Computational Study on the Kinetics and Mechanism of the Carbaryl +

Aug 21, 2014 - Postal 73, Cordemex, 97310 Mérida, Yucatán, México. J. Phys. Chem. A , 2014, 118 (36), pp 7776–7781. DOI: 10.1021/jp507244s. Publicatio...
0 downloads 0 Views 839KB Size
Subscriber access provided by UNIV MASSACHUSETTS BOSTON

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

1

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

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.

ACS Paragon Plus Environment

2

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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 cm3—molecule-1—s-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

ACS Paragon Plus Environment

3

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

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

ACS Paragon Plus Environment

4

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

5

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

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.

ACS Paragon Plus Environment

6

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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 kcal—mol-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 kcal—mol-1) than the value computed for H25 (7.7 kcal—mol-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 kcal—mol-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)

ACS Paragon Plus Environment

PC-HA24

7

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

RC-HA25

TS-HA25 (λmin=1358i cm-1)

PC-HA25

RC-HA26

TS-HA26 (λmin=1683i cm-1)

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

ACS Paragon Plus Environment

8

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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 kcal—mol-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).

ACS Paragon Plus Environment

9

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

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 kcal—mol-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

ACS Paragon Plus Environment

10

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

RC-Add1 = RC-Add2

TS-Add2 (λmin=470i cm-1)

P-Add2

RC-Add3 = RC-Add4 = RC-Add5

TS-Add5 (λmin=412i cm-1)

P-Add5

RC-Add10 ≈ RC-Add13

TS-Add10 (λmin=400i cm-1)

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

ACS Paragon Plus Environment

11

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

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 kcal—mol-1. All the abstraction channels are exothermic and exergonic (∆HI = -23.3, ∆HII = 8.5, ∆GI = -23.8, and ∆GII = -10.3 kcal—mol-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 kcal—mol-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.

ACS Paragon Plus Environment

12

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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:

ACS Paragon Plus Environment

13

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

(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.

ACS Paragon Plus Environment

14

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Table 2. Rate coefficients (x 10-12 cm3—molecule-1—s-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

ACS Paragon Plus Environment

15

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

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

ACS Paragon Plus Environment

16

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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 cm3—molecule-1—s-1. The agreement with the available experimental data (3.3 x 10-11 cm3—molecule-1—s-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.

ACS Paragon Plus Environment

17

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

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.

References 1.

Interim Reregistration Eligibility Decision (Ired) Carbaryl. EPA, Ed. Washington,

DC, 2004. 2.

Maggio, R. M.; Damiani, P. C.; Olivieri, A. C., Four-Way Kinetic-Excitation-

Emission Fluorescence Data Processed by Multi-Way Algorithms. Determination of Carbaryl and 1-Naphthol in Water Samples in the Presence of Fluorescent Interferents. Anal. Chim. Acta 2010, 677, 97-107. 3.

Tehrani, M. S.; Givianrad, M. H.; Akhoundi, L.; Akhoundi, M., Preconcentration

and Determination of Carbaryl and Carbofuran in Water Samples Using Ionic Liquids and in Situ Solvent Formation Microextraction. Anal. Methods 2013, 5, 2406-2412. 4.

Miller, P. L.; Chin, Y. P., Photoinduced Degradation of Carbaryl in a Wetland

Surface Water. J. Agric. Food. Chem. 2002, 50, 6758-6765. 5.

Gunasekara, A. S.; Rubin, A. L.; Goh, K. S.; Spurlock, F. C.; Tjeerdema, R. S.,

Environmental Fate and Toxicology of Carbaryl. Rev. Environ. Contam. Toxicol. Vol. 196 2008, 196, 95-121.

ACS Paragon Plus Environment

18

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

6.

van Dijk, H. G.; Guicherit, R., Atmospheric Dispersion of Current-Use Pesticides:

A Review of the Evidence from Monitoring Studies. Water, Air, Soil Pollut. 1999, 115, 21-70. 7.

Sun, F.; Zhu, T.; Shang, J.; Han, L., Gas-Phase Reaction of Dichlorvos, Carbaryl,

Chlordimeform, and 2,4-D Butyl Ester with Oh Radicals. Int. J. Chem. Kinet. 2005, 37, 755-762. 8.

Sun, S.; Zhang, K.; Lu, Y.; Wang, A.; Zhang, H., Theoretical Study on the

Reaction Mechanism of Carbaryl with OH Radicals. J Mol Model 2014, 20, 1-10. 9.

Yang, B.; Meng, J.; Zhang, Y.; Liu, C.; Gan, J.; Shu, J., Experimental Studies on

the Heterogeneous Reaction of NO3 Radicals with Suspended Carbaryl Particles. Atmos. Environ. 2011, 45, 2074-2079. 10.

Seakins, P. W., Product Branching Ratios in Simple Gas Phase Reactions. Ann.

Rep. Prog. Chem. Sect. C 2007, 103, 173-222. 11.

Butkovskaya, N. I.; Kukui, A.; Le Bras, G., Branching Fractions for H2O Forming

Channels of the Reaction of OH Radicals with Acetaldehyde. J. Phys. Chem. A 2004, 108, 1160-1168. 12.

Butkovskaya, N. I.; Setser, D. W., Product Branching Fractions and Kinetic

Isotope Effects for the Reactions of OH and OD Radicals with CH3SH and CH3SD. J. Phys. Chem. A 1999, 103, 6921-6929. 13.

Zhao, Y.; Schultz, N. E.; Truhlar, D. G., Design of Density Functionals by

Combining

the

Method

of

Constraint

Satisfaction

with

Parametrization

for

Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions. J. Chem. Theory Comput. 2006, 2, 364-382.

ACS Paragon Plus Environment

19

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14.

Page 20 of 23

Gonzalez, C.; Schlegel, H. B., Reaction-Path Following in Mass-Weighted

Internal Coordinates. J. Phys. Chem. 1990, 94, 5523-5527. 15.

Zhao, Y.; Truhlar, D. G., Hybrid Meta Density Functional Theory Methods for

Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions: The MPW1B95 and MPWB1K Models and Comparative Assessments for Hydrogen Bonding and Van Der Waals Interactions. J. Phys. Chem. A 2004, 108, 6908-6918. 16.

Zhao, Y.; Truhlar, D. G., Design of Density Functionals That Are Broadly

Accurate for Thermochemistry, Thermochemical Kinetics, and Nonbonded Interactions. J. Phys. Chem. A 2005, 109, 5656-5667. 17.

Zhao, Y.; Truhlar, D. G., Benchmark Databases for Nonbonded Interactions and

Their Use to Test Density Functional Theory. J. Chem. Theory Comput. 2005, 1, 415432. 18.

Zhao, Y.; Gonzalez-Garcia, N.; Truhlar, D. G., Benchmark Database of Barrier

Heights for Heavy Atom Transfer, Nucleophilic Substitution, Association, and Unimolecular Reactions and Its Use to Test Theoretical Methods. J. Phys. Chem. A 2005, 109, 2012-2018. 19.

Zavala-Oseguera, C.; Alvarez-Idaboy, J. R.; Merino, G.; Galano, A., Oh Radical

Gas Phase Reactions with Aliphatic Ethers: A Variational Transition State Theory Study. J. Phys. Chem. A 2009, 113, 13913-13920. 20.

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;

Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Gaussian, Inc.: Wallingford, CT, USA, 2009.

ACS Paragon Plus Environment

20

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

21.

Eyring, H., The Activated Complex in Chemical Reactions. J. Chem. Phys. 1935,

3, 107-115. 22.

Truhlar, D. G.; Hase, W. L.; Hynes, J. T., Current Status of Transition-State

Theory. J. Phys. Chem. 1983, 87, 2664-2682. 23.

Zavala-Oseguera, C.; Ramírez-Manzanares, A.; Merino, G. Eyring 1.0, 1.0;

Guanajuato, GTO, México., 2013. 24.

Corchado, J. C.; Coitino, E. L.; Chuang, Y. Y.; Fast, P. L.; Truhlar, D. G.,

Interpolated Variational Transition-State Theory by Mapping. J. Phys. Chem. A 1998, 102, 2424-2438. 25.

Corchado, J. C.; Chuang, Y.-Y.; Fast, P. L.; Villà, J.; Hu, W.-P.; Liu, Y.-P.; Lynch,

G. C.; Nguyen, K. A.; Jackels, C. F.; Melissas, V. S., et al. Polyrate-Version 9.1, 9.1; University of Minnesota: Minneapolis., 2002. 26.

Isaacson, A. D.; Truhlar, D. G., Polyatomic Canonical Variational Theory for

Chemical-Reaction Rates - Separable-Mode Formalism with Application to Oh+H2H2o+H. J. Chem. Phys. 1982, 76, 1380-1391. 27.

Truhlar, D. G.; Garrett, B. C., Variational Transition-State Theory. Annu. Rev.

Phys. Chem. 1984, 35, 159-189. 28.

Chuang, Y. Y.; Cramer, C. J.; Truhlar, D. G., The Interface of Electronic Structure

and Dynamics for Reactions in Solution. Int. J. Quantum Chem 1998, 70, 887-896. 29.

Zavala-Oseguera,

C.;

Galano,

A.,

Cbs-Qb3+Vtst

Study

of

Methyl

N-

Methylcarbamate Plus Oh Gas-Phase Reaction: Mechanism, Kinetics, and Branching Ratios. J. Chem. Theory Comput. 2009, 5, 1295-1303.

ACS Paragon Plus Environment

21

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30.

Page 22 of 23

Eckart, C., The Penetration of a Potential Barrier by Electrons. Phys. Rev. 1930,

35, 1303-1309. 31.

Jacox, M. E., Vibrational and Electronic Energy Levels of Polyatomic Transient

Molecules. Supplement A. J. Phys. Chem. Ref. Data 1998, 27, 115-393. 32.

Ayala, P. Y.; Schlegel, H. B., Identification and Treatment of Internal Rotation in

Normal Mode Vibrational Analysis. J. Chem. Phys. 1998, 108, 2314-2325. 33.

Alvarez-Idaboy, J. R.; Mora-Diez, N.; Boyd, R. J.; Vivier-Bunge, A., On the

Importance of Prereactive Complexes in Molecule-Radical Reactions: Hydrogen Abstraction from Aldehydes by Oh. J. Am. Chem. Soc. 2001, 123, 2018-2024. 34.

Masgrau, L.; Gonzalez-Lafont, A.; Lluch, J. M., Variational Transition-State

Theory Rate Constant Calculations with Multidimensional Tunneling Corrections of the Reaction of Acetone with Oh. J. Phys. Chem. A 2002, 106, 11760-11770. 35.

Smith, I. W. M.; Ravishankara, A. R., Role of Hydrogen-Bonded Intermediates in

the Bimolecular Reactions of the Hydroxyl Radical. J. Phys. Chem. A 2002, 106, 47984807. 36.

Hansen, J. C.; Francisco, J. S., Radical-Molecule Complexes: Changing Our

Perspective on the Molecular Mechanisms of Radical-Molecule Reactions and Their Impact on Atmospheric Chemistry. Chemphyschem 2002, 3, 833-840.

ACS Paragon Plus Environment

22

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

TOC

ACS Paragon Plus Environment

23