Computational Study on the Kinetics and Mechanism of the Carbaryl +

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A Computational Study on the Kinetics and Mechanism of the Carbaryl + OH Reaction Claudia Zavala-Oseguera, Annia Galano, and Gabriel Merino J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp507244s • Publication Date (Web): 21 Aug 2014 Downloaded from http://pubs.acs.org on August 21, 2014

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The Journal of Physical Chemistry

A Computational Study on the Kinetics and Mechanism of the Carbaryl + OH Reaction Claudia Zavala-Oseguera,1 Annia Galano,2,* Gabriel Merino.3,* 1

Departamento de Química, Universidad de Guanajuato, Noria Alta s/n C.P. 36050, Guanajuato, Gto. México.

2

Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa. San

Rafael Atlixco 186, Col. Vicentina. Iztapalapa. C. P. 09340. México D. F. México. 3

Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados,

Unidad Mérida. Km 6 Antigua Carretera a Progreso. Apdo. Postal 73, Cordemex, 97310 Mérida, Yuc., México.

[email protected] [email protected]

ACS Paragon Plus Environment

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Abstract Carbaryl is released into the atmosphere as a spray drift immediately following the application. In order to evaluate its fate in the atmosphere, a computational study on the kinetics of the OH radical reaction with carbaryl is presented. Different reaction paths are studied at the M05-2X/6-311++G(d,p) level. A complex mechanism involving the formation of a stable reactant complex is proposed and the temperature dependence of the rate coefficients is studied in the 280-650 K temperature range. The principal degradation path is the hydroxyl radical addition to naphthalene, but hydrogen abstractions from the methyl group are identified as a secondary significant path. The rate coefficients, compute using the conventional transition state theory, reproduce quite well the scarce experimental data available.

Keywords: Reaction mechanism; Carbaryl; Rate coefficients; Branching ratios.


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Introduction Carbaryl (1-naphthyl-N-methylcarbamate, 1) is one of the most widely used broadspectrum insecticides in agriculture, professional turf management, ornamental production, nursery, landscapes, golf courses, lawn, and garden markets.1 This molecule has a low molecular weight, moderate solubility in water, and low volatility, hence most studies are mainly focused on carbaryl detection methods, degradation, and transformation in aqueous environment.2-4 However, 1 could become airborne through volatile particles or when it is applied by airblast or aerosol cans.5 Atmosphere is a major receptacle and vehicle for pesticide residues and carbaryl is not exception.6 Once in the atmosphere, 1 is degraded after the reaction with oxidants such as hydroxyl radicals (OH), ozone (O3), or nitrate radicals (NO3), as well as by photolysis. The major loss carbaryl process during daytime is the reaction with OH radicals. Sun et al. reported the experimental rate constant for the reaction between 1 and OH at room temperature and 101 kPa total pressure of (3.3 ± 0.5) x 10-11 cm3molecule-1s-1, but the products were not defined.7 In a very recent paper, Sun et al. studied on paper several paths for the same reaction,8 suggesting that the hydrogen abstraction reactions from the carbamate group are the most important reaction channels. The reaction of 1 with NO3 radicals was also analyzed, but this process is to slow at room temperature to contribute to the carbaryl degradation.9 Herein, we decided to study the mechanism of carbaryl + •OH reaction in the gas phase. The kinetics and mechanism of this reaction are studied using density functional theory (DFT) computations. The conventional transition state theory is used to calculate


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the rate constants in a temperature range of 280-650 K. On the other hand, it is well known that for understanding the fundamental mechanisms of a chemical reaction, the product branching ratios in multi-channel reactions are as important as the overall rate of reaction.10-12 So, we computed the branching ratios for the different channels. Our computations show that the most important contribution to the kinetic constant comes from the OH-addition to the naphthalene ring.

Computational Details Full geometry optimizations are performed using the M05-2X13 functional in conjunction with a 6-311++G(d,p) basis set. The harmonic vibrational analysis is done at the same level to verify the nature of the stationary points and to provide the zero point energy (ZPE), the thermodynamic contributions to the enthalpy and free energy at 298.15 K. Intrinsic reaction coordinate (IRC) computations14 are done at the same level to ensure that the transition states connect to the correct local minima. The M05-2X functional has been optimized to reproduce a large kinetics database that includes both forward and reverse barrier heights and reaction energies. This functional yields good results for thermochemistry and kinetics and also provides excellent geometries for a variety of chemical systems.15-19 All computations are performed using the Gaussian 09 program package.20 The rate constant calculations for all the multichannel reactions are computed using the conventional transition state theory (TST),21-22 which is non-expensive for ab initio computations. For this purpose, the Eyring 0.1 program is used.23 Eyring is a userfriendly application to calculate chemical reaction rates in the gas phase. TST does not


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include the recrossing effects, which are important for reactions with low barriers. The carbaryl + •OH reaction has relative high barriers, so in order to prove that the recrossing effects are negligible in this reaction, the rate constant for the lower path is also computed using direct dynamics with Interpolated Variational Transition-State Theory by Mapping (IVTST-M),24 as implemented in POLYRATE 9.1.25 In the IVTST-M algorithm, rate constant calculations, evaluated by Canonical Variational Theory (CVT)26-28 with Small-Curvature Tunneling (SCT) corrections, are based on reactionpath data. The minimum energy paths (MEP) of the reactions are interpolated from geometries, gradients, and Hessian information of small number of points and fitted to splines under tension as functions of a mapped independent variable that is a nonlinear function of the reaction coordinate. For this path, IVTST-M-4/4 is used, meaning that interpolations are based on the stationary points (reactants, reactant complex, transition state, and products) plus the additional energies and gradients of four non-stationary points along the MEP and also the Hessian information of the same non-stationary points. The efficiency of the IVTST-M algorithm when a small number of points along the reaction path is explicitly included was confirmed in a previous work.29 The tunneling correction to the rate constants of all the reaction paths is considered and computed by the zero-order approximation to the vibrational adiabatic PES with zero curvature. The unsymmetrical Eckart potential energy barrier, as implemented in Eyring, is used to approximate the potential energy curve.30 Since accurate rate constant calculations require the proper computation of the partition functions (Q), the hindered rotor approximation is used to correct the internal rotation partition function with torsional barriers comparable to RT. Direct inspection of


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the low-frequency modes of the stationary points indicates that some of them are related to hindered rotations.31 Eyring program calculates the hindered rotor partition function (QHR) and replaces the vibrational partition function of the corresponding species by the analytical approximation to QHR for one-dimensional hindered internal rotation proposed by Ayala and Schlegel.32

Results and Discussion Figure 1 depicts the most stable conformer of 1, including the atom numbering used throughout the discussion. For the OH radical attack on 1, thirteen different reaction channels were investigated, which can be classified in three principal pathways: the hydrogen abstraction from methyl group, the hydrogen abstraction from NH, and the OH-addition to naphthalene ring. Based on the Sun et al.’ findings,8 the hydrogen transfer reactions from the naphthalene ring were not included.

Figure 1. Carbaryl local minimum conformer.


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I. H-abstraction from CH3. Using the O20 atom as a reference, two possible Habstraction paths from CH3 emerge (see Figure 2).33-34 The abstraction of H23 and H24 is considered as one channel because both have the very similar transition states. The other channel is the abstraction of H25. Given that the CH3 rotational barrier is too small (less than 1.0 kcalmol-1, Table 1), both transition states (TS-HA24 and TS-HA25) have a similar structure. Additionally, there is a strong hydrogen bond between the OH radical and O20. The hydrogen bond in TS-HA25 is 0.048 Å shorter than in TS-HA25, and consequently, the activation free energy (∆GTS) at 298.15 K for the H24-abstraction is smaller (7.3 kcalmol-1) than the value computed for H25 (7.7 kcalmol-1).

II. H-abstraction from NH. Like in pathway I, the transition state (TS-HA26) involves a hydrogen bond between the hydroxyl radical and O18. This hydrogen bond is 0.152 Å longer than that computed in TS-HA25. The activation free energy of this H-abstraction is 3.1 kcalmol-1 higher than the value computed for H25. Thus, path II is not expected to contribute significantly to the carbaryl decomposition.

RC-HA24

TS-HA24 (λmin=1376i cm-1)


PC-HA24

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RC-HA25


PC-HA25

RC-HA26


PC-HA26

Figure 2. Optimized structures of reactant complexes (RC), transition states (TS), and product complexes (PC) from pathways I and II. Bond lengths in Angstroms. The imaginary frequencies are in parentheses.

III. OH-addition to naphthalene carbon atoms. Ten channels corresponding to the OH-addition to the carbon atoms of the naphthalene fragment were considered. Hence, the OH-addition to C1 is labeled as channel Add1, the addition to C2 is Add2, and so on. In Figure 3, the structures of reactant complexes, transition states, and products for the three most viable channels are depicted. The stationary points for the rest of addition channels are reported in the supplementary information (Figure 1S, 2S, and 3S). For the TSs, the distances of the forming C-O bonds are in the range of 1.8672.103 Å. An interaction between the H atom of the hydroxyl radical and O18 is present in the OH-addition to C2, C3, and C4,. Note that the OH-addition to C10 (Add10) is the


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the lowest barrier path, in terms of Gibbs free energy, therefore, this is the most relevant one. Computed barriers of the OH-addition to C2 (Add2), C5 (Add5), and C14 (Add14) indicate that they are also significant. It is well know that the reactant complexes (RC) play an important role in atmospheric bimolecular reactions, so these complexes could have a key influence on the rate constant.35-36 These species were obtained by optimizing the final structures from the IRC computations. In several cases, more than one reaction channel lead to the same RC. Figure 2 depicts RC-HA24, RC-HA25, and RC-HA26 optimized structures. In the first one, three weak interactions are found, two between the O atom from the radical and H23 and H24, and last one between the proton of the OH radical and O20. In RC-HA25, the most important interaction is found between the hydrogen of the hydroxyl radical and the oxygen of the carbonyl group (O20). Both RC structures are almost degenerated; the relative free energy between them is only 0.3 kcalmol-1. For path II, RC-HA26 shows also two interactions, one relatively strong between the H atom from the OH radical and O18, and the second one between the oxygen atom of the hydroxyl radical and H26. In terms of enthalpy, the RCs in pathways I and II are more stable than the separate reactants. However, the stability of these RCs changes, in terms of Gibbs free energy, as a consequence of the unfavorable activation entropies associated with the formation of these adducts (Table 1).


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Table 1. M05-2x/6-311++G(d,p) relative enthalpies (∆H) and free energies (∆G) at 298 K for the carbaryl reaction with OH radical. All energies values are in kcalmol-1. ∆HRC

∆GRC ∆HTS

∆GTS

∆Hrxn

∆Grxn

I HA24

-7.3

0.6

-3.0

7.3

-23.3

-23.8

HA25

-7.9

0.9

-3.4

7.7

-23.3

-23.8

-5.9

2.0

1.7

10.8

-8.5

-10.3

Add1

-8.0

0.8

-1.1

8.6

-21.9

-11.8

Add2

-8.0

0.9

-3.5

7.3

-29.0

-18.1

Add3

-8.3

0.1

5.0

15.6

-4.1

6.5

Add4

-8.3

0.1

5.6

15.9

-3.4

6.6

Add5

-8.3

0.1

-2.2

7.3

-27.0

-17.3

Add6

-4.2

2.6

-1.0

8.6

-21.6

-11.5

Add9

-4.0

3.5

-1.0

8.5

-31.4

-20.9

Add10

-8.5

-1.7

-2.9

6.5

-27.5

-17.9

Add13

-8.6

-0.8

-1.6

8.1

-22.3

-12.4

Add14

-4.0

3.5

-2.4

7.4

-23.8

-14.0

II HA26 III


10

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RC-Add1 = RC-Add2

TS-Add2 (λmin=470i cm-1)

P-Add2

RC-Add3 = RC-Add4 = RC-Add5


P-Add5

RC-Add10 ≈ RC-Add13


P-Add10

Figure 3. Optimized structures of reactant complexes (RC), transition states (TS), and products (P) of Add2, Add5, and Add10 paths. Bond lengths in Angstroms. The imaginary frequencies are in parentheses.

For Add1 and Add2 channels, the RC is the same (Figure 3). For Add3, Add4, and Add5, the IRCs stop in an identical RC, this is also noticed for Add9 and Add14 (see Figure 1S). However, for Add10 and Add13, the RC structures are almost the


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same but the Gibbs free energy is slightly different (Table 1). The reactant complex formation of OH-addition is also an exothermic process, but the formation is disfavored due to the entropic term (an endergonic process), although the RC-Add10 and RCAdd13 formation is slightly exergonic. The product complexes (PC) are also depicted in Figures 2 and 3. Both PCHA24 and PC-HA25 have an interaction involving one hydrogen atom from water and O20. The secondary interaction in PC-HA24 and PC-HA26 involves the oxygen of water and one hydrogen atom of naphthalene. The PCs formation for all the abstraction routes is exothermic and endergonic with values of ∆HHA24 = -29.3, ∆HHA25 = -29.0, ∆HHA26 = 13.6, ∆GHA24 = -21.4, ∆GHA25 = -19.8, and ∆GHA26 = -6.1 kcalmol-1. All the abstraction channels are exothermic and exergonic (∆HI = -23.3, ∆HII = 8.5, ∆GI = -23.8, and ∆GII = -10.3 kcalmol-1), being the H-abstraction from NH the least favored one. Almost all the OH-addition pathways are also exothermic. However, addition to the tertiary carbon atoms of naphthalene (C3 and C4) is endergonic, therefore both of them do not contribute to the total reaction. The most thermodynamic product corresponds to the addition to C9.

Kinetics and Branching Ratios. From Table 1, it is apparent that all paths have barriers higher than 6.5 kcalmol-1. So, the kinetic study was performed using TST and the tunneling factor was computed assuming an unsymmetrical one dimensional Eckart function barrier. Only the Add10 path (the lower barrier path) was also computed using IVTST-M-4/4 with SCT correction.


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It has been assumed that neither mixing nor crossover between different pathways occurs. Thus, the global rate constant (k) from the carbaryl reaction with OH radical is determined as the sum of each rate coefficients path:

kglobal = (kHA24 + kHA25) + kHA26 + (kAdd1 + kAdd2 + kAdd3 + kAdd4 + kAdd5 + kAdd6 + kAdd9 + kAdd10 + kAdd13 + kAdd14)

(1)

For the rate coefficients (k) calculation, a complex mechanism has been proposed for the reaction between 1 and OH (vide supra). It initially involves the barrierless formation of a reactant complex:

carbaryl + OH ⇌ RC

(2)

The second step leads to the formation of the corresponding PC (in the Habstraction cases) or to the products formation:

RC → PC

or

RC → P

(3)

Being Keq the equilibrium constant of the fast pre-equilibrium between the reactants and the RC for the equilibrium reaction (Equation 2) and k the rate constant for the formation of the products or product complexes of equation 3, the overall rate constant for the ith path can be expressed as:


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(4)

ki = Keqk

Herein, all the rate coefficients were calculated taking into account equation (4). In a classical treatment, the overall rate coefficient calculation depends only on the reactants and TS properties. However, when there is a possibility of quantum mechanical tunneling, the existence of the RC might change the tunneling factor. In the present work, the RC and PC (in the H-abstraction channels) have been explicitly considered in the rate constant calculations. The values of the rate coefficients for the most important channels are reported in Table 2 (the others channels are reported in Table 1S). The corresponding value of path Add10 was obtained using both the TST and IVTST-M-4/4 methodologies, with Eckart and SCT tunneling, respectively. In the present work, the relevant paths do not show tunneling effects.


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Table 2. Rate coefficients (x 10-12 cm3molecule-1s-1) for the main reaction paths between carbaryl and the OH radical. T(K)

kHA24

kAdd5

kAdd10

k*Add10

kglobal

280.00

2.64

1.31

5.70

5.36

13.83

290.00

2.36

1.23

5.11

4.80

12.49

296.00

2.20

1.19

4.80

4.52

11.79

298.15

2.16

1.17

4.70

4.42

11.57

310.00

1.92

1.10

4.21

3.96

10.47

320.00

1.76

1.05

3.87

3.64

9.70

340.00

1.50

1.02

3.34

3.14

8.58

360.00

1.36

1.14

3.12

2.94

8.13

380.00

1.49

1.25

3.33

3.14

8.48

400.00

1.60

1.35

3.52

3.32

8.92

450.00

1.87

1.58

3.91

3.68

10.16

500.00

2.08

1.76

4.20

3.94

11.47

550.00

2.26

1.92

4.40

4.14

12.63

600.00

2.41

2.06

4.54

4.29

13.67

650.00

2.54

2.19

4.70

4.43

14.69

*

IVTST-M-4/4

HA24, Add5, Add10, and global: TST


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To quantify the different paths contributions to the overall rate coefficient, the branching ratios (Γ), expressed as percent, were calculated in the temperature range of 280-650 K (Table 3). Note that the major contribution comes from the OH-addition to the naphthalene ring (more than 75%). The contributions of the H-abstraction from the methyl group are smaller but still significant, ranging from 24.6% to 20.0%. In contrast, the H-abstraction from the NH site does not contribute to the OH + carbaryl reaction (Γ < 1%).

Table 3. Branching ratios (Γ) in percentage for the reaction between carbaryl and OH. T (K)

ΓI

ΓII

ΓIII

280.00

24.65

0.04

75.31

290.00

24.25

0.05

75.70

296.00

23.90

0.06

76.04

298.15

23.87

0.06

76.07

310.00

23.35

0.08

76.57

320.00

22.92

0.09

76.99

340.00

21.92

0.13

77.96

360.00

20.68

0.16

79.16

380.00

20.81

0.19

79.01

400.00

20.74

0.21

79.05

450.00

20.82

0.27

78.91


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500.00

20.67

0.46

78.87

550.00

20.50

0.73

78.76

600.00

20.31

1.09

78.60

650.00

20.03

1.51

78.45

Conclusions Different reaction paths for the carbaryl + •OH reaction in the gas phase have been computed at the M05-2x/6-311++g(d,p) level. Based on the computed branching ratios, the addition of the OH radical to the naphthalene ring is the most important process for the reaction between 1 and OH. The OH-addition to the para position to the carbamate group is the main reaction path. On the other hand, the H-abstraction from the NH site is the less favorable path, while the contribution to H-abstraction from the methyl group is small but still significant. The overall rate coefficient for the OH reaction with the carbaryl, at 296 K is computed like 1.17 x 10-11 cm3molecule-1s-1. The agreement with the available experimental data (3.3 x 10-11 cm3molecule-1s-1) is very well.

Supporting information Full author list for references 19, and 24, fully optimized structures (Figure 1S, 2S and 3S), rate coefficients (Table 1S), and Cartesian coordinates of all structures are given. This material is available free charge via the Internet at http://pubs.acs.org.


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Acknowledgements The

authors

gratefully

thank

Conacyt

(Grants

INFRA-2013-01-204586).

Moshinsky Foundation supported the work in Mérida. The CGSTIC (Xiuhcoalt) at Cinvestav is gratefully acknowledged for generous allocation of computational resources. C.Z.-O acknowledges Conacyt for the PhD fellowship.

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Computational Study on the Kinetics and Mechanism of the Carbaryl +

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