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Exploration of Unimolecular Gas-Phase Detoxication Pathways of Sarin and Soman: A Computational Study from the Perspective of Reaction Energetics and Kinetics Tamalika Ash, Tanay Debnath, Tahamida Banu, and Abhijit Kumar Das* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India S Supporting Information *

ABSTRACT: A mechanistic investigation has been carried out to explore all possible gas phase unimolecular isomerization as well as decomposition pathways of toxic organophosphorus compounds (OPCs), namely, sarin (GB) and soman (GD), which are better known as nerve agents. We have identified a total of 13 detoxication pathways for sarin, where the α-H, β-H, and γ-H take part in the H-transfer process. However, for soman, due to the presence of ω-H, three additional detoxication pathways are obtained, where the ω-H is involved in the H-transfer process. Among all the pathways, the D3 decomposition pathway, where the phosphorus oxoacid derivative and alkene are generated via the formation of a six-membered ring in the transition state, is identified as the most feasible pathway from the perspective of both activation barrier and reaction enthalpy values. Moreover, we have studied the feasibility of the isomerization and decomposition pathways by performing the reaction kinetics in the temperature range of 300 K−1000 K using the one-dimensional Rice−Ramsperger−Kassel−Marcus (RRKM) master equation. From the RRKM calculation also, D3 pathway is confirmed as the most feasible pathway for both OPCs. The rate constant values associated with the D3 pathway within the temperature range of 600 K−700 K imply that the degradation of the OPCs is possible within this temperature range via the D3 pathway, which is in good agreement with the earlier reported experimental result. It is also observed that at higher temperature range (∼900 K), the increased rate constant values of other detoxication pathways indicate that along with D3, all other pathways become more or less equally feasible. Therefore, the entire work provides a widespread idea about the kinetic as well as thermodynamic feasibility of the explored detoxication pathways of the titled OPCs. processes such as hydrolysis,5,6 thermal decomposition,7,8 combustion,9 catalytic decomposition,10 and so forth. Degradation of OPCs using solid surfaces such as Brucite,11 MgO(001),12 TiO 2 , 13−18 SiO 2 , 19 Rh(100), 20 Mo(111), 21 Pt(111), 22 Ni(111),23 Pd(111),23 Al2O3,24−26 Fe2O3,27 Y2O3,28 and aluminum-supported iron oxide29 are also performed. In the year 1962, Gustafson et al.10 kinetically studied the decomposition of O-isopropyl methylphosphonofluoridate (sarin) through hydrolysis using copper(II) chelates of diamine as catalysts. Henderson et al.22 studied the adsorption as well as the decomposition of dimethyl methylphosphonate (DMMP) on Pt(111) surface using various spectroscopic techniques. The base-catalyzed hydrolysis of 1,2,2-trimethylpropyl methylphosphonofluoridate (soman) was studied by Ward et al.,5 where the nucleophilic substitution of F− by OH− reveals the way of deactivation of acetylcholinesterase enzyme. Alvim et al.12

1. INTRODUCTION Recent years have witnessed a sharp increase of concern about the organophosphorus compounds1−30 (OPCs), one of the most nefarious synthetic chemical derivatives. The use of OPCs as agricultural chemicals such as insecticides, pesticides, herbicides, etc. is well-known. They are also used as “chemical warfare agents”,3,4 most commonly known as nerve agents. The wellknown nerve agents are tabun (GA), sarin (GB), soman (GD), and VX. These substances became known as nerve agents due to their effects on the disruption of nerve impulses in human beings. They act as potent inhibitors of the enzyme acetylcholinesterase (AChE),31−33 which is responsible for the hydrolysis of the neurotransmitter acetylcholine. The inhibition of the activity of acetylcholinesterase leads to excess accumulation of acetylcholine which may cause death.34,35 These nerve agents are lethal chemical warfare agents selected for their extreme and acute mammalian toxicity via inhalation or skin penetration. The decomposition of OPCs has been of great interest from last several years. OPCs can be decomposed through different © XXXX American Chemical Society

Received: April 20, 2016

A

DOI: 10.1021/acs.chemrestox.6b00132 Chem. Res. Toxicol. XXXX, XXX, XXX−XXX

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Chemical Research in Toxicology

decomposition pathways of sarin and soman, the kinetics study is carried out using RRKM theory. By calculating the rate constants (kT) over a temperature range of 300 K−1000 K, our aim is to predict the feasibility of the detoxication pathways, which is further clarified by calculating the branching ratio values. Overall, the work is intended to provide a fundamental and extensive understanding of the isomerization as well as decomposition pathways of the OPCs by which they can get decomposed into nontoxic compounds.

investigated the hydrolysis of VX-like OPCs (O,S-dimethyl methylphosphonothioate, DMPT) on two types of MgO(001) surfaces, namely, the terrace and the Al-doped surfaces through dissociative chemisorptions using density functional theory (DFT). Templeton et al.25 examined the adsorption and decomposition of diisopropyl methylphosphonate (DIMP), DMMP, and diphenyl methylphosphonate (DPMP) on an aluminum oxide surface via tunneling spectroscopy. Šečkutė et al.36 studied the potential energy diagram for the alkaline hydrolysis of sarin and DMPT using ab initio and DFT methods. As far as our knowledge goes, there are very few theoretical works related to the gas phase unimolecular decomposition of OPCs. Yang et al. 37 investigated the gas phase unimolecular decomposition of DMMP. They mainly studied the effect of method and basis set on the whole reaction mechanism. In our previous work,38 we have also investigated all possible gas phase unimolecular decomposition and isomerization pathways of dimethyl ethylphosphonate (DMEP) using ab initio and DFT molecular orbital theory. To get a clear idea about the detoxication pathways, kinetics study has also been carried out using RRKM theory. Like DMMP and DMEP, sarin and soman are also typical toxic OPCs and known as nerve agents. Because of the toxic nature of sarin and soman, they are harmful to human beings and need to be destroyed. Despite this fact, until now very few works have been done regarding the decomposition of these two OPCs. According to Baier et al.,8 thermal and catalytic decomposition of sarin by means of dialkylation can occur via the formation of a sixmembered ring. They experimentally studied the decomposition of sarin in both borosilicate glass tubes as well as under N2 atmosphere. The main objective of this work is to focus on the annihilation of the toxicity of these OPCs by exploring unimolecular gas phase isomerization and decomposition pathways using high level computational techniques. As depicted in the schematic diagram (Figure 1), sarin and soman, being

2. COMPUTATIONAL DETAILS All of the electronic structure calculations have been carried out using Gaussian 09, revision D.0139 suite of the quantum chemistry program. The geometry optimization of the species involved in the isomerization as well as decomposition pathways, i.e., the reactants, transition states (TSs), and products have been carried out by employing density functional theory (DFT) with the M06-2X40 functional in conjunction with the 6-311++G(d,p)41−43 basis set. The M06-2X, a hybrid metaGGA functional belonging to the M06 family, was developed by Zhao and Truhlar. The M06 family of functionals shows promising performance for the kinetic and thermodynamic calculations without the need to refine the energies by post Hartree−Fock methods. The M06-2X functional is reported to be one of the best functionals to study the main-group thermochemistry, kinetics, and noncovalent interactions. The TS connecting the two minima is obtained by using the Synchronous Transit-guided Quasi-Newton (STQN) method. The normal-mode analyses have been performed at the same level of theory for reactants and products as well as TS geometries, and the minima are characterized with no imaginary frequency, whereas the presence of one imaginary frequency is the characteristic of TS. To confirm whether these TSs connect to the right minima, a parallel intrinsic reaction coordinate calculation (IRC)44,45 has been performed with all TSs. For the prediction of highly accurate energy and thermodynamic parameters, we have reoptimized all the structures, obtained from the M06-2X method, by using the CBS-QB346 method. In the CBS-QB3 method, the geometry optimization and the frequency calculations are carried out using B3LYP/CBSB7, which are further refined by single point CCSD(T) calculations. Moreover, the frequencies used in the CBS-QB3 method are scaled by a factor of 0.99, and these frequencies are further used for thermodynamic calculations. The enthalpies of formation for all of the reactants, TSs, and products at 298.15 K (ΔHf° 298.15) are calculated using CBS-QB3 enthalpies and the required atomization enthalpies of atoms are collected from available database.47 The rate constants, kT (where, T represents the temperature) for the titled reaction pathways are calculated with the microcanonical RRKM theory48−52 by solving a one-dimensional master equation (using Nesbet method of iteration53). The entire kinetic study is performed with ChemRate54 software using the CBS-QB3 geometries, frequencies, and enthalpies of formation. In ChemRate software, the microcanonical rate constant is evaluated by the following equation:

k T = k(E) = l + Figure 1. Schematic diagram of the detoxication processes of sarin and soman.

G+(E) hN (E)

(1)

+

where, G (E) is the number of states of the transition structure having an energy less than or equal to E, N(E) is the density of states of the dissociating or isomerizing molecule, and l+ is the reaction path degeneracy. It should be mentioned that the above-mentioned equation is the reduced form of the two-dimensional master equation which depends on angular momentum. The angular momentum conservation is included approximately via the constant l+ in the one-dimensional master equation used in the Chemrate software. According to Marcus,55 the one-dimensional master equation can predict the high pressure rate constant exactly, but it gives approximate results at low pressure region (P → 0). However, at moderate pressure also the equation gives satisfactory results. During the calculation of kT, the collision energy transfer is treated with an exponential down model with ΔEdown = 500 cm−1, where argon is used as a bath gas with σ = 3.542 Å and ε = 39.95 K (σ and ε are the

structurally analogous, exhibit almost similar types of isomerization and decomposition pathways. The decomposition processes led to the production of various alkenes, ketones, alcohols, HF, etc. along with the phosphorus containing nontoxic compounds, whereas by isomerization processes the toxic OPCs convert into some nontoxic compounds keeping the molecular formula intact. The thermochemical analysis of the species involved in the whole reaction pathways has been performed using CBS-QB3 composite quantum chemical methods. To get a clear idea about the feasibility of the explored isomerization and B


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Figure 2. Optimized geometries of sarin and soman at M06-2X/6-311++G(d,p) level. collision parameters). The effect of quantum mechanical tunneling is incorporated by using Eckart’s tunneling correction.56 The tunneling barrier width is calculated by the nonlinear curve fitting of the IRC curve with Eckart’s potential (eq 2), and the calculated barrier width is used to estimate the tunneling coefficient

V (x) =

e u ⎛⎜ B ⎞⎟ A+ 1 + eu ⎝ 1 + eu ⎠

6, 7, 8, and 9 for both sarin and soman at the M06-2X level of theory. 3.1. Section I. 3.1.1. Isomerization Pathways of Sarin. In this study, we have identified a total of three isomerization pathways (IM1sa, IM2sa, and IM3sa) of sarin, where the reactions take place via intramolecular H-transfer. In the case of the IM1sa pathway, the α-H of the methyl group attached with the P(+5) center is transferred to the -PO through the formation of a four-membered TS (TS1iso sa ), and consequently, the -P-Me is converted into -PCH2 and -PO turns into -P−OH. This isomerization process is mechanistically similar to the keto−enol tautomerization, and as a result, the phospho-enol type (P1sa) product is formed. As evident from Table 1, the activation barrier height for IM1sa pathway is found to be 61.8 kcal/mol for M06-2X and 63.1 kcal/mol for CBS-QB3. The reaction enthalpy values obtained from both methods suggest that the process is highly endothermic in nature. In the IM2sa pathway, the β-H of the alkoxy group is involved in the H-transfer process. The simultaneous conversion of -P O into -P−OH and bridging between Cβ and P(+5) center occur through a well-defined four-membered TS (TS2iso sa ) of barrier height 66.2 kcal/mol (for M06-2X), which leads to the production of a three-membered phospho-epoxy type product (P2sa). As seen from Table 1, the barrier height obtained from the CBS-QB3 calculation (67.0 kcal/mol) almost matches with the M06-2X value. This process is also found to be endothermic in nature as the reaction enthalpy values calculated by both methods show positive values. Although the mechanism of the IM3sa pathway is likely to be similar to IM2sa, the basic difference arises from the perception of H-transfer sources. Unlike the previous one, here the γ-H of the alkoxy group is transferred to -PO instead of β-H. In this process also, the -PO becomes -P−OH and Cγ forms a bond with P(+5) through a four-membered TS (TS3iso sa ) of activation barrier 61.9 and 63.3 kcal/mol for M06-2X and CBS-QB3, respectively. In this isomerization process, a four-membered phospho-epoxy type product (P5sa) is obtained, for which the reaction enthalpy value is also positive for both methods. From Figures 6 and 7, it is apparent that both the phosphoepoxy type products obtained from IM2sa and IM3sa pathways further undergo decomposition through the breakage of threeand four-membered phospho-epoxy rings, respectively. In the second step of the IM2sa pathway, the three-membered phosphoepoxy ring (P2sa) is decomposed into phospho-alcohol (P3sa) and acetone (P4sa) via a three-membered TS (TS2′iso sa ), where the P-center is reduced to the +3 oxidation state in P3sa. For the IM3sa pathway, the four-membered phospho-epoxy ring (P5sa)

(2)

where u = 2πx/l, A = E1 − E−1, and B = (E1 + E−1 ) . The constants, E1 and E−1 represent the barrier heights relative to the reactants and products, respectively. In our calculation, the kT is calculated from 300 to 1000 K with an interval of 100 K, keeping the pressure constant at 1 atm. During the calculation of kT, we have also determined the branching ratios (b.r.) for each pathway at a particular temperature. The calculated rate constants obtained at different temperatures are fitted using a modified form of the Arrhenius equation, and the Arrhenius parameters, i.e., the pre-exponential factor (A) and n, are calculated for all reaction pathways. 1/2

⎛ E ⎞ k T = AT nexp⎜− a ⎟ ⎝ RT ⎠

1/2 2

(3)

3. RESULTS AND DISCUSSION We have divided the entire Results and Discussion into two sections. In the first section, we have explored all possible uncatalyzed unimolecular isomerization and decomposition pathways of sarin (GB) and soman (GD) in gas phase and finally interpreted the feasibility of the explored pathways kinetically in the second section. It is noteworthy that most of the isomerization and decomposition pathways unveiled here for sarin are identical to those of soman due to their structural resemblance. However, some additional pathways are also obtained for soman because of the presence of ω-H. After a thorough analysis of the optimized structures obtained from both M06-2X and CBS-QB3 methods, it is noticed that though the geometrical parameters differ slightly on varying the methods, overall the structures look almost same. Thus, only the optimized geometries of the reactants obtained from M06-2X calculation are given in Figure 2. All of the detoxication pathways of sarin and soman, along with their corresponding TS and product geometries obtained from M06-2X calculation, are given in Figures 3 and 4, respectively. In Tables 1 and 2, the activation barrier heights and the reaction enthalpy values calculated at both M06-2X and CBS-QB3 levels for each detoxication pathway of sarin and soman, respectively, are given by mentioning the respective detoxication pathways along with their associated TSs and products. The potential energy surfaces (PESs) for the aforementioned detoxication pathways are depicted in Figures 5, C


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Figure 3. continued

D


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Figure 3. All of the isomerization and decomposition pathways of sarin with their corresponding optimized geometries of TSs and products at the M062X/6-311++G(d,p) level.

Analyzing all these isomerization pathways, it is observed that in the first step all of them lead to the production of fourmembered rings in the TS and that the activation barriers calculated by both methods are within 60−70 kcal/mol. The sources of H-transfer are different for each case, as in IM1sa the αH of methyl is transferred to form a phospho-enol type product, whereas for the other two, phospho-epoxy type products are formed through β- and γ-H transfers, respectively. The oxidation

splits into the phosphorus oxoacid derivative (P6sa) and propene (P7sa) through the formation of a well-defined four-membered TS (TS3′iso sa ). The second step of both IM2sa and IM3sa pathways are exothermic in nature as the calculated reaction enthalpy values are negative at both M06-2X as well as CBS-QB3 methods. The noticeable thing obtained for the IM2′sa pathway is the significant difference in the reaction enthalpy values between the two methods. E


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Figure 4. continued

F


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Figure 4. continued

G


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Figure 4. All of the isomerization and decomposition pathways of soman with their corresponding optimized geometries of TSs and products at the M06-2X/6-311++G(d,p) level.

state of phosphorus remains unchanged in the first step for all of these isomerization pathways. However, for the IM2sa pathway, in the second step, further decomposition of P2sa into P3sa leads

to the reduction of the oxidation state of phosphorus from +5 to +3. From the above discussion, it is clear that all of the three isomerization pathways are kinetically as well as thermodynamiH


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Table 1. Activation Barrier Heights (ΔEa) and Reaction Enthalpy (ΔHrxn) Related to Different Isomerization and Decomposition Pathways of Sarin at M06-2X and CBS-QB3 Levels in kcal/mol M06-2X

CBS-QB3

reaction pathways

transition states

products

ΔEa

ΔHrxn

ΔEa

ΔHrxn

IM1sa IM2sa IM3sa D1sa D2sa D3sa D4sa D5sa D6sa D7sa D8sa D9sa D10sa IM2′sa IM3′sa D5′sa D6′sa

TS1iso sa TS2iso sa TS3iso sa TS1decomp sa TS2decomp sa TS3decomp sa TS4decomp sa TS5decomp sa TS6decomp sa TS7decomp sa TS8decomp sa TS9decomp sa TS10decomp sa TS2′iso sa TS3′iso sa TS5′decomp sa TS6′decomp sa

P1sa P2sa P5sa P8sa + P9sa P3sa + P4sa P6sa + P7sa P10sa + HF P11sa + HF P13sa + HF P4sa + P12sa + HF P7sa + P14sa + HF P15sa + methane P16sa + ethane P3sa + P4sa P6sa + P7sa P4sa + P12sa P7sa + P14sa

61.8 66.2 61.9 69.7 87.2 39.7 70.5 84.3 84.6 108.3 58.2 89.2 140.6 11.1 37.5 26.1 33.5

38.6 45.3 25.0 60.9 31.3 14.7 65.8 54.9 39.2 70.4 57.0 73.0 87.1 −14.0 −10.2 15.6 17.8

63.1 67.0 63.3 72.8 91.9 39.1 76.3 86.9 88.2 111.1 62.6 87.4 140.2 14.6 34.0 32.8 30.4

41.8 48.4 29.9 67.2 45.9 16.7 70.5 57.3 42.3 83.8 57.5 72.5 86.6 −2.6 −13.2 26.5 15.2

Table 2. Activation Barrier Heights (ΔEa) and Reaction Enthalpy (ΔHrxn) Related to Different Isomerization and Decomposition Pathways of Soman at M06-2X and CBS-QB3 Levels in kcal/mol M06-2X

CBS-QB3

reaction pathways

transition states

products

ΔEa

ΔHrxn

ΔEa

ΔHrxn

IM1so IM2so IM3so IM4so D1so D2so D3so D4so D5so D6so D7so D8so D9so D10so D11so D12so IM2′so IM3′so IM4′so D5′so D6′so D11′so

TS1iso so TS2iso so TS3iso so TS4iso so TS1decomp so TS2decomp so TS3decomp so TS4decomp so TS5decomp so TS6decomp so TS7decomp so TS8decomp so TS9decomp so TS10decomp so TS11decomp so TS12decomp so TS2′iso so TS3′iso so TS4′iso so TS5′decomp so TS6′decomp so TS11′decomp so

P1so P2so P5so P17so P8so + P9so P3so + P4so P6so + P7so P10so + HF P11so + HF P13so + HF P4so + P12so + HF P7so + P14so + HF P15so + methane P16so + isobutane P20so + HF P21so + neopentane P3so + P4so P6so + P7so P3so + P18so + P19so P4so + P12so P7so + P14so P12so + P18so + P19so

64.4 68.0 61.6 63.1 69.4 86.6 39.8 72.3 85.6 83.5 110.4 59.1 90.4 89.0 78.7 145.8 10.4 27.2 55.0 24.4 33.4 77.6

39.1 46.0 34.5 28.2 60.8 33.2 15.4 67.4 57.1 38.4 72.4 57.4 73.9 73.6 24.0 86.6 −12.7 −19.2 33.2 15.2 19.0 76.5

67.8 69.3 64.1 66.6 75.1 92.1 39.7 79.1 88.5 88.0 113.9 62.6 88.8 87.1 84.1 145.8 14.9 34.8 62.8 31.4 30.7 84.0

42.5 49.7 29.5 33.0 68.4 48.2 18.3 73.6 60.2 42.3 86.1 58.9 73.8 79.2 28.5 87.1 −1.6 −11.2 41.9 25.9 16.6 84.4

ring. The barrier height associated with this conversion is calculated to be 69.7 kcal/mol for the M06-2X method, whereas for CBS-QB3, the respective value is 72.8 kcal/mol. The endothermic nature of this decomposition pathway is also evident from the reaction enthalpy values collected in Table 1. In the D2sa decomposition pathway, sarin gets decomposed to phospho-alcohol (P3sa) and acetone (P4sa) through the ), formation of a five-membered ring in the TS (TS2decomp sa where the barrier height is 87.2 kcal/mol for M06-2X and for CBS-QB3 the respective value is 91.9 kcal/mol. Here, the β-H is transferred to -PO to form -P−OH, and simultaneously the phospho alkoxy (−P-O) bond breaks to produce P4sa. This

cally disfavored at room temperature due to their high activation barriers as well as high endothermic nature. 3.1.2. Decomposition Pathways of Sarin. The exploration of all possible decomposition pathways of sarin is another aspect of our work. Here, we have identified a total of 10 decomposition pathways of sarin. The D1sa decomposition pathway is associated with the transfer of α-H of the methyl group to the oxygen of the alkoxy group, which leads to the formation of a phospho-ketene type ), the product (P8sa) and propanol (P9sa). In the TS (TS1decomp sa simultaneous transfer of α-H and the breakage of the phosphoalkoxy bond occur through the formation of a four-membered I


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Figure 5. Potential energy surface (PES) for the isomerization and decomposition pathways of both sarin and soman for α-H transfer at the M06-2X/6311++G(d,p) level. Relative energy values are given in kcal/mol.

Figure 6. Potential energy surface (PES) for the isomerization and decomposition pathways of both sarin and soman for β-H transfer at the M06-2X/6311++G(d,p) level. Relative energy values are given in kcal/mol.

decomposition pathway is highly endothermic in nature, and the calculated reaction enthalpy value in M06-2X is 31.3 kcal/mol,

which is 14.6 kcal/mol lower than the CBS-QB3 calculated reaction enthalpy. So, both the kinetic as well as thermodynamic J


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Figure 7. Potential energy surface (PES) for the isomerization and decomposition pathways of both sarin and soman for γ-H transfer at the M06-2X/6311++G(d,p) level. Relative energy values are given in kcal/mol.

Figure 8. Potential energy surface (PES) for the isomerization and decomposition pathways of soman for ω-H transfer at the M06-2X/6-311++G(d,p) level. Relative energy values are given in kcal/mol.

K


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Figure 9. Potential energy surface (PES) of D9sa and D10sa pathways of sarin and D9so, D10so, and D12so pathways of soman at the M06-2X/6-311+ +G(d,p) level. Relative energy values are given in kcal/mol.

As portrayed in Figure 5, the D4sa pathway corresponds to the production of a phospho-ketene product (P10sa) via the removal of HF, where the α-H of methyl forms a bond with fluorine to produce HF. In this pathway, the reaction takes place via the ) of activation formation of a four-membered TS (TS4decomp sa barrier 70.5 and 76.3 kcal/mol for M06-2X and CBS-QB3, respectively. The endothermic nature of this pathway is also evident from its ΔHrxn value, which is highly positive for both methods. In the D5sa decomposition pathway (see Figure 6), the β-H of the alkoxy group and fluorine become bonded with each other to form HF, and accordingly, Cβ attaches with the P(+5) center to generate a three-membered phospho-epoxy type product (P11sa), where the phosphorus remains in the +5 oxidation state. Here also, a well-defined four-membered TS ) is detected, where the barrier height is significantly (TS5decomp sa high (84.3 and 86.9 kcal/mol for M06-2X and CBS-QB3, respectively). Further decomposition of P11sa, obtained from the first step of the D5sa pathway, leads to the formation of OPCH3 (P12sa) and acetone (P4sa) via a three-membered TS ), in which the phosphorus center gets reduced from (TS5′decomp sa +5 to +3. When the reaction enthalpy of this step is considered, it is observed that the value of reaction enthalpy calculated with the CBS-QB3 method is significantly higher than the M06-2X value. Now looking at the D6sa pathway shown in Figure 7, it is noticed that the mechanism of this pathway is almost similar to the D5sa pathway; the only difference is the participation of γ-H instead of

parameters indicate that the process is unfeasible at room temperature. From Figure 7, it is apparent that in the D3sa decomposition pathway the reactant sarin passes through the formation of a well), where the barrier height is defined six-membered TS (TS3decomp sa 39.7 kcal/mol for M06-2X. In the case of CBS-QB3 method, the calculated barrier height (39.1 kcal/mol) is almost identical to the M06-2X one. From the mechanistic viewpoint, it is noticed that in this decomposition pathway the γ-H takes part in the Htransfer process to produce -P−OH from -PO and that concurrently the -Cβ-O bond breaks to generate the phosphorus oxoacid derivative (P6sa) and propene (P7sa). This pathway is also endothermic in nature, but the endothermicity value (ΔHrxn = 14.7 kcal/mol for M06-2X and 16.7 kcal/mol for CBS-QB3) is the lowest among all other detoxication pathways for both methods. Interestingly, in 1967 Baier et al.8 proposed a similar mechanism while investigating the decomposition of sarin experimentally. Therefore, it is also manifested from our study that the mechanism of the explored D3sa pathway is in good agreement with the mechanism proposed earlier by Baier et al.8 Among the explored decomposition pathways, there are several pathways (D4sa, D5sa, D6sa, D7sa, and D8sa) where the decompositions occur through the elimination of HF as a side product. All of the pathways are different from each other not only from the perspective of energetics but also from a mechanistic viewpoint. L


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Chemical Research in Toxicology β-H. Here, the γ-H of the alkoxy group participates in the bond formation with fluorine during the removal of HF, and simultaneously, Cγ connects with P(+5) to form a fourmembered hetero cyclic phospho-epoxy type (P13sa) product. In this case also, the bond formation and bond breaking take place through the formation of cyclic four-membered TS (TS6decomp ), where the associated barrier height is 84.6 and sa 88.2 kcal/mol for M06-2X and CBS-QB3, respectively. Further ) decomposition of P13sa via the four-membered TS (TS6′decomp sa produces phosphorus containing diketone (P14sa) and propene (P7sa) as products. Both the D5sa and D6sa pathways are characterized as endothermic processes. Analyzing the mechanism of the D7sa pathway, it is apparent that the removal of HF takes place through a five-membered TS ) and that OP−CH3 (P12sa) and acetone (P4sa) are (TS7decomp sa obtained as products along with HF. The associated barrier height obtained from both the methods is too high to make the reaction kinetically feasible. Here also, the discrepancy of reaction enthalpy value between the two methods is noticeable. Therefore, it can be affirmed that in the pathways, where the phosphorus center is reduced from its +5 oxidation state to +3, the method sensitivity arises during the calculation of reaction enthalpy values. For the D8sa pathway, it is noticed that the γ-H of the alkoxy group forms HF through the six-membered TS (TS8sadecomp) and that concomitantly diketone (P14sa) and propene (P7sa) are formed. Here, the barrier height is 58.2 for M06-2X and 62.6 kcal/mol for CBS-QB3, which is the lowest among all HF removal pathways in the respective methods. Therefore, from a kinetic viewpoint, this pathway is the most feasible among all the HF removal pathways. Similar to all other decomposition pathways, this pathway is also characterized as the endothermic one, where the value of ΔHrxn is 57.0 kcal/mol at the M06-2X level. There are two decomposition pathways (D9sa, D10sa) where the basic structure of the OPC remains unaffected. In the D9sa process, the β-H transfers to the Cγ, and simultaneously, the CβCγ bond breaks to form P15sa along with the by-product methane. Like D9sa pathway, D10sa pathway also follows a similar mechanism, where ethane gets removed as a by-product and P16sa is obtained as the main product. In both cases, the product formation takes place via three-membered TS, where the TSs are designated as TS9decomp and TS10decomp . Both the M06-2X and sa sa CBS-QB3 calculations show that the pathways are not kinetically feasible at room temperature due to their very high barrier heights. The associated ΔHrxn values for D9sa and D10sa pathways, shown in Table 1, also indicate their thermodynamic unfeasibility. 3.1.3. Isomerization Pathways of Soman. Although most of the isomerization pathways (IM1so, IM2so, and IM3so) generated from soman are mechanistically similar to sarin, one additional isomerization pathway is obtained for soman due to the presence of ω-H. The barrier heights associated with IM1so, IM2so, and IM3so isomerization processes are 64.4, 68.0, and 61.6 kcal/mol for M06-2X and 67.8, 69.3, and 64.1 for CBS-QB3, respectively. The IM4so isomerization pathway is the newly introduced pathway for soman, where the ω-H participates in the H-transfer process. In this pathway, the -PO is converted into -P−OH, and Cω forms bond with P(+5) to produce a five-membered phospho-epoxy type product (P17so) via a four-membered TS (TS4iso so ). The associated activation barrier for this conversion is 63.1 and 66.6 kcal/mol for M06-2X and CBS-QB3, respectively. The pathway is endothermic, where the ΔHrxn value is 28.2 kcal/ mol at M06-2X. Apart from the H-transfer sources, the basic

mechanism of IM4 so is similar to the IM2 so and IM3 so isomerization processes. In the second step, the isomerized product, P17so is further decomposed into P3so, isobutene (P18so), and acetaldehyde (P19so). Like sarin, for soman also, the activation barriers of all the isomerization pathways are within 60−70 kcal/mol, and due to significantly high reaction enthalpy values, the pathways are not feasible at 298 K. 3.1.4. Decomposition Pathways of Soman. Like the isomerization pathways, for decomposition also, most of the pathways (from D1so to D9so) of soman match well with the decomposition pathways of sarin from the viewpoint of reaction mechanisms. In case of the D10so pathway, unlike sarin, isobutane is removed from soman by keeping the basic structure of the OPC unaltered. The additional decomposition pathways explored for soman are D11so and D12so, where the ω-H actively participates in the reaction mechanism. The D11so decomposition pathway is also categorized as a HF removal process. In this pathway, the Cω-H bond breaks, and consequently, ω-H forms a bond with fluorine; finally, the fivemembered phospho-epoxy type product (P20so) is produced along with HF. As tabulated in Table 2, the barrier height for this decomposition pathway is calculated to be 78.7 and 84.1 kcal/ mol for M06-2X and CBS-QB3, respectively, and the associated TS (TS11decomp ) for this pathway is characterized by a fourso membered ring. Like all other decomposition pathways, this process is also endothermic in nature, and the calculated ΔHrxn is 24.0 kcal/mol for M06-2X. The P20so product further undergoes decomposition to form P12so, P18so, and P19so products. However, the D12so pathway looks similar to the D9so and D10so pathways, where the basic structures of the OPCs remain unchanged. In the case of D9so and D10so pathways, methane and isobutane are eliminated as by-products, whereas in the case of D12so, neopentane is eliminated along with the formation of ) is very P21so. The barrier height associated with TS (TS12decomp so high for this reaction pathway. Therefore, from the entire study of soman, it can be concluded that the D3so decomposition pathway is the most feasible among all the decomposition and isomerization processes not only from the perspective of activation barrier but also from the viewpoint of reaction enthalpy. Similar to sarin, for soman also, the significant discrepancy of reaction enthalpy values between the M06-2X and CBS-QB3 methods arises when the phosphorus center is reduced from the +5 oxidation state to +3. 3.1.5. Pathway Analyses Based on the H-Transfer Sources. On the basis of H-transfer sources (α-H, β-H, and γ-H), we have classified all of the isomerization and decomposition pathways of sarin into three categories. In the case of soman, along with all these aforementioned H-transfer sources, an additional Htransfer takes place due to the presence of ω-H. In the following section, all of these detoxication pathways are categorized on the basis of their H-transfer sources for both sarin and soman. 3.1.5.1. α-H source. As evident from the PES depicted in Figure 5, there are a total of three detoxication pathways where the α-Hs are involved in the H-transfer process, and those are IM1sa, D1sa, and D4sa for sarin. Comparing all of these three pathways, it is evident that the IM1sa pathway is the most feasible among all of the α-H transfer pathways for both methods. In the case of decomposition pathways, the HF removal pathway, i.e., D4sa experiences a considerably higher energetic barrier. In the case of soman also, IM1so, D1so, and D4so pathways are characterized as the α-H transfer processes. Similar to sarin, here also, the IM1so is found to be the most feasible pathway, whereas the D4so is detected as the least feasible one. M


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Chemical Research in Toxicology Table 3. Values of Rate Constants (in s−1) for Different Detoxication Pathways Obtained from RRKM Calculation in a Temperature Range of 300 K−1000 K for Sarin reactions IM1sa

IM2sa

IM3sa

D1sa

T 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000

kT

log kT −32

1.50 × 10 4.93 × 10−21 4.48 × 10−14 2.08 × 10−09 4.66 × 10−06 1.56 × 10−03 1.47 × 10−01 5.61 × 1000 2.99 × 10−36 6.06 × 10−24 1.61 × 10−16 1.52 × 10−11 5.64 × 10−08 2.74 × 10−05 3.43 × 10−03 1.64 × 10−01 5.00 × 10−34 1.77 × 10−22 1.63 × 10−15 7.52 × 10−11 1.66 × 10−07 5.49 × 10−05 5.07 × 10−03 1.91 × 10−01 1.77 × 10−41 4.00 × 10−28 4.36 × 10−20 1.03 × 10−14 7.26 × 10−11 5.67 × 10−08 1.02 × 10−05 6.42 × 10−04

reactions

−31.82 −20.31 −13.35 −8.68 −5.33 −2.81 −0.83 0.75 −35.52 −23.22 −15.79 −10.82 −7.25 −4.56 −2.47 −0.79 −33.30 −21.75 −14.79 −10.12 −6.78 −4.26 −2.29 −0.72 −40.75 −27.40 −19.36 −13.99 −10.14 −7.25 −4.99 −3.19

D3sa

D4sa

D8sa

3.1.5.2. β-H Source. Analyzing all of the detoxication pathways of sarin, it is observed that the IM2sa, D2sa, D5sa, and D7sa pathways occur via the intramolecular β-H transfer. As depicted in Figure 6, in this case also the IM2sa is found to be the most favorable one among the aforementioned β-H-participated pathways for both methods. It is also clear that due to very high activation barriers (>85.0 kcal/mol), all of the decomposition pathways (D2sa, D5sa, and D7sa) are not kinetically feasible. So, the detoxication of the studied OPCs via β-H elimination can only be achieved through IM2sa, which further undergoes degradation through the cleavage of a three-membered phospho-epoxy ring. For soman, the participation of the β-H is observed for IM2so, D2so, D5so, and D7so pathways, among which the IM2so pathway experiences the least activation barrier for both methods. Hence, like sarin, for soman also, the detoxication via β-H transfer exclusively proceeds through the IM2so pathway. 3.1.5.3. γ-H Source. The involvement of γ−H is detected in IM3sa, D3sa, D6sa, and D8sa pathways of sarin (Figure 7). By analyzing their activation barriers, it is found that D3sa is the most feasible pathway among all γ−H transfer pathways for both methods. The existence of the D6sa decomposition pathway is insignificant due to its high activation barrier (>80 kcal/mol). It is also observed that the detoxication via γ-H transfer is comparatively more feasible than other H-transfer processes as most of the γ-H transfer processes experience 80 kcal/mol are not feasible at all. Thus, only the pathways for which the barrier height is ≤80 kcal/mol are considered for the RRKM study. As mentioned in section 2, the standard enthalpy of formation, coordinates, and frequencies of all the species involved in the RRKM study are obtained from the CBS-QB3 optimization process. As mentioned earlier, for both sarin and soman, we have determined the values of the rate constants (kT) for the selected N


Article

Chemical Research in Toxicology Table 4. Values of Rate Constants (in s−1) for Different Detoxication Pathways Obtained from RRKM Calculation in a Temperature Range of 300 K−1000 K for Soman T

reactions IM1so

kT 1.37 × 10 2.66 × 10−24 7.18 × 10−17 6.94 × 10−12 2.64 × 10−08 1.31 × 10−05 1.68 × 10−03 8.28 × 10−02 3.53 × 10−38 1.75 × 10−25 7.94 × 10−18 1.07 × 10−12 5.14 × 10−09 3.04 × 10−06 4.42 × 10−04 2.40 × 10−02 1.53 × 10−34 7.63 × 10−23 8.67 × 10−16 4.62 × 10−11 1.14 × 10−07 4.09 × 10−05 4.04 × 10−03 1.62 × 10−01 3.09 × 10−37 3.68 × 10−25 6.84 × 10−18 5.01 × 10−13 1.54 × 10−09 6.51 × 10−07 7.31 × 10−05 3.23 × 10−03

300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000

IM2so

IM3so

IM4so

log kT −36

reactions

−35.86 −23.58 −16.14 −11.16 −7.58 −4.88 −2.77 −1.08 −37.45 −24.76 −17.10 −11.97 −8.29 −5.52 −3.36 −1.62 −33.82 −22.12 −15.06 −10.34 −6.94 −4.39 −2.39 −0.79 −36.51 −24.43 −17.16 −12.30 −8.81 −6.19 −4.14 −2.49

D1so

D3so

D4so

D8so

isomerization as well as decomposition pathways for a temperature range of 300 K−1000 K, keeping the pressure fixed at 1 atm. During the RRKM rate calculations, the values of critical parameters (temperature and pressure) and acentric factor predicted by Sokkalingam et al.57 are used to calculate the σ and ε values for sarin and soman. The values of rate constants for different detoxication pathways obtained from the RRKM calculation at different temperatures are given in Tables 3 and 4 for sarin and soman, respectively. The values of Arrhenius parameters are tabulated in Table 5.

sarin A

IM1sa IM2sa IM3sa D1sa D3sa D4sa D8sa

9.55 × 10 1.48 × 109 7.76 × 108 8.51 × 109 9.12 × 108 1.86 × 1010 2.24 × 109

soman n

8

1.78 1.53 1.35 0.93 0.05 0.93 0.35

reactions

A

n

IM1so IM2so IM3so IM4so D1so D3so D4so D8so

2.39 × 10 3.80 × 108 3.02 × 108 1.29 × 108 1.55 × 1010 2.04 × 108 1.48 × 109 9.33 × 108 7

300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000

kT

log kT −42

1.40 × 10 8.13 × 10−29 1.56 × 10−20 5.40 × 10−15 5.00 × 10−11 4.80 × 10−08 1.01 × 10−05 7.38 × 10−04 2.21 × 10−21 4.29 × 10−14 1.01 × 10−09 8.39 × 10−07 1.02 × 10−04 3.74 × 10−03 6.15 × 10−02 5.77 × 10−01 1.35 × 10−46 3.964 × 10−32 2.01 × 10−23 1.32 × 10−17 1.93 × 10−13 2.60 × 10−10 7.17 × 10−08 6.47 × 10−06 3.37 × 10−37 1.24 × 10−25 1.10 × 10−18 4.75 × 10−14 9.80 × 10−11 3.01 × 10−08 2.59 × 10−06 9.15 × 10−05

−41.85 −28.09 −19.81 −14.27 −10.30 −7.32 −4.99 −3.13 −20.66 −13.36 −9.00 −6.08 −3.99 −2.43 −1.21 −0.24 −45.87 −31.40 −22.70 −16.88 −12.72 −9.58 −7.14 −5.19 −36.47 −24.91 −17.96 −13.32 −10.01 −7.52 −5.59 −4.04

As evident from earlier discussions, due to very high activation barriers, the isomerization as well as decomposition pathways are practically not feasible at room temperature. From Tables 3 and 4, it is noticed that the value of k300 of the isomerization pathways varies within ∼10−32−10−36 s−1 and ∼10−34−10−38 s−1 for sarin and soman, respectively. Thus, it is apparent that the sufficiently low kT values make all of the isomerization pathways practically inaccessible at room temperature. As the temperature increases, the kT value of each isomerization pathway also increases significantly, and at 1000 K, it becomes in the order of ∼10−1 and ∼10−2 for sarin and soman, respectively, which implies that at higher temperature all of the isomerization pathways become almost equally probable. While analyzing the decomposition pathways, it is observed that apart from the D3 pathway, for all other decomposition pathways the k300 value is very low and that for most of the cases, the values are even lower than those of the isomerization pathways. From Tables 3 and 4, it is apparent that k300 of the D3 pathway is at least ∼1012 times higher than that of any other decomposition as well as isomerization pathways, but still, the pathway is too slow to make the reaction feasible at room temperature as k300 is only in the order of ∼10−20 and ∼10−21 s−1 for sarin and soman, respectively. As the temperature increases, kT of the D3 pathway also increases, and for both the OPCs, it becomes ∼10−6 s−1 at 600 K, which is ∼1015 times greater than k300. From this perspective, it should be mentioned here that

Table 5. Values of Arrhenius Parameters for Different Detoxication Pathways Obtained from RRKM Calculation for Both Sarin and Soman

reactions

T

2.00 1.60 1.52 1.23 1.01 0.06 0.95 0.28 O


Article

Chemical Research in Toxicology Baier et al.8 also showed that the decomposition of sarin can be achieved at ∼600 K and 1 atm pressure under N2 atmosphere and at ∼668 K and 1 atm pressure in a borosilicate glass tube. Therefore, from our RRKM study also it is evident that the uncatalyzed decomposition of sarin becomes feasible at a temperature of ∼600 K−700 K, which clearly shows that the result obtained by our calculation is in good agreement with the earlier reported experimental result. From the kinetic study of soman also, it is observed that the detoxication of soman via the D3 pathway is achievable within a temperature range of 600 K− 700 K. Moreover, from the graphical representations (Figures 10,

Figure 11. (a) Graphical representation of log kT vs temperature (K) for the detoxication pathways of sarin for β-H transfer along with the D3sa pathway. (b) Graphical representation of log kT vs temperature (K) for the detoxication pathways of soman for β-H transfer along with the D3so pathway.

Figure 10. (a) Graphical representation of log kT vs temperature (K) for the detoxication pathways of sarin for α-H transfer along with the D3sa pathway. (b) Graphical representation of log kT vs temperature (K) for the detoxication pathways of soman for α-H transfer along with the D3so pathway.

also reveals that at temperatures up to 700 K, D3 is the exclusive pathway for detoxication, but as the temperature rises above 700 K, the b.r. values of the other detoxication pathways also increase. As shown in the graphical representation (Figure 14), above 700 K the b.r. for D3 pathway starts to decrease slowly, and the value becomes ∼0.70 for sarin and soman at 900 and 1000 K, respectively. At higher temperature, the significant b.r. values of the isomerization pathways clearly indicate that detoxication of the titled OPCs will also take place via the isomerization pathways along with the D3 pathway.

11, 12, and 13), it is evident that at a lower temperature range the kT of the D3 pathway is the highest among all but that with the increase of temperature (>700 K) the isomerization pathways are found to be more or less equally kinetically feasible like the D3 pathway. The kinetic feasibility of the detoxication pathways can also be well justified by determining the branching ratio (b.r.) values at different temperatures for the selected pathways during the RRKM calculation. The detailed examination of the detoxication pathways from the perception of b.r. values elucidates that at room temperature, for both the OPCs, the b.r. value of D3 pathway is 1, whereas for other detoxication pathways it remains almost zero. The calculation of the b.r. values

4. CONCLUSIONS In this article, we have explored all possible decomposition and isomerization pathways of sarin and soman in order to detoxify the above-mentioned nerve agents and further analyzed the feasibility of the explored pathways kinetically using an RRKM one-dimensional master equation. For sarin, a total of 13 detoxication pathways are identified; among them, three are isomerization pathways, and the rest are decomposition pathways. However, for soman, apart from these pathways, three additional detoxication pathways are obtained due to the presence of ω-H. The explored pathways are further categorized according to the intramolecular H-transfer sources, and those are P


Article


Figure 14. (a) Graphical representation of branching ratio vs temperature (K) for different detoxication pathways of sarin. (b) Graphical representation of branching ratio vs temperature (K) for different detoxication pathways of soman.

Figure 12. (a) Graphical representation of log kT vs temperature (K) for the detoxication pathways of sarin for γ-H transfer along with the D3sa pathway. (b) Graphical representation of log kT vs temperature (K) for the detoxication pathways of soman for γ-H transfer along with the D3so pathway.

feasible pathway among all for both OPCs. The associated barrier height for the D3 pathway is at least ∼20 kcal/mol lower than that of any other decomposition and isomerization pathways. Apart from the D3 pathway, the greater feasibility of the isomerization pathways over the decomposition pathways is also apparent from our study. Another important finding obtained from our present work is that the detoxication pathways involving γ-H are favored over other pathways. During the RRKM calculation also, it is observed that the rate constant of the D3 pathway is the highest among all of the pathways, which indicates its kinetics feasibility over other pathways. Like Baier et al.,8 from our work also, it is clear that the uncatalytic decomposition of the OPCs occurs within the temperature range of 600 K−700 K and exclusively follows the D3 pathway, which is further clarified by estimating the b.r. values. At higher temperature, although the rate constant as well as the b.r. value of the D3 pathway remains sufficiently high, the increased values of the rate constants and b.r. of other detoxication pathways suggest that along with D3 the OPCs may follow the other pathways in order to get detoxified. Therefore, by exploring the detoxication pathways theoretically we have given an idea that if the detoxication processes are carried out experimentally at higher temperature, there might be a possibility of getting different types of products including the products associated with the D3 pathway. Overall, our work is thus expected to offer a fundamental idea about the detoxication processes of the titled

Figure 13. Graphical representation of log kT vs temperature (K) for the detoxication pathways of soman for ω-H transfer along with the D3so pathway.

α-H, β-H, and γ-H sources (for both OPCs) and ω-H source (for soman only). After the complete exploration of all of the detoxication pathways, it is noticed that from the perspective of both activation barrier and reaction enthalpy, D3 is the most Q


Article


Alkaline Hydrolysis Reaction As in Phosphotriesterase? J. Phys. Chem. B 112, 9982−9991. (7) Liang, S., Hemberger, P., Neisius, N. M., Bodi, A., Grützmacher, H., Levalois-Grützmacher, J., and Gaan, S. (2015) Elucidating the Thermal Decomposition of Dimethyl Methylphosphonate by Vacuum Ultraviolet (VUV) Photoionization: Pathways to the PO Radical, a Key Species in Flame-Retardant Mechanisms. Chem. - Eur. J. 21, 1073−1080. (8) Baier, R. W., and Weller, S. W. (1967) Catalytic and Thermal Decomposition of Isopropyl Methyl Fluorophosphonate. Ind. Eng. Chem. Process Des. Dev. 6, 380−385. (9) Glaude, P. A., Curran, H. J., Pitz, W. J., and Westbrook, C. K. (2000) Kinetic study of the combustion of organophosphorus compounds. Proc. Combust. Inst. 28, 1749−1756. (10) Gustafson, R. L., and Martell, A. E. (1962) A Kinetic Study of the Copper(II) Chelate-catalyzed Hydrolysis of Isopropyl Methylphosphonofluoridate (Sarin). J. Am. Chem. Soc. 84, 2309−2316. (11) Vaiss, V. S., Borges, I., and Leitão, A. A. (2011) Sarin Degradation Using Brucite. J. Phys. Chem. C 115, 24937−24944. (12) Alvim, R. S., Vaiss, V. S., Leitão, A. A., and Borges, I. (2013) Hydrolysis of a VX-like Organophosphorus Compound through Dissociative Chemisorption on the MgO(001) Surface. J. Phys. Chem. C 117, 20791−20801. (13) Moss, J. A., Szczepankiewicz, S. H., Park, E., and Hoffmann, M. R. (2005) Adsorption and Photodegradation of Dimethyl Methylphosphonate Vapor at TiO2 Surfaces. J. Phys. Chem. B 109, 19779−19785. (14) Panayotov, D. A., and Morris, J. R. (2008) Catalytic Degradation of a Chemical Warfare Agent Simulant: Reaction Mechanisms on TiO2Supported Au Nanoparticles. J. Phys. Chem. C 112, 7496−7502. (15) Rusu, C. N., and Yates, J. T. (2000) Photooxidation of Dimethyl Methylphosphonate on TiO2 Powder. J. Phys. Chem. B 104, 12299− 12305. (16) Trubitsyn, D. A., and Vorontsov, A. V. (2005) Experimental Study of Dimethyl Methylphosphonate Decomposition over Anatase TiO2. J. Phys. Chem. B 109, 21884−21892. (17) Yang, L., Taylor, R., de Jong, W. A., and Hase, W. L. (2011) A Model DMMP/TiO2 (110) Intermolecular Potential Energy Function Developed from ab Initio Calculations. J. Phys. Chem. C 115, 12403− 12413. (18) Yang, L., Tunega, D., Xu, L., Govind, N., Sun, R., Taylor, R., Lischka, H., DeJong, W. A., and Hase, W. L. (2013) Comparison of Cluster, Slab, and Analytic Potential Models for the Dimethyl Methylphosphonate (DMMP)/TiO2(110) Intermolecular Interaction. J. Phys. Chem. C 117, 17613−17622. (19) Bermudez, V. M. (2007) Computational Study of the Adsorption of Trichlorophosphate, Dimethyl Methylphosphonate, and Sarin on Amorphous SiO2. J. Phys. Chem. C 111, 9314−9323. (20) Hegde, R. I., Greenlief, C. M., and White, J. M. (1985) Surface chemistry of dimethyl methylphosphonate on rhodium(100). J. Phys. Chem. 89, 2886−2891. (21) Smentkowski, V. S., Hagans, P., and Yates, J. T. (1988) Study of the catalytic destruction of dimethyl methylphosphonate(DMMP): oxidation over molybdenum(110). J. Phys. Chem. 92, 6351−6357. (22) Henderson, M. A., and White, J. M. (1988) Adsorption and decomposition of dimethyl methylphosphonate on platinum(111). J. Am. Chem. Soc. 110, 6939−6947. (23) Guo, X., Yoshinobu, J., and Yates, J. T. (1990) Decomposition of an organophosphonate compound (dimethylmethylphosphonate) on the nickel(111) and palladium(111) surfaces. J. Phys. Chem. 94, 6839− 6842. (24) Bermudez, V. M. (2007) Quantum-Chemical Study of the Adsorption of DMMP and Sarin on γ-Al2O3. J. Phys. Chem. C 111, 3719−3728. (25) Bermudez, V. M. (2009) Computational Study of Environmental Effects in the Adsorption of DMMP, Sarin, and VX on γ-Al2O3: Photolysis and Surface Hydroxylation. J. Phys. Chem. C 113, 1917−1930. (26) Templeton, M. K., and Weinberg, W. H. (1985) Adsorption and decomposition of dimethyl methylphosphonate on an aluminum oxide surface. J. Am. Chem. Soc. 107, 97−108.

nerve agents, which will further invite experimental and theoretical investigations on organophosphorus compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemrestox.6b00132. Values of standard enthalpy of formation (ΔHf°) of the transition states in CBS-QB3 method, values of standard enthalpy of formation (ΔHf°) of the reactants and products in the CBS-QB3 method, and the atomic coordinates of the reactant, TS, and products associated with the D3 pathway of sarin and soman in both M06-2X and CBS-QB3 methods (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91-33-2473-4971, ext. 1257. Fax: 91-33-24732805. Email: [email protected]. Funding

A.K.D. gratefully acknowledges a research grant under scheme number SB/S1/PC-79/2012 from the Department of Science and Technology (DST), Government of India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.A., T.D., and T.B. are grateful to the Council of Scientific and Industrial Research (CSIR), Government of India, for providing them research fellowships.

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ABBREVIATIONS OPC, organophosphorus compounds; AChE, acetylcholinesterase; sarin, O-isopropyl methylphosphonofluoridate; DMMP, dimethyl methylphosphonate; soman, 1,2,2-trimethylpropyl methylphosphonofluoridate; DMPT, O,S-dimethyl methylphosphonothioate; DIMP, diisopropyl methylphosphonate; DPMP, diphenyl methylphosphonate; DMEP, dimethyl ethylphosphonate; DFT, density functional theory; TS, transition state; STQN, synchronous transit-guided quasi-Newton; IRC, intrinsic reaction coordinate; RRKM, Rice−Ramsperger−Kassel−Marcus; b.r., branching ratio

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REFERENCES

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