0

(5)

where R′ is the ideal gas constant in liter atmosphere units, Kp is the equilibrium constant in pressure units and ∆G 0T is the standard Gibbs free energy at pressure of 1 atm. The kinetic calculations were performed by using GAUSSRATE 2009A26 program, which is an interface program between GAUSSSIAN 0921 and POLYRATE 2010A27 programs.

RESULTS AND DISCUSSION Reaction Mechanism and Pathways As shown in Figure 1, the monochloroacetic acid (MCAA) exists in two forms: cis and trans, the carboxylic hydrogen atom pointing towards the carbonyl group corresponds to cis conformer and the carboxylic hydrogen atom pointing away from the carbonyl group corresponds to trans conformer. Figure 2 shows the potential energy surface scan for monochloroacetic acid (MCAA) as a function of dihedral angles (O2-C1-O1-H1) at M06-2X/6311+G(d,p) level of theory which predicts that cis conformer with Cs symmetry is the most stable conformer by 7 kcal/mol and is in agreement with the previous molecular mechanics28,29 and ab initio studies.30 Therefore the reaction paths R1, R2 and R3 were studied for the cis conformer. The optimized structural parameters of MCAA are summarized in Table S1 along with the values obtained by R.Fausto et al.,29 and Derison et al.31 The results show that the 5



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structure of the titled molecule obtained from the present study is closer to the values reported by R.Fausto et al.,29 and Derison et al.31

Figure 1: The optimized structure of cis and trans conformers of monochloroacetic acid.

Figure 2: Potential energy surface scan along the dihedral angle (O2-C1-O1-H1) at M06-2X/6311+G(d,p) level of theory. 6


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As discussed earlier, the reaction between OH radical and the cis conformer of MCAA is expected to proceed via three different pathways, R1, R2 and R3. Pathway, R1 is initiated by hydrogen atom abstraction from the -C(O)OH site, pathway, R2 is initiated by hydrogen atom abstraction from α-carbon and pathway, R3 is initiated by the chlorine atom abstraction from αcarbon of MCAA. Hydrogen bonded complexes were located on the reactant and product sides of the three reaction pathways, indicating that the reaction between MCAA and OH proceeds via an indirect mechanism through the formation of pre and post-reactive complexes. These reactant complexes were formed due to weak hydrogen bonding interaction between MCAA and OH radical, and the product complexes were formed due to the hydrogen bonding interaction between the radical formed and H2O/HOCl. The optimized geometry of the transition states obtained at M06-2X/6-311+G(d,p) level of theory are shown in Figure 3. The structural parameters of the reactive species obtained from M06-2X and MPW1K functional differ slightly. The average root mean square deviation between the internal coordinates obtained from the two DFT functional for the pathway, R1 is 0.03 Å for the reactant complex (RC1), 0.02 Å for TS1 and 0.04 Å for the intermediate complex (IC1) and the rmsd for the pathway, R2 is 0.2 Å for the reactant complex (RC2), 0.07 Å for TS2 and 0.07 Å for the intermediate complex (IC2). The rmsd for the pathway, R3 is 0.05 Å for the reactant complex, 0.1 Å for TS3 and 0.3 Å for the intermediate complex (IC3). The above mentioned rmsd values include interactions of both bonded and non-bonded atoms.

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Figure 3: Optimized geometry of the transition states (TS1, TS2, TS3, TS4, TS5 and TS6), obtained at M06-2X/6- 311+G(d,p) level of theory. The bond lengths on the structures are in Å. The reaction enthalpies and free energies of the pathways R1, R2 and R3 are calculated using DFT, coupled cluster and composite methods and the results are summarized in Table 1. Earlier studies on atmospheric degradation of species reveal that most of the H atom abstraction processes are exothermic in nature.32-35 In regard with these studies, both the H-atom abstraction 8


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pathways R1 and R2 are exothermic. The H-atom abstraction from the –OH group of MCAA forming a O-centred radical (I1) and H2O (R1) is exothermic by -8.73 and -7.74 kcal/mol at the best level thermochemistry methods CBS-QB3 and G3B3. The H-atom abstraction from the α-C of MCAA forming a C-centered radical (I2) along with H2O (R2) is exothermic by -27.42 and -26.6 kcal/mol at CBS-QB3 and G3B3 methods. Chlorine atom abstraction from α-carbon results in formation of radical intermediate I3 and HOCl in a highly endothermic reaction with the reaction enthalpy of 20.61 and 20.91 kcal/mol at CBS-QB3 and G3B3 methods. As noted from Table 1, in terms of thermochemistry the MPWB1K/6-311+G(d,p) level produces results comparable with the composite methods. The results obtained at M06-2X/6-311+G(d,p) and CCSD(T)/aug-cc-pVDZ levels are in closer agreement with each other, but underestimates the reaction energies obtained using MPWB1K/6-311+G(d,p) level and composite methods. The transition state search along the minimum energy path reveals that TS1 and TS2 are the transition states involved during the hydrogen atom abstraction reactions, R1 and R2 and TS3 is the transition state involved during the chlorine atom abstraction reaction, R3. The important structural parameter that has to be observed during the formation of transition state in the reactions R1 and R2 are the bond lengths r(O1-H1), r(C2-H2) and the newly formed bond between cleaved H/Cl atom and O atom of O-H radical. As shown in Figure3, the breaking (O1H1) bond in TS1 stretches from 0.978 to 1.203 Å and (C2-H2) in TS2 elongated from 1.089 to 1.184 Å. The elongation of the breaking bonds in transition states TS1 and TS2 corresponding to the reactions R1 and R2 is about 23% and 9%, respectively when compared with the corresponding equilibrium bond distances in cis conformer of MCAA. On the other hand, elongation of the newly formed H-O bond in transition states TS1 and TS2 is about 26% (1.206 Å) and 43% (1.367 Å) of the equilibrium bond distance of 0.959 Å in H2O molecule. The results show that for the reactions R1 and R2 the extent of elongation of newly forming H-O bond is greater than the extent of elongation of the breaking bonds which indicates that the transition state is nearer to the corresponding reactant and the reactions proceeds via early transition state structure in accordance with Hammond’s36 postulate applied to an exothermic reaction. In TS3, the breaking (C2-Cl1) bond is elongated by about 26% of the (C-Cl) bond length in the reactant, whereas the length of newly formed (Cl1-O3) bond is about 10% of the (Cl-O) bond in

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HOCl,which envisages that the reaction R3 is an endothermic reaction and is supported by the data summarized in Table 1.

Figure 4: Potential Energy diagram for the reactions, R1, R2 and R3 obtained at M06-2X/6311+G(d,p) level in kcal/mol. The values in the paranthesis are in kcal/mol obtained at MPWB1K/6-311+G(d,p) level of theory.

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Table 1: Relative energies ∆ETot (kcal/mol), enthalpy ∆H298 (kcal/mol) and Gibbs free energy ∆G298 (kcal/mol) for the proposed reaction of MCAA with OH radical calculated at M06-2X, MPWB1K, CCSD(T), CBS-QB3 and G3B3 levels of theory. CCSD(T)/aug-cc-pvDZ//

Stationary points

R+OH

M06-2X/6-311+G(d,p)

MPWB1K/6-311+G(d,p)

CBS-QB3

G3B3

M06-2X/6-311+G(d,p)

∆ETot

∆H298

∆G298

∆ETot

∆H298

∆G298

∆ETot

∆H298

∆G298

∆ETot

∆H298

∆G298

∆ETot

∆H298

∆G298

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

RC1

-9.75

-7.99

0.41

-8.80

-6.42

2.87

-8.92

-7.16

1.24

-6.83

-7.42

1.20

-6.71

-7.30

1.58

TS1

0.69

0.70

10.54

3.48

12.88

22.90

2.28

2.29

12.13

2.23

1.64

11.45

2.52

1.93

11.72

IC1

-7.44

-5.94

1.30

-5.32

-5.47

12.34

-6.63

-5.13

2.11

-4.86

-5.45

-1.94

-10.35

-10.94

-4.16

I1+H2O

-2.43

-3.07

-4.60

-1.76

-6.46

-8.24

-3.26

-3.9

-5.43

-8.74

-8.73

-9.76

-7.74

-7.74

-8.73

RC2

-4.68

-3.36

3.74

-3.26

-6.02

2.12

-3.88

-2.56

4.54

-1.76

-2.35

5.20

-1.65

-2.25

5.45

TS2

2.95

1.50

10.69

4.84

12.57

21.55

3.20

1.75

10.94

1.41

0.81

10.00

2.41

1.82

10.76

IC2

-31.12

-29.46

-22.07

-27.34

-27.42

-28.1

-28.48

-26.82

-19.43

-29.72

-30.31

-22.78

-28.87

-29.46

-21.91

I2+ H2O

-24.86

-25.12

-25.95

-23.34

-27.42

-28.10

-23.04

-23.3

-24.13

-27.42

-27.42

-28.04

-26.59

-26.59

-27.20

RC3

-4.70

-3.39

3.70

-2.24

-2.08

3.50

-3.21

-1.9

5.19

-1.79

-2.38

5.72

-1.80

-2.40

5.01

TS3

43.67

44.92

53.45

49.81

60.00

68.20

41.67

42.92

51.45

38.67

38.07

46.05

42.03

41.43

49.29

IC3

14.59

16.00

23.35

19.64

20.0

37.13

14.78

16.19

23.54

13.54

12.95

20.54

14.19

13.60

21.02

I3+HOCl

25.06

24.94

23.18

28.20

23.78

21.99

24.25

24.13

22.37

20.61

20.61

18.86

20.92

20.91

19.24

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A potential energy surface of the CH2ClCOOH + OH reactions obtained at M06-2X/6311+G(d,p) level is depicted in Figure 4. The energies are plotted with respect to ground state energy of isolated reactants arbitrarily taken as zero. The energy barrier calculated using M062X/6-311+G(d,p) for hydrogen abstraction from O1 (TS1) and C2 (TS2) is 10.5 and 7.6 kcal/mol, whereas the energy barrier for the chlorine atom abstraction from C2 (TS3) is 48.4 kcal/mol. At MPW1K/6-311+G(d,p) level of theory the energy barrier associated with TS1, TS2 and TS3 is 12.3, 8.1, 52 kcal/mol, and at CCSD(T)/aug-cc-pVDZ//M06-2X/6-311+G(d,p) level of theory, the energy barrier is 11.2, 7.1, 44.9 kcal/mol, respectively. The energy barrier calculated from various level of theories for the three reaction channels indicate that hydrogen atom abstraction from the α-Carbon of MCAA is more feasible. The presence of a Cl atom on the α-Carbon site of MCAA reduces the C-H bond energy, thereby leading to the lower energy barrier for the abstraction of H-atom from α-Carbon.37 There is no theoretical (or) experimental data available in the literature to compare the energy barriers associated with the three reaction pathways considered during the present investigation. However, to ascertain the reliability of the calculated values a comparison is made with the single point energy calculation at CCSD(T)/ccpVTZ, M06-2X/cc-pVTZ, MPWB1K/cc-pVTZ based on the geometries optimized at M06-2X/6311+G(d,p) and their values are summarized in Table S3. On comparing the results obtained using DFT methods and CCSD(T) calculations, the most favourable reaction and the nature of reaction mechanism remains unchanged with respect to the methods of calculation. The energy barrier associated with the three reaction pathways are summarized in Table 1. The result show that the energy barrier obtained using M06-2X is comparable with the values obtained using MPW1K and CCSD(T) level of theories. Hence, the geometry and the energetics obtained using M06-2X functional with 6-311+G(d,p) basis set are discussed in detailed and are used in further kinetic calculations. The energetics summarized in Table 1 shows that the hydrogen abstraction from α-carbon of MCAA is both thermodynamically as well as barrierwise favorable producing a radical intermediate I2 along with H2O. As shown in Scheme 2, this radical I2 formed in the primary reaction is prone to undergo reaction with the molecular oxygen present in the atmosphere, producing peroxy radical intermediate, I4. Further, the formed peroxy radical would react with

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NO and HO2 producing alkoxy radical intermediate, I5 and a stable product chlorohydroxyperoxy acetic acid (P).

Scheme 2 : Possible secondary reactions from OH-initiated reactions of MCAA

Theoretical investigations were carried out using M06-2X/6-311+G(d,p) level of theory for the atmospheric degradation mechanism of I2 radical. The optimized geometries of the transition states involved in the secondary reaction is shown in Figure 3. The relative energy profile for the secondary reaction is shown in Figure 5. The relative energy, enthalpy and Gibbs free energy of the reactive species calculated at M06-2X/6-311+G(d,p) level of theory are summarized in Table S4. As presented in Scheme 2, under atmospheric conditions, the energized radical intermediate I2 is expected to react with molecular oxygen producing peroxy radical intermediate I4. Due to the existence of conformational complexity the potential energy surface scan was carried out along the dihedral angle (H2-C2-O3-O4) of the peroxy radical intermediate, I4 using M06-2X/6-311+G(d,p) level of theory and is presented in Figure S4 of supporting information. Two conformers I4 and I4' of the peroxy radical intermediates were identified with the dihedral angle (H2-C2-O3-O4) of -166o and 34o and it is predicted from the scan that I4 is the most stable minimum energy conformer. Therefore the reaction pathways R5 and R6 were studied for the stable conformer I4. A transition state TS4 with a barrier of 4.4 kcal/mol was identified for the formation of peroxy radical I4. The intermediate, I4 is formed exothermically 13



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with the enthalpy of -15.4 kcal/mol and exoergic with free energy of -11.6 kcal/mol. The main chemical fate of the peroxy radical I4 depend on the levels of NO in the atmosphere. When the levels of NO are sufficiently high, the loss of I4 is dominated by the reaction with NO (pathway R5) leading to the formation of alkoxy radical intermediate I5. This intermediate is formed through a transition state TS5 with a barrier of 10.5 kcal/mol, with an exothermicity of -27.6 kcal/mol, and the formation is exoergic with ∆G of -27 kcal/mol. Reactions of organic peroxy radicals with HO2 are of central importance in the atmosphere, as they serve as sinks for HO2 radicals and terminators for ozone-generating chain reactions.38 At lower NO level, the next most significant pathway is the reaction of I4 with hydroperoxy radical producing a stable product chloro-hydoxyperoxy-acetic acid (P) and O2. In order to verify whether the optimized product, P belongs to global minima, we performed the potential energy scan along the dihedral angle (C2-O3-O4-H3, Figure S5). The potential energy surface scan reveals that the product P4, indeed correspond to global minima with the dihedral angle of -90o. This product is formed through the transition state, TS6 with a barrier of 5.1 kcal/mol. The chloro-hydoxyperoxy-acetic acid formation is an exothermic reaction with a reaction enthalpy of -43 kcal/mol and exoergic by -45 kcal/mol. The energy barrier for the reaction of peroxy radical I4 with HO2 is less compared to that of the reaction with NO. That is, the reaction of peoxy radical with HO2 is the dominant exit pathway for the primary reaction resulting in the formation of a stable product P.

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Figure 5:

Relative Energy Profile for the reaction of radical I2 with O2, leading to the

formation of peroxy radical intermediate, I4 and subsequent reactions of I4 with NO and HO2.

KINETICS The rate constant for the hydrogen atom abstraction reactions R1 and R2 are calculated using canonical variational transition state theory (CVT) with small curvature tunneling correction (SCT) (CVT/SCT) in the temperature range from 278 to 350 K with zero point corrected energies, gradients and hessians calculated at M06-2X/6-311+G(d,p) level of theory. The CVT/SCT rate constant for the hydrogen abstraction reactions are designated as kR1 and kR2 15



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and are summarized in Table 2. For comparison, the rate constant calculated using transition state theory (TST), canonical variational transition state theory (CVT), TST with SCT (TST/SCT) and CVT with SCT (CVT/SCT) methods for the reaction pathways R1 and R2 over the temperature range of 278-350 K are summarized in Table S5 and S6. The ratio between CVT with SCT rate constant and CVT rate constant, is of the order of 1, showing a negligible tunneling effect for the reaction pathway R1 and approximately of the order of 101 for the reaction pathway R2, that is for the pathway R2, SCT effect plays an important role at low temperature,

and

it

decreases

with

the

increase

of

temperature.

For

example

k(CVT/SCT)/k(CVT) ratios are 22 at 278 K and 9.6 at 350 K for the reaction pathway R2. Further the ratio between CVT and TST rate constants is in the order of 1, showing negligible variational effect for the studied reaction over the temperature range of 278- 350 K. The rate constant calculated for reaction channel R1 (hydrogen atom abstraction from –C(O)OH site) is found to be 8.18 x 10-14 cm3 molecule

-1 -1

s while that for the reaction channel

R2 (hydrogen atom abstraction from α-carbon site) is found to be 2.42 x 10-13 cm3 molecule -1 s-1 at 298 K and 1 atm, respectively. The overall rate constant for the hydrogen atom abstraction reaction of MCAA by OH radical is obtained by summing up the rate constant corresponding to each reaction channels R1 and R2. The calculated overall rate constant k Overall = 3.24x10-13 cm3 molecule

-1 -1

s is in agreement with the available experimental value. The rate constant for the

similar species39-41 with OH radicals are summarized in Table S7 and are comparable with the present work. Figure 6 shows the Arrhenius plot for the rate constant obtained for the hydrogen abstraction reactions and overall rate constants over the temperature range of 278-350 K, which shows a positive temperature dependence as reported by Daguat et al.42 The branching ratio for the reaction channels that represent their individual contribution towards overall reaction rate constant has been determined using the expression,

ΓR1 / R 2 =

k R1 (or )k R 2 × 100 k

(6)

The branching ratio for reaction channels R1 and R2 in the temperature range of 278 to 350 K are summarized in Table 2. The branching ratio for hydrogen atom abstraction at –C(O)OH group is 25% and at α-carbon is 75% respectively at 298 K. Theoretical and experimental study by De Smedt et al.,43 on the reaction of acetic acid with OH radical showed that the abstraction of 16


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acetic hydrogen atom is dominant over the abstraction of –CH3 hydrogen with a branching ratio of 64

14 % . In the present work methyl hydrogen (-CH2Cl) abstraction is the dominant

pathway with a branching ratio of 74

2 % as the methyl C-H bond strength is weaker than the

acidic O-H bond strength.43,44 The rate constant value reported by De Smedt et al., is 7.8 x 10-13 cm3molecule-1s-1 agrees with the present theoretical study. Experimental study by Butkovskaya et al.,45 reported an overall rate constant of 6.6x10-13 cm3molecule-1s-1 at 298 K. The atmospheric lifetime of any compound which is released into the atmosphere depends on the rate at which it degrades via various physical and chemical processes. The tropospheric lifetime of CH2ClC(O)OH can be estimated by its removal from the troposphere which occurs through the reaction with OH radicals is estimated using the following equation46,47

τ OH =

1 kOH [OH ]

(7 )

where, kOH is the overall rate constant for the degradation of CH2ClC(O)OH by the reaction with OH radical as discussed earlier. Taking the average concentration of OH in the troposphere as 2x 106 molecules cm-3,48,49 the atmospheric lifetime of CH2ClC(O)OH is determined to be 18 days.

17



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Table 2: The calculated CVT/SCT rate constant values (in cm3 molecule-1 s-1) and branching ratio for hydrogen abstraction reactions of MCAA with OH radical

Rate constant (cm3 molecule -1 s-1)

Temperature

Branching ratio (%)

(K)

kR1 x 10-14

kR2 x 10-13

kOverall x 10-13

ΓR1

ΓR2

278

7.55

2.36

3.12

24

76

288

7.87

2.39

3.18

25

75

298

8.18

2.42

3.24

25

75

308

8.50

2.45

3.3

26

74

318

8.82

2.48

3.36

26

74

328

9.14

2.52

3.43

27

73

338

9.45

2.56

3.51

27

73

348

9.77

2.60

3.58

27

73

350

9.84

2.61

3.59

27

73

CONCLUSIONS The mechanism, thermochemistry and kinetics of the gas-phase reactions of CH2ClC(O)OH with OH radicals were investigated for the first time using high level quantum chemical methods and variational transition state theory. Three reaction pathways, two hydrogen atom abstraction and one chlorine atom abstraction reactions, were identified for the title reaction. The results indicate that the hydrogen atom abstraction from α-carbon site of MCAA is the most favourable reaction both kinetically and thermodynamically with a branching ratio contribution of 75%. The secondary reactions were studied with O2, NO and HO2 for the favorable hydrogen atom abstraction pathway (R2). Our results reveal that reaction between peroxy radical intermediate (I4) with HO2 is the dominant exit pathway leading to the formation of product chloro-hydroperoxy-acetic acid. The calculated overall rate constant for the hydrogen atom abstraction reaction of MCAA by OH radical is 3.24x10-13cm3molecule-1 s-1 at 298 K and is in agreement with the available experimental data. Atmospheric life time of CH2ClC(O)OH is estimated as 18 days. 18


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Figure 6: Arrhenius plot for the rate constant obtained for the hydrogen abstraction reaction at –COOH site and –CH2Cl of MCAA over the temperature range of 278-350 K at M06-2X/6311+G(d,p)

19



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Page 20 of 24

ACKNOWLEDGEMENTS The authors thankful to UGC and Department of Science and Technology (DST), India for funding to establish the high performance computing facility under the SAP and PURSE programs. We thank the reviewers for the valuable suggestions to improve the manuscript. SUPPORTING INFORMATION The bond distances, bond angles and dihedral angles of Monochloroacetic acid molecule is summarized in Table S1. The harmonic vibrational frequencies of the reactant complexes, transition states, radical intermediate, product and product complexes obtained at M06-2X/6311+G(d,p) level of theory are summarized in Table S2. IRC plots for the transition states TS1TS6 involved in the OH radical initiated reactions of Monochloroacetic acid are presented in Figures S1-S3. Relative energies ∆ETot (kcal/mol) for the proposed reaction of MCAA with OH radical calculated at M06-2X, MPWB1K and CCSD(T) methods using cc-pVTZ basis set are summarized in Table S3. The relative energy, enthalpy and Gibbs free energy for the reaction pathways R4, R5, R6 are summarized in Table S4. Potential energy surface scan for the peroxy radical intermediate (I4) and chloro-hydoxyperoxy-acetic product (P) are shown in Figure S4S5. The rate constant calculated using Transition state theory (TST), Canonical variational transition state theory (CVT), TST with SCT (TST/SCT) methods for the reaction pathways R1 and R2 over the temperature range of 278-350 K are summarized in Table S5 and S6. Comparison of the rate constant values for the reaction of OH radical with acetic acid and haloacetic acid at 298 K are summarized in Table S7. Optimized geometry of the species involved in primary and secondary reactions obtained at M06-2X/6-311+G(d,p) level of theory are shown in Figure S6. Cartesian Coordinates (Gaussian Standard Orientation) of the reactant, reactant complex, transition, intermediate complexes and intermediates involved in the primary reaction at the M06-2X/6-311+G(d,p) level of theory is given in Section S8.

20


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Graphial Abstract

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Theoretical Investigations on the Mechanism and ... - ACS Publications

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