Hydrolysis of a VX-like Organophosphorus Compound through

Article pubs.acs.org/JPCC

Hydrolysis of a VX-like Organophosphorus Compound through Dissociative Chemisorption on the MgO(001) Surface Raphael S. Alvim,† Viviane S. Vaiss,† Alexandre A. Leitaõ ,† and Itamar Borges, Jr.*,‡ †

Departamento de Química, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-330, Brazil Departamento de Química, Instituto Militar de Engenharia, Praça General Tibúrcio 80, Rio de Janeiro, RJ 22290-270, Brazil

‡

ABSTRACT: Organophosporous VX agent O-ethyl S-(2diisopropylethylamino)ethyl methylphosphonothioate is one of the main nerve agents. For this reason, the search for ways to deactivate it is very important. In this work, hydrolysis and adsorption reactions of a VX-like compound (O,S-dimethyl methylphosphonothioate, DMPT) on the MgO(001) surface were studied by density-functional theory (DFT) using periodic boundary conditions. A degradation reaction mechanism was proposed and theoretically investigated on two types of MgO(001) surfaces: the terrace and the Aldoped. Conformations, free-energy differences, transition states, reaction barriers, and minimum-energy paths were computed. We found that the P−S bond, related to the agent toxicity, breaks via hydrolysis occurring spontaneously throughout the analyzed temperature range, 100−600 K. In the dissociative chemisorption of the DMPT molecule, the formation of the MgO: [PO(CH3)(OCH3)]+[SCH3]− intermediate is catalytically favored from a temperature of about 335 K for the Al-doped surface, a value considerable smaller than the 500 K value for the same process on the terrace. At 335 K, the dissociation fragments on the Al-doped surface are less stable in comparison to the hydrolysis products. The possible reconstitution of the P−S bond on both surfaces does not occur according to kinetic analysis; however, the electronic energy barrier for the direct dissociation reaction on the Al-doped sites is about 49.0 kJ/mol lower than the value for the terrace. After recombination with the OH− and H+ ions, the HOPO(CH3)(OCH3) and HSCH3 products do not accumulate on either surface because these molecules desorb below the DMPT dissociation temperatures. The Al-doped sites of MgO(001) are thus more active in the catalytic hydrolysis process of the VX-like organophosphorus compound than is the nondoped surface.

1. INTRODUCTION Phosphorus is a vital element in living organisms because it is essential to cellular processes and it is usually found in phosphoric esters. Moreover, the chemical stability of the P−O bonds is crucial for DNA and RNA phosphodiesters. In particular, organophosphorus compounds having ester groups resistant to water, soil, and air action were selected long ago for developing chemical warfare agents that can reach a target without chemical modification.1 These substances react with enzymes having amino acid residues that are usually phosphorylated, such as acetylcholinesterase.2−4 Consequently, this process can lead to death from organ failure through accumulation of acetylcholine in synaptic receptors, which irreversibly impair and prevent nerve impulse transmission.4 Therefore, the study of the deactivation of chemical warfare agents has important consequences in both defense and civil issues. Safety and the environmental impact of organophosphate stockpile destruction processes are of great interest and concern. In the past decade, there has been growing interest in the chemistry of these compounds and their degradation mechanisms. The techniques used for organophosphorus compound degradation and analyses have to comply with regulations imposed by conventions and agreements. Because © XXXX American Chemical Society

of their extreme toxicity and restricted use imposed by the 1992 Chemical Weapons Convention, organophosphorus agents are regulated even for research in academic laboratories.5 Concerning the required knowledge of warfare agent degradation, molecular modeling can provide physical− chemical data of great accuracy. This approach, besides not involving the dangerous manipulation of such substances, allow detailed atomistic understanding of the involved physical and chemical processes. Recently, there has been a considerable progress on the study of adsorption, dissociation6 and antidotes against nerve agent poisoning7 derived from the increasing use of computational simulations either based on Hartree−Fock or density-functional theory (DFT) methods. Along these lines, we investigated another nerve agent, Sarin (C4H10FO2P), and its deactivation on the layered hydroxide brucite (Mg(OH)2).8 VX agent [O-ethyl S-(2-diisopropylethylamino)ethyl methylphosphonothioate] (Figure 1A) is one of the main nerve agents and belongs to the class of so-called V agents, being also a model system for the synthesis of organophosphorus insecticide Malathion because of structural similarities.9 There Received: July 29, 2013 Revised: September 12, 2013

A

dx.doi.org/10.1021/jp4075477 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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other oxides26 for this purpose. There are theoretical studies of dickite using the ONIOM method,27 of tetrahedral edge clay mineral fragments by MP2,28 of CaO29 and (ZnO)n30 clusters using DFT and MP2 methods, and of γ-Al2O3 clusters by DFT calculations.31 In these works, desorption occurs as a result of the coordination of the PO bond to the acidic surface sites.15,32 Because the reactant is bonded to the surface, it can react, an essential feature of the catalytic properties of soils.33 Consequently, the relevant neurotoxic bond can be broken. Magnesium oxide (MgO) has a single valence state, and its most stable low-index surface is the (001). MgO is also a characteristic rock salt material. This mineral specimen is an ideal model for ionic metal oxides because it consists of layers of almost flat planes composed of equal numbers of cations and anions, tending not to have spontaneous surface reconstruction along the (001) direction. MgO can destroy phosphorus compounds, and it has been successfully used as a destructive adsorbent for toxic agents in experiments employing dimethyl methylphosphonate (DMMP),15,34,35 VX,36 and mustard gas compounds.37,38 In particular, Li et al.35 showed experimentally that the addition of water to a phosphorus compound dramatically enhances the facility of MgO to destroy it. There have been also theoretical works using MP2 and representative cluster models to calculate the electronic structure of these systems.32,39 In heterogeneous catalysis, several reactions involving the acid−base properties of the MgO surface are well known. Moreover, the Mg52+ and O52− sites, even with extremely low reactivity (the subscript indicates the coordination number), are very important in heterogeneous catalysis for understanding different environmental phenomena, such as their interaction with gases.40,41 However, the existence of dopant point defects is essential to enhancing the reactivity in oxides. This situation promotes the formation of low-coordination superficial sites including different morphological irregularities and structural properties directly related to the oxide surface. Another important characteristic of dopants in oxides is that a knowledge of local acid−base properties of these sites is fairly simple, as is the case for doped MgO. On the MgO(001) surface, the Mg52+ sites may be substituted by different kinds of dopants or removed to produce vacancies, respectively called Vs-doped centers and Vs vacancies,42,43 with the subscript s indicating the surface. Vs2− vacancies, corresponding to the removal of Mg2+, are mainly generated when there are Vs-doped centers with an ionic radius larger than that of Mg2+, in this way forming O42− sites that are more reactive because of an enhanced superficial molecular interaction. Therefore, surface models combining vacancies and dopant atoms can be useful in examining and predicting many superficial processes of the adsorption and dissociation of molecules. The nature of electrostatic interactions in modified specific sites is essential to the heterolytic dissociation of covalent bonds without intermolecular interactions.44,45 Point defects result in the largest change in charge redistribution through the surface atoms and are energetically dominant for dissociative processes in acid−base reactions.46−49 In particular, because of the higher reactivity of Mg/Al mixed oxides compared to that of MgO,50,51 the electronic characteristics of VAl and O42− sites are essential to acid−base-catalyzed reactions. However, the structure of the Mg/Al mixed oxide is still not well described in the literature because this oxide is obtained in powder form and has low crystallinity.52 Consequently, the X-ray powder

Figure 1. Optimized structures: (A) O-ethyl S-(2diisopropylethylamino)ethyl methylphosphonothioate (VX), (B) O,S-dimethyl methylphosphonothioate (DMPT), (C) HOPO(CH3)(OCH3) (P1), (D) HSCH3 (P2), (E) HOPO(CH3)(SCH3) (P3), and (F) HOCH3 (P4) molecules. The substitution of some radicals by the methyl groups generates the DMPT analogue molecule. Subscripts 1 and 2 represent oxygens with one single bond and double bonds in VX and consequently DMPT molecules. Subscript w represents oxygen from water after hydrolysis.

is also a VX analogue and isomer called R-VX (Russian-VX, S(2-(diethylamino)ethyl O-isobutyl methylphosphonothioate).10 The toxicity of VX in the nervous system is related to the P−S bond. The main physical−chemical properties of pure VX are low volatility, boiling point of 571 K, and relative stability toward spontaneous hydrolysis. Cleavage of the P−O bond produces thioic acid that is almost as toxic as VX.10 Therefore, oxidation and nucleophilic substitution lead to products with a high degree of toxicity: these products are susceptible to extremely slow hydrolysis.11−13 Many of the reactions that might be useful in neutralizing the VX agent need to be bondspecific for P−S bond cleavage in order not to form toxic byproducts.12 For solvolysis, P−O and P−S bond cleavage processes are kinetically competitive, and the products resulting from P−S bond breakage are thermodynamically favored.14 Therefore, a catalytic process to degrade VX could lead to good results for practical applications.15 In 1988, Ekerdt et al.16 discussed the multifunctional characteristics of surfaces and reactant species, the surface acid−base character, and defect generation on surfaces used in organophosphorus compound degradation. There are also experimental studies of the degradation of VX molecules by autocatalytic hydrolysis,17 photoassisted reaction,18 and catalytic specificities of organophosphorus enzymes using enzyme engineering to produce sufficiently active catalysts13,19 and phosphonothioate hydrolysis of VX by an organometallic reagent.20 We also mention DFT calculations with the B3LYP functional for chelation21 and hydrolysis,22 and Møller−Plesset second-order perturbation theory (MP2) and Hartree−Fock (HF) results for the solvolysis14,23 of the VX molecule main groups. The interaction of organophosphorus compounds with clay minerals and metal oxide surfaces is particularly attractive because of their high surface areas and unique morphological features that shows good results for degradation.24 There are, for example, experimental studies on nanotubular titania25 and B



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doped surface the adsorbed products P1 and P2 are represented respectively by P15D and P25D. Elementary reaction R4 will not be studied in this work, but it is part of the proposed reaction mechanism. Afterward, P1 and P2 are desorbed from the terrace (R5T) and Al-doped (R5D) surfaces; these desorption processes are represented by general elementary reaction R5. The global (H1 and H2) and elementary (R1, R2, R3, R4, and R5) reactions are thus represented as

diffraction technique is limited to the proper characterization of the types of sites occupied by the metals. Another structural issue is the possibility of finding different structures of this oxide, which depends on the calcination heating rate and final temperature.52 Because calcined hydrotalcite is obtained in molar fractions of 0.20 ≤ x ≤ 0.33,53 the Al-doped MgO surface may be representative of superficial sites of the Mg/Al mixed oxides with a low molar fraction.54 Thus, the Al-doped MgO surface based on the simple MgO(001) terrace is a good model for a reactional study of these types of sites present in Mg/Al mixed oxides. In this work, we used DFT calculations with periodic boundary conditions to obtain the most probable hydrolysis proposal of a VX-like organophosphorus compound, O,Sdimethyl methylphosphonothioate, DMPT (Figure 1B), through dissociative chemisorption on two types of MgO(001) surfaces: terrace and Al-doped. We computed conformations, free energies, and reaction barriers, thereby allowing a thorough understanding of the interaction nature between the DMPT molecule and the Mg52+, O52−, VAl, and O42− superficial sites in order to examine the degradation reaction of the VX agent.

DMPT(g) + H 2O(g) → P1(g) + P2(g)

(H1)

DMPT(g) + H 2O(g) → P3(g) + P4 (g)

(H2)

MgO(s) + DMPT(g) → MgO: DMPT(s)

(R1)

MgO: DMPT(s) → MgO: [PO(CH3)(OCH3)]+ [SCH3]− (s)

(R2)

MgO(s) + H 2O(g) → MgO: [OH−][H+](s)

(R3)

MgO: [PO(CH3)(OCH3)]+ [SCH3]− (s) + MgO: [OH−][H+](s)

2. THEORETICAL METHODS 2.1. Mechanistic Proposal. In the first step of our mechanistic proposal, we examine the DMPT hydrolysis resulting from two competitive processes: P−S (H1) and P− O (H2) bond cleavage. The H1 process generates HOPO(CH3)(OCH3) [P1] (Figure 1C) and HSCH3 [P2] (Figure 1D) nontoxic products, and H2 leads to the HOPO(CH3)(SCH3) [P3] (Figure 1E) and HOCH3 [P4] (Figure 1F) products. In a real MgO(001) surface, most hydroxyls appears on sites characterized by defects; there are free Mg52+ and O52− sites because this surface is not wholly hydroxylated and this process is possible only under high pressure or for a MgO(111) surface that is naturally OH-terminated.55 For this reason, we investigate dissociation processes of the DMPT and water molecules separately even when the surface is Al-doped. Therefore, the second step is the adsorption process of the DMPT molecule on the terrace (R1T) and the Al-doped (R1D) surfaces, represented by the general elementary reaction R1 that has as an adsorption product the MgO:DMPT superstructure. Thus, R1T and R1D produce the I1T and I1D intermediates, respectively, in the proposed reaction mechanism. In the third step, the DMPT molecule dissociates on the terrace (R2T) and on the Al-doped (R2D) surfaces, thus leading to the cleavage of the P−S bond followed by the formation of I2T and I2D intermediates, respectively. This general elementary reactional is represented by R2. We recently studied the fourth step,56 represented by the general elementary reaction R3 consisting of adsorption and partial dissociation processes of water molecules on the MgO(001) terrace followed by the formation of OH− and H+ ions. We note that adsorption and dissociative chemisorption of water molecules are spontaneous on lowcoordinated sites57,58 as well on an Al-doped surface model similar to ours. In the fourth step, after dissociation processes R2 and R3, there is an ionic recombination on the catalyst surface leading to the formation of harmless P1 and P2 products adsorbed on the surface. The P1 and P2 products adsorbed on the terrace are represented by P15T and P25T, respectively, whereas on the Al-

→ MgO: [P1][P2](s)

MgO: [P1][P2](s) → P1(g) + P2(g) + MgO(s)

(R4) (R5)

2.2. Computational Approach. We used a unit cell rotated 45° on the z axis from vectors of the crystallographic unit cell of the MgO bulk. Thus, the unit cell used in this work has vectors of (a/2, b/2)R45, with lattice parameters calculated with a = b = c = 4.23 Å and α = β = γ = 90°. Slab models of the MgO(001) surface using periodic boundary conditions were constructed; this approach eliminates some of the problems associated with edge effects present when cluster models of the surface are used.59 For terrace calculations, the unit cell of the MgO(001) surface is replicated four times on the x axis (a = 8.46 Å) and three times on the y axis (b = 6.35 Å). This supercell contains 12 Mg and 12 O atoms on the surface. However, for calculations of the Al-doped surface, the unit cell of the MgO(001) surface is replicated four times on the x and y axes (a = b = 8.46 Å). This supercell contains 16 Mg and 16 O atoms on the surface. For both surface models, there are three MgO monolayers and a vacuum layer of about 15.00 Å along the z axis (c = 19.23 Å). This vacuum layer was introduced to generate the surface and to isolate the top of one slab from the bottom of the next. The supercells are then large enough to avoid interactions among the adsorbed DMPT molecule and its periodic images; supercells of the same dimensions were used to optimize the free molecules on each surface. The molecules were adsorbed on only one side of the slab. Considering that it is necessary to maintain the system electroneutrality, on the Al-doped MgO(001) surface one Mg2+ vacancy (Vs2−) is induced when there are two Al53+ sites. We tested some relative positions of the Al53+ ions based on the total electronic energy of these surfaces. The most stable structure was the one having these two Al53+ ions positioned according to Figure 2. Low-energy electron diffraction surface experiments indicate that there is only a relaxation of no more than 2.5% of the lattice constant in the superficial monolayer and a rumpling of less than about 2.0% of the O52− ions compared to that of the Mg52+ ions.33 Therefore, in the case of the MgO(001) terrace, C



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under different stress conditions, transition states, and phonons, among other atomistic properties. The electronic wave functions were expanded in a planewave basis set using the tested cutoff energy of 408.2 eV for Kohn−Sham orbitals and 4 times this cutoff energy for charge density and potential calculations. The electronic density was obtained from the Kohn−Sham orbitals calculated at the Γ point (0, 0, 0) in the first Brillouin zone. The transition state for the dissociation reaction of DMPT was calculated from the minimum-energy paths (MEPs). In this approach, the MEP is found by constructing a set of images of the system between the initial and final states; the saddle points are on the top of the potential energy. We computed MEPs connecting the different minimum geometries using the climbing-image nudged elastic band (CI-NEB) method,68,69 which can satisfactorily describe the dissociative adsorption processes at the DFT level. Ten images were used to compute each MEP; their geometries were optimized to establish the minimum-energy path on the potential surface of the system until energy variations were less than 0.05 eV/Å. For all of the optimized structures, the vibrational modes and the respective frequencies were calculated using the harmonic approximation. The vibrational data were also used to calculate both the contribution of the lattice thermal vibration to the total energy and the zero-point energy (ZPE) as previously described.56,70−72 We calculated in the 100−600 K temperature range, at 1 atm, the electronic and Gibbs free energy differences between products and reactants for DMPT hydrolysis reactions in H1 and H2, DMPT adsorption process in R1, DMPT dissociation in R2, reaction R3 on the terrace previously investigated,56 and P1 and P2 desorption processes in R5, according to the expression

Figure 2. Optimized structure of the Al-doped surface with two Al53+ sites, four O42− sites, and one VMg2− vacancy.

only the geometric parameters of the superficial monolayer are affected by the molecular adsorption processes and suffer from reconstruction. This rumpling on the top layer is repeated in the bottom layers with decreasing amplitude.33 However, in spite of these indications, we tested the relaxation of the first two atomic layers for the terrace model used here and in our previous work.56 We then verified that there is considerable variation in the geometric parameters relative to the optimization of the superficial monolayer. However, the second monolayer can be affected by the vacancy generated in the case of the doped surface model. Therefore, in our model surfaces, the first layer in the terrace, the first two atomic layers in the Aldoped surface, and all atoms of the molecules are allowed to relax during geometry optimization until the residual force components were less than 0.08 eV/Å. All other atoms of MgO(001) were held at their theoretical bulk positions. We studied an analogue of the VX compound, DMPT, that has some radicals replaced by methyl groups. This analog has been shown to be chemically identical to VX14 once the organic radicals are the first groups to be eliminated at low temperature (about 323 K).15 This simplification of the VX molecule structure drastically reduces the computational effort because it would be necessary to use a supercell replicating six and five times on the x (a = 12.69 Å) and y (b = 10.58 Å) axes, respectively, to keep these adsorbed VX molecules sufficiently distant in order to have neglectible dipole interactions between the images in our calculations. Likewise, organophosphorus nerve agents have different stable conformations,60,61 and their chemical or biological environments can change the conformational energy.60 In this way, the DMPT molecule has fewer conformers and avoids steric effects of P−S bond cleavage on the MgO surface, thus allowing us to investigate the degradation of the typical V-agent group without substantially affecting the chemistry of the problem. The adsorption and decomposition of the DMPT compound on the terrace and Al-doped surfaces were investigated using DFT62,63 with the GGA exchange-correlation functional of Perdew and Wang (PW91).64 The PWscf code was used65,66 for all calculations. A plane-wave basis set and a Vanderbilt ultrasoft pseudopotential67 were used. The PWscf package is able to calculate the energy of the ground state, optimize structures, and compute properties such as the atomic forces

ΔG =

∑ Gp − ∑ Gr p

(1)

r

where index p or r refers to products or reactants, respectively. The Gibbs free energy can be calculated using the well-known equation G = H − TS. We consider the Helmholtz energy to be approximately equal to the Gibbs energy because in solid-state processes the volume change is not significant. Therefore, the enthalpy Hs(T) and the entropy Ss(T) for the system in the solid state were calculated through the following approximations73 Hs(T ) = E elec + EZPE + Evib(T )

(2)

Ss(T ) = S vib(T )

(3)

elec

ZPE

vib

vib

where E , E , E (T), and S (T) are the total electronic energy at 0 K, the zero-point energy ZPE (linear sum of the fundamental harmonic frequencies), and the vibrational contributions for the enthalpy and entropy, respectively. The molecules in the gas phase are treated according to the formalism described previously.74 In this case, enthalpy Hg(T) and entropy Sg(p, T) contributions were calculated from73 Hg(T ) = E elec + EZPE + Evib(T ) + Etrans(T ) + Erot(T ) + pV

(4)

Sg(p , T ) = S vib(T ) + Strans(p , T ) + S rot(T ) trans

(5)

rot

where E (T) and E (T) are the translational and rotational contributions, respectively, to the enthalpy equal to 3/2(RT) D



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Table 1. Main Geometric Parameters of the Calculated VX, DMPT, P1, P2, P3, and P4 Molecules Compared to Theoretical Values of the VX Molecule18,a calc.18

calc. VX C−H P−C P−S PO2 P−O1 O1−C S−C P−Ow Ow−H S−H O1−H ∠C−P−S ∠C−P−O1 ∠C−PO2 ∠O1−PO2 ∠O1−P−S ∠O2P−S ∠P−S−C ∠P−O1−C ∠C−P−Ow ∠Ow−PO2 ∠Ow−P−S ∠C−O1−H

DMPT

1.10 1.81 2.11 1.49 1.62 1.46 1.83

P1

1.10 1.81 2.11 1.49 1.62 1.45 1.83

P2

1.10 1.80

P3

P4

VX

1.10

1.10 1.81 2.10

1.10

1.82

1.83 1.63 0.98

1.06 1.86 2.10 1.45 1.76 1.43 1.78

1.48 1.62 1.45

1.43

1.62 0.98 1.35

0.97 108.70

109.05 99.65 116.00 116.16 106.62 108.70 102.32 117.53

117.16

103.04 119.91

109.12 101.05 116.25 116.30

116.33

108.84 103.02 118. 61 106.29

99.58 115.64 106.65 108.38

a

The distances and the angles are in angstroms and degrees, respectively. Subscripts 1 and 2 represent oxygens with one single bond and double bonds in VX and, consequently, DMPT molecules. Subscript w represents oxygen from water after hydrolysis.

Table 2. Thermodynamic Analysis for DMPT Hydrolysis (H1 and H2) and for Adsorption (R1), Dissociation (R2), and Desorption (R5) Processes on the Terrace (T) and Al-Doped (D) MgO(001) Surfaces at Room Temperature (298 K)a R1 ΔE ΔS ΔH ΔG

R2

R5

H1

H2

R1T

R1D

R2T

R2D

P1R5T

P2R5T

P1R5D

P2R5D

−3.7 1.1 −30.1 −29.7

−2.9 2.1 8.7 10.8

10.2 −0.1 −20.7 13.3

7.1 −0.1 −54.7 −15.0

3.1

−1.1

−40.1 −54.4

−23.2 −22.6

−9.8 0.1 48.4 15.1

−7.5 0.1 36.4 1.9

−2.7 0.2 31.6 −16.5

−1.4 0.2 14.4 −30.2

a We considered ΔEvib+trans+rot and ΔSvib+trans+rot for gas state. Likewise, we considered ΔEvib and ΔSvib for solid state. The energy and entropy variations are in kJ/mol and kJ/mol·K, respectively.

and R is the gas constant. The pV term is equal to RT. Strans(p, T) and Srot(T) are the translational and rotational contributions to the entropy that were also taken into account to compute the thermodynamic properties of the molecules in the gas phase.73 More details of the thermodynamic analysis can be found in our previous paper.56

approach; dP−S = 2.11 and dP−O1 = 1.62 Å are the distances for the VX and DMPT molecules (Table 1). The calculated global Gibbs free energies of the P−S and the P−O1 bond hydrolyses according to reactions H1 (ΔGH1) and H2 (ΔGH2) are respectively −29.7 and 10.8 kJ/mol at room temperature (298 K). At this temperature, we calculated the enthalpy variation ΔHH1 = −30.1 and ΔHH2 = 8.7 kJ/mol for the H1 and H2 processes. The contributions of the vibrational, rotational, and translational energies (ΔEvib+trans+rot) for H1 and H2 were calculated to be −3.7 and −2.9 kJ/mol at room temperature, respectively. Likewise, the entropic contributions (ΔSvib+trans+rot) for H1 and H2 were calculated to be 1.1 and 2.1 kJ/mol·K, respectively. The thermodynamic analysis is shown in Table 2. Therefore, in contrast to the P−O1 bond, the P−S heterolytic bond rupture through the incorporation of OH− and H+ ions is thermodynamically favorable over the whole 100−600 K temperature range (Figure 3). In this case, we presented only the results for the lattice parameters of the

3. RESULTS AND DISCUSSION In Table 1, we present computed geometric parameters of VX (Figure 1A), DMPT (Figure 1B), and free-molecule products P1 (Figure 1C), P2 (Figure 1D), P3 (Figure 1E), and P4 (Figure 1F). The computed VX and DMPT geometries are very similar and compare very favorably with other theoretical data for the VX molecule:18 calculated distances were dP−S = 2.10 and dP−O1 = 1.76 Å . Likewise, our results are very close to the theoretical results of S̆ec̆kutė et al.14 for the similar O,S-dimethyl methylphosphonothioate compound: they found dP−S = 2.11 and dP−O1 = 1.64 Å . Therefore, the main bonds in the molecules of interest are well described in our computational E



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Figure 3. Gibbs free energy differences for hydrolysis H1 (ΔGH1) and H2 (ΔGH2) and for the combined adsorption and dissociation processes of DMPT on the terrace (ΔGsum T = ΔGR1T + ΔGR2T).

supercell used in the terrace model because they exhibit the same behavior as in the Al-doped model. Although proposed VX detoxification reaction H1 can occur spontaneously according to our thermodynamic calculations, experimentally the VX agent has relative kinetic stability toward spontaneous hydrolysis (50% degradation in 78 h at 295 K),10 so its P−S cleavage is possible only in highly alkaline aqueous solutions at high temperature.13 However, without competitive P−O1 bond cleavage (H2), the MgO(001) surface may be specific to favor kinetically the P−S bond cleavage in the VX compound. Thus, we computed the DMPT interaction with the terrace and Al-doped MgO(001) surfaces. The first step in the catalyzed hydrolysis process starts from nondestructive adsorption according to elementary reaction R1. In this case, the DMPT molecule adsorbs on the surface with its S and O2 atoms oriented toward the surface Mg atoms because other adsorption modes are sterically hindered. In this way, adsorption on MgO(001) was essential to stabilizing a single type of conformer of DMPT even at low temperatures, and it did not affect the kinetics or the thermodynamics of the hydrolysis reactions. For the terrace (Figure 4A), the dP−S, dP−Os, dS−Mg, and dO2−Mg bond distances in I1T are 2.10, 3.60, 3.01, and 2.45 Å, respectively (Table 3). We also tested some different structural configurations for DMPT adsorption on the Al-doped surface. For the energetically most stable configuration (Figure 4C), the dP−S, dP−Os, dS−Mg, and dO2−Mg bond distances in I1D are 2.10, 3.72, 3.10, and 2.26 Å, respectively (Table 3). The main geometric parameters of the molecule do not change after this interaction; only the methyl groups are displaced in the direction opposite to the adsorption site. The Gibbs free energy variation (ΔGR1T), adsorption enthalpy (ΔHR1T), and vibrational contribution (ΔEvib R1T) for DMPT adsorption on the terrace are respectively 13.3, −20.7, and 10.2 kJ/mol at room temperature. This adsorption occurs spontaneously up to 190 K, and at this temperature, there is an unfavorable, though small, vibrational entropy change (ΔSvib R1T) of −0.1 kJ/mol·K because adsorption confines the gas to the MgO(001) surface. However, ΔGR1D, ΔHR1D, and ΔEvib R1D for DMPT adsorption on the Al-doped surface are respectively −15.0, −54.7, and 7.1 kJ/mol at room temperature, being spontaneous up to 420 K. Similar to the terrace, ΔSvib R1D for adsorption on the Al-doped surface is −0.1 kJ/mol·K at room temperature. Thermodynamic analysis is shown in Table 2.

Figure 4. Dissociation processes of the DMPT molecule on the terrace and Al-doped surfaces through P−S bond cleavage: (A) I1T intermediate, (B) I2T intermediate, (C) I1D intermediate, and (D) I2D intermediate.

Therefore, we see that point defects of the Al-doped type provide greater thermodynamic stability in the initial adsorption step of the organophosphate. Moreover, the adsorption of DMPT on the Al-doped surface model did not produce any change in the geometrical parameters of this molecule relative to the terrace model. From adsorbed DMPT, according to elementary reaction R2, I2T and I2D intermediates are produced by a P−S bond heterolytic break on both the MgO(001) terrace (Figure 4B) and Al-doped (Figure 4D) surface. The DMPT molecule is not formed again because of the dissociative chemisorption of the dissociated fragments on the surface. For this process on the terrace, ΔGR2T = −55.4 kJ/mol at room temperature and occurs spontaneously throughout the studied temperature range of 100−600 K. The computed dP−S distance is 3.86 Å, a value larger in comparison to that of isolated DMPT (2.11 Å). The [PO(CH3)(OCH3)]+ group interacts with the surface through the P−Os and O2−Mg bonds with dP−Os and dO2−Mg distances of 1.58 and 2.11 Å, respectively. The [SCH3]− group is located between two Mg atoms with dS−Mg distances of 2.63 and 2.74 Å (Table 3). ΔHR2T and ΔEvib R2T are about −40.1 and 3.1 kJ/mol at room temperature, with negligible ΔSvib R2T. The thermodynamic analysis is shown in Table 2. We tested some different structural configurations for dissociation on the Al-doped surface. For the most stable configuration (Figure 4D), ΔGR2D is −22.6 kJ/mol at room temperature and is a spontaneous process over the whole temperature range from 100 to 600 K. The dP−S, dP−Os, and dO2−Mg distances are 5.00, 1.57, and 2.22 Å, respectively (Table 3), with ΔHR2D and ΔEvib R2D calculated to be −23.2 and −1.1 kJ/ mol at room temperature, with negligible ΔSvib R2D. Thermodynamic analysis is shown in Table 2. This situation is possibly a stable position for the phosphate group chemisorbed on superficial defects, similarly to the work of Michalkova et al.39 for the decomposition of Sarin on magnesium oxide clusters. F



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Table 3. Main Geometric Parameters of the Intermediates and Transition States Formed in the Elementary Reactions on the Terrace (T) and Al-Doped (D) MgO(001) Surfacesa R1 P−S P−Os S−Mg O2−Mg

R2

R5

I1T

I1D

TS2T

TS2D

I2T

I2D

2.10 3.60 3.01 2.45

2.10 3.72 3.10 2.26

2.36 2.18 2.60 2.00

2.30 1.83 2.59 2.04

3.86 1.58 2.63 2.74 2.11

5.00 1.57 2.45 2.22

P15T

P25T

3.71

P15D 3.62

3.06 2.43

P25D

2.95 2.30

a

The distances are in angstroms. Subscript 2 represents oxygens with a double bond in the DMPT molecule, and subscript s represents one site on the MgO(001) surface.

However, the [SCH3]− group bonds only to a single Mg atom with dS−Mg = 2.45 Å ; this structural configuration could favor a lower thermodynamic stabilization of the I2D intermediate relative to that of the I2T intermediate because the difference in the ΔHR2T and ΔHR2D enthalpies depends mainly on the interaction between dissociation fragments and the reconstruction of the MgO(001) surface after dissociation. Some experimental results34,35 indicate that the main DMMP bonds can break at lower temperatures, but in particular, at 463 K more DMMP molecules decompose on the MgO surface in the presence of water. Considering the sum ΔGsum T = ΔGR1T + ΔGR2T for adsorption and dissociation processes of DMPT on the terrace, we verified that only from about 500 K onward is the formation of the I2T intermediate in reaction R1 thermodynamically less stable in comparison to P1 and P2 hydrolysis products (Figure 3). The overall situation is illustrated by Figure 5, which shows the Gibbs energy for the

steps of the proposed reaction mechanism at room temperature and also at about 500 K for the terrace. Therefore, our results indicate that from about 500 K the DMPT compound decomposes on superficial regions related to the sites on the MgO(001) terrace. Considering the sum ΔGDsum = ΔGR1D + ΔGR2D for adsorption and dissociation processes of DMPT on the Aldoped surface, the I2D intermediate is thermodynamically less stable than H1 products at a temperature below 500 K, namely, 335 K (Figure 6); here we show the result of H1 on the Al-

Figure 6. Gibbs free energy differences for hydrolysis H1 (ΔGH1) in the supercell with the dimensions used for the Al-doped surface and for the sum of the adsorption and dissociation processes of DMPT on the Al-doped surface (ΔGsum D = ΔGR1D + ΔGR2D).

doped model surface. In Figure 5, for the Al-doped surface from 335 K onward, the process is thermodynamically more efficient than the formation of the intermediate in the DMPT dissociation by the terrace. Accordingly, even at room temperature, sites on the doped surface have a convenient thermodynamic stability with respect to the chemical deactivation process of DMPT, which mainly favors the adsorption step (Figure 5). However, we stress that in this discussion the analysis of the kinetic barrier is also a determining step for this comparison. Experimental results10,13 indicated that in alkaline reactions the P−S bond break occurs in greater proportions than the unwanted P−O1 cleavage. Theoretically, S̆ec̆kutė et al.14 showed that P−S and P−O1 bond cleavages, respectively represented by H1 and H2 reactions, are kinetically competitive but that the products of P−S bond cleavage are thermodynamically favored when involving alkaline hydrolysis for the same VX model molecule used in this work. For this reason, the H2 reaction was discarded in our kinetic study because we

Figure 5. Gibbs free energy for each composition of the DMPT reaction on the terrace and Al-doped surfaces. The first composition refers to the free DMPT and H2O molecules plus the isolated MgO surface (comp0 = DMPT + H2O + MgO). The second composition is intermediate I1 obtained from R1 plus the free H2O molecule (comp1 = I1 + H2O). The third composition contains dissociation intermediate I2 obtained from R2 plus the free H2O molecule (comp2 = I2 + H2O). The fourth composition refers to product P1 adsorbed on the MgO surface (P15) plus free product P2 (comp3 = P15 + P2). The fifth composition refers to product P2 adsorbed on the MgO surface (P25) plus free product P1 (comp4 = P25 + P1). Finally, the last composition contains free products P1 and P2 plus the isolated MgO surface at the end of the reaction (comp5 = P1 + P2 + MgO). The energy of comp0 = DMPT + H2O + MgO was used because the energy reference is equal to zero, and this value was subtracted from the others. Thus, R1 = comp0 → comp1, R2 = comp1 → comp2, P1R5 = comp3 → comp5, and P2R5 = comp4 → comp5. G



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desorption of products P1 and P2 and the reconstitution of the MgO catalyst at the end of the proposed reaction mechanism. Desorption processes are represented by elementary reaction R5. For P1 and P2 adsorbed on the terrace, we calculated the dP−Os, dS−Mg, and dO2−Mg distances as 3.71, 3.06, and 2.43 Å (Table 3). Gibbs free energy variations of P1 and P2 desorption for the terrace at room temperature are ΔGP1 R5T = 15.1 and ΔGP2 R5T = 1.9 kJ/mol, respectively. Accordingly, for the terrace at room temperature, desorption enthalpies of P1 (ΔHP1 R5T) and ) molecules were calculated to be 48.4 and 36.4 kJ/ P2 (ΔHP2 R5T = 0.1 kJ/mol·K mol, respectively, thus there is positive ΔSvib R5T because desorption releases the gas from the MgO(001) surface. ΔEvib R5T was calculated to be −9.8 and −7.5 kJ/mol for the desorption of P1 and P2 from the terrace at room temperature, respectively. Thermodynamic analysis is shown in Table 2. The spontaneous desorption of P1 and P2 molecules can take place only at temperatures respectively above 442.3 and 314.2 K from the regular surface (Figure 8).

considered the selectivity of P−S cleavage for catalyzed hydrolysis to be more relevant than the catalyzed reaction of the P−O1 bond, and we examine here a simple hydrolysis. Because the thermodynamic stability of I2T and I2D intermediates toward DMPT hydrolysis products is essential for the catalytic reaction on MgO(001), we calculated electronic energy barriers for the R2T and R2D elementary reactions (Figure 7). On both terrace and Al-doped surfaces,

Figure 7. Calculated reaction paths for the dissociation of the DMPT molecule represented by reactions R2T and R2D. The energy of the reagent was used as the energy reference, and this value was subtracted from the other image values.

the kinetic barriers for DMPT dissociation after adsorption have a direct barrier that is smaller than the reverse barrier. Accordingly, possible P−S bond reconstitution leading to DMPT formation can be hindered. Direct and reverse barriers for the terrace are respectively 141.0 and 179.2 kJ/mol. However, I2D intermediate formation is kinetically more favorable than I2T formation because on the Al-doped surface direct and reverse barriers are 91.7 and 105.6 kJ/mol, respectively. Geometric parameters of the transition states are not very different for the two surfaces. In elementary reaction R2, transition state TS2T has dP−S, dP−Os, dO2−Mg, and dS−Mg distances equal to 2.36, 2.18, 2.00, and 2.60 Å, respectively, whereas these distances for TS2D are 2.30, 1.83, 2.04, and 2.59 Å, respectively (Table 3). For the P−S bond break on the terrace and Al-doped surfaces, the wavenumbers of the imaginary frequencies are 210.9i and 240.9i cm−1, respectively. Even the transition state has fewer energy levels that can be occupied at the given temperature in comparison to the reactant ground state; the decrease in the kinetic barrier is not affected by the transition-state geometry on the Al-doped surface compared to the terrace. Consequently, these barrier results indicate that the Al-doped surface is also kinetically more active for DMPT decomposition as a result of the reconstruction of the doped MgO(001) surface after the dissociation process. After the dissociative chemisorption of water molecules represented by the R3 reaction,56 according to elementary reaction R4, the OH− and H+ ions recombine with the [PO(CH3)(OCH3)]+ and [SCH3]− ions on both terrace and Al-doped surfaces. This situation leads to the last step of the possibly catalyzed hydrolysis process of DMPT induced by the

Figure 8. Gibbs free energy variations for the desorption of products P1 and P2 from structures P15 and P25, respectively. For the terrace, desorption reactions of P1 and P2 are P1R5T and P2R5T, respectively. Likewise, for the Al-doped surface, desorption reactions of P1 and P2 are P1R5D and P2R5D, respectively.

For P1 and P2 adsorbed on the Al-doped surface, we calculated the dP−Os, dS−Mg, and dO2−Mg distances to be 3.62, 2.95, and 2.30 Å (Table 3). At room temperature, P1 1 1 desorption has parameters of ΔGPR5D = −16.5 and ΔHPR5D = P2 31.6 kJ/mol, whereas P2 has parameters of ΔGR5D = −30.2 and vib ΔHP2 R5D = 14.4 kJ/mol. In this way, ΔSR5D = 0.2 kJ/mol·K. vib ΔHR5D was calculated to be −2.7 and −1.4 kJ/mol for the desorption of P1 and P2 from an Al-doped surface at room temperature, respectively. Thermodynamic analysis is shown in Table 2. Therefore, P1 and P2 molecules can spontaneously desorb from the doped surface above 197.0 K and over the entire studied temperature range of 100−600 K, respectively (Figure 8). This situation is the greatest disadvantage in the molecular desorption enthalpy from the terrace because the doped surface tends to undergo new reconstruction and return to its initial state after the ionic recombination step in R4. The geometric parameters of products P1 and P2 in both superficial situations are very similar to free-molecule values; see Table 1. The above results show that the temperatures for the stability of P1 and P2 products after the desorption from both the terrace and Al-doped surfaces are lower in comparison to those H



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spontaneous desorption of P1 and P2 and final MgO reconstitution in the last step of the catalytic process. However, regular surface sites also participate in catalyzed hydrolysis processes and, consequently, increases the DMPT decomposition from 500 K. To summarize, in this work we have shown theoretically that a MgO(001) surface can function as a catalyst for the safe degradation of DMPT, an analogous of the VX nerve agent. The Al-doped sites have higher selectivity in comparison to the terrace sites. These results can also serve as a benchmark for further studies of VX and VX-like decomposition processes on other surfaces with other kinds of defects and dopant atoms.

necessary to form their respective intermediaries in the DMPT dissociation process. However, on the basis of the present results, the Al-doped MgO surface, compared to the terrace, has the most favorable catalytic sites for DMPT dissociative chemisorption and the spontaneous desorption of products without neurotoxic activity.

4. CONCLUSIONS We proposed a reaction mechanism for the hydrolysis of O,Sdimethyl methylphosphonothioate, DMPT, catalyzed by the MgO(001) surface. We examined the reaction mechanism using DFT, the PW91 exchange-correlation functional, and periodic boundary conditions. The DMPT molecule was used to represent the larger VX molecule in the hydrolysis mechanism. We have shown that although the VX detoxification reaction is slow it can occur because the investigated hydrolysis global reaction mechanism indicated that P−S bond heterolytic cleavage is spontaneous at room temperature (ΔGH1 = −29.7 kJ/mol). This rupture is thermodynamically more stable and occurs in greater proportion in comparison with P− O1 bond breakage. P−O1 cleavage is possible only in alkaline reactions. In catalyzed hydrolysis, DMPT spontaneously adsorbes on the MgO(001) terrace up to a temperature of 190 K with a small unfavorable entropy change of −0.1 kJ/mol·K. The main geometric parameters of the molecule remain essentially the same after adsorption. At room temperature, this process may take place but at very small rates: ΔGR1T = 13.3 kJ/mol. However, on the Al-doped surface at room temperature, DMPT adsorption happens in a very high proportion, ΔGR1D = −15.0 kJ/mol, spontaneously up to about 420 K because of a favorable adsorption enthalpy, ΔHR1D = −54.7 kJ/mol, and the same entropy change of the terrace. Therefore, this adsorption step on the doped surface was thermodynamically crucial for the next chemisorption process, also being more active. In contrast to dissociation with I2D intermediate formation at room temperature, ΔGR2D = −25.0 kJ/mol, the [SCH3]− and [PO(CH3)(OCH3)]+ fragments of the I2T intermediate are more strongly trapped on the surface, with ΔGR2T = −55.4 kJ/ mol at room temperature; this process occurs spontaneously throughout the studied temperature range of 100−600 K. The reconstruction on the Al-doped surface was essential to decreasing the dissociation enthalpy (ΔHR2D = −23.2 kJ/ mol) compared to the terrace (ΔHR2T = −40.1 kJ/mol). On the terrace, the direct electronic energy barrier of the dissociation process, 140.9 kJ/mol, is lower than for the reverse reaction, 179.3 kJ/mol, but on the Al-doped surface these values are respectively equal to 91.7 and 105.6 kJ/mol. This kinetic feature does not allow for the reconstitution of the P−S bond in both surfaces. Moreover, the lowest dissociation barrier for the Al-doped surface establishes that a reaction on this type of site is also kinetically favored. Therefore, from the ΔGsum D sum, the formation of the I2D intermediate above 335 K ensures the catalytic action of the MgO(001) surface on the Al-doped sites. For regular surface sites, this process occurs only above 500 K. For this reason, after recombination with the OH− and H+ ions, the HOPO(CH3)(OCH3) (P1) and HSCH3 (P2) products do not accumulate on the Al-doped surface because these molecules desorb at temperatures lower than 335 K. Therefore, if compared to the hydrolysis global reaction, 335 K would be an ideal temperature, close to room temperature, for avoiding product accumulation on the point defect followed by the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a CAPES-Brazilian Ministry of Defense grant. We acknowledge the computational support of CENAPAD-SP and also thank CNPq, Vale S.A. (CEX-RDP00138-10), FAPEMIG, and FAPERJ, Brazilian agencies, for support.

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