Sarin Degradation Using Brucite - The Journal of Physical Chemistry

ARTICLE pubs.acs.org/JPCC

Sarin Degradation Using Brucite Viviane S. Vaiss,† Itamar Borges, Jr.,*,‡ and Alexandre A. Leit~ao† † ‡

Departamento de Química, Universidade Federal de Juiz de Fora, Juiz de Fora, MG, 36036-330, Brazil Departamento de Química, Instituto Militar de Engenharia, Prac-a General Tiburcio, 80, Rio de Janeiro, RJ, 22290-270, Brazil ABSTRACT: Density Functional Theory (DFT) calculations with periodic boundary conditions were used to investigate a degradation reaction mechanism of the Sarin molecule (isopropyl methylphosphonofluoridate, C4H10FO2P) using the layered hydroxide brucite (Mg(OH)2). We computed the adsorption of Sarin on brucite including the infrared vibrational spectrum of gas and adsorbed Sarin, reaction paths, activation energies, and the Gibbs free energy differences (ΔG). The isopropyl methylphosphonate molecule and the Mg(OH)2
xFx compound are the global reaction products. The entire process has ΔG =
19.7 kcal/mol. Two intermediates and two transition states were found. The two transition states correspond to a hydroxyl anion motion toward the phosphorus atom and to a fluoride moving toward the hydroxyl vacant position. From the converged structures, we propose four elementary reactions for the global process. The activation barrier for the rate-limiting step indicates that the degradation reaction of Sarin using brucite is slow. The results of the Sarin deactivation process on brucite show the potential of a layered hydroxide to degrade organophosphorus compounds.

1. INTRODUCTION Nerve agents (NAs) are organophosphorus compounds which are potent inhibitors of serine esterases, especially the enzyme acetylcholinesterase, responsible for the hydrolysis of the neurotransmitter acetylcholine. The inhibition of acetylcholinesterase leads to the accumulation of acetylcholine in synapses and neuromuscular junctions causing a collapse in the central nervous system and death of the individual. NAs are esters of phosphoric acid similar to many commercially available pesticides; the most known NAs are Tabun (GA), Sarin (GB), Soman (GD), and VX.1
3 The first reported use of NAs was in 1988 during the first Persian Gulf War when Iraq used Sarin against Iran and the Kurd people. In June 1994 and March 1995, a Japanese cult utilized Sarin to attack a building in Matsumoto and the Tokyo subway.4,5 The Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction (the Chemical Weapons Convention, CWC) was issued on April 29, 1997. Since then, many efforts have been made to develop and implement methods for the identification of NAs, their precursors, or products originated from decomposition because such analysis can play an important role in verifying the Treaty, as well as enable monitoring the destruction of NAs. Due to the extreme toxicity of the NAs and the restricted use determined by the CWC, there are few experimental studies on these compounds. Wagner and collaborators6
8 using solid state nuclear magnetic resonance found that Sarin, VX, and Soman hydrolyze on the surface of reactive MgO, CaO, and Al2O3 nanoparticles. Hydrolysis of VX and Soman produces nontoxic complexes on the surfaces, namely, ethyl methylphosphonate and pinacolyl methylphosphonate, respectively. Sarin yields isopropyl methylphosphonate in the reaction with nanosize Al2O3. The reaction kinetics for all three agents on nanosize MgO is characterized by a fast initial reaction followed by a gradual slowing to a first-order steady-state reaction. The fast r 2011 American Chemical Society

reaction is consistent with liquid spreading through the porous nanoparticle aggregates. On the other hand, the steady-state reaction is identified as a gas-phase reaction, mediated by evaporation, once the liquid achieves its volume in the smallest available pores. Ewing and Lerner9 investigated the detection of Sarin using diffuse reflectance infrared spectroscopy with magnesium oxide (MgO) as the preconcentrating medium. In this method, magnesium hydroxide is first produced from magnesium oxide and water, which then hydrolyzes the Sarin to isopropyl methylphosphonic acid; the excess magnesium hydroxide reacts with the isopropyl methylphosphonate product leading to an insoluble salt. Gas chromatography mass spectrometry analysis of the Sarin/water solution indicates hydrolysis of the Sarin after 5 min of addition of the Nantek MgO and 15 min after addition of the Aldrich MgO. As an alternative to study experimentally the interactions of phosphate esters with the surface of materials, the dimethyl methylphosphonate molecule is widely used as a model system.10
14 Additionally, theoretical studies can even assist the investigation of antidotes against NA poisoning.15 Computer simulations also allow one to investigate the deactivation of NAs by a variety of materials which can be important for counter-measures.16
23 Layered hydroxides are well-known for their industrial application as adsorbents, catalysts, and catalyst supports. Brucite, Mg(OH)2, is the layered hydroxide of simplest structure, being composed by individual layers of Mg2+ in octahedral sites surrounded by six hydroxyl groups. Each OH
anion is surrounded by three metal (M) cations, resulting in neutral layers.24 When a fraction of the Mg2+ ions of the brucite layers is isomorphically substituted by M3+ ions, a positive charge excess Received: September 6, 2011 Revised: November 7, 2011 Published: November 07, 2011 24937

dx.doi.org/10.1021/jp208598c | J. Phys. Chem. C 2011, 115, 24937–24944

The Journal of Physical Chemistry C is formed in the layers, which is neutralized by interlayer anions. The outcome of these compositional changes is a series of compounds called layered double hydroxides (LDHs) or hydrotalcite-type compounds, described by the general formula 3+ x+ m
)x/m 3 nH2O, where M3+ and M2+ [M2+ (1
x)Mx (OH)2] (A m
are metal cations and A is a counterion with m
negative charge.25,26 Layered hydroxides have recently received attention in view of their potential usefulness as adsorbents of inorganic and organic contaminants. Experimental works concerning the uptake from aqueous solutions of surfactants, trinitrophenol, acid herbicides, dicamba, chromate, arsenate, and nitrate, and many other anions, have been reported.27
32 Recently, we studied the potential of the layered hydroxide brucite as adsorbent of fluoride and the mechanism of this reaction using Density Functional Theory (DFT) calculations.33 We have been using the same methodology to study brucite to investigate the formation energy of mixed neutral layered hydroxides, propose a model to LDHs, and discuss the changes that occur in the dehydration process of hydrotalcites containing Cl
and CO32
counteranions.34
36 There are few theoretical studies of the interaction between phosphates and clay minerals. The adsorption of Sarin and Soman on tetrahedral and octahedral dickite surfaces was investigated using the ONIOM method and cluster models by Michalkova et al.37 The adsorption of Sarin and Soman on dickites occurs through the formation of multiple hydrogen bonds between the adsorbed molecules and the hydroxyl groups in the octahedral side and the basal oxygen atoms in the tetrahedral side. Sarin and Soman are preferably adsorbed on the octahedral aluminum hydroxide surface rather than on the tetrahedral silica surface. In another work, Michalkova and collaborators38 studied the adsorption mechanism of both molecules on the edge tetrahedral fragments of clay minerals containing Si4+ and Al3+ as central cations. Sarin and Soman were found to interact similarly with the edge mineral fragments containing distinct central cations. The chemisorption occurs between the molecules and the AlO(OH)32
and SiO(OH)31
clusters through the formation of a P
O chemical bond. In neutral complexes, the physisorption of Sarin and Soman on such clay fragments is mediated by C
H 3 3 3 O and O
H 3 3 3 O
H bonds. According to their Gibbs free energy difference values, only strongly interacting complexes containing Al3+ are stable in room temperature. Thus, Sarin and Soman will preferably adsorb on these types of edge mineral fragments. Calculations by Murashov and Leszczynski39 of dihydrogen and dimethyl phosphate anions interacting with orthosilicic acid showed that the phosphate groups can form strong hydrogenbonded complexes with the silanols of the silica surface. A molecular dynamics study of the large tributyl phosphate complex of europium provides a test of the sensitivity of force-field calculations to predict the behavior of molecules within the interlayer of a trioctahedral smectite clay hectorite.40 In this work, we used DFT methods to study the degradation of the nerve agent Sarin, isopropyl methylphosphonofluoridate (C4H10FO2P), using a brucite (Mg(OH)2) surface. The main focus was to conceive and confirm computationally the reaction mechanism for the global reaction Mg(OH)2 + xC4H10O2PF f Mg(OH)2
xFx + xC4H11O3P. For this purpose, we proposed a formation mechanism composed of four elementary reactions and computed the corresponding formation Gibbs free energy differences, the infrared vibrational spectrum of the gas and

ARTICLE

Figure 1. Optimized Sarin structure.

adsorbed Sarin, reactions paths, transition states, activation energies, and optimized geometries.

2. METHODOLOGY The degradation of the Sarin molecule on a brucite surface was investigated with the PWscf code,41 which is a computer package for DFT42,43 electronic structure calculations that uses pseudopotentials (to describe ion cores), periodic boundary conditions, and plane-wave basis sets. The periodic boundary conditions allow the simulation of extensive surfaces without the border effects present in cluster models. PWscf is part of the Quantum ESPRESSO software distribution for the quantum simulation of matter at the atomic scale. PWscf can be used to calculate a variety of properties, such as atomic forces, stresses, structural optimizations, energy barriers, reaction paths, phonon frequencies, effective charges, dielectric tensors, scanning tunneling microscopy images, and density of states. We adopted the generalized gradient approximation GGA-PBE for the exchange-correlation functional44 and the Vanderbilt ultrasoft pseudopotentials45 to describe the ion cores of the H, O, Mg, P, C, and F atoms. The energy cutoff for the planewave basis set was fixed to 50 Ry (680 eV), and the electron density was obtained at the Γ point in the first Brillouin zone.46 We used periodic slab geometries consisting of one brucite layer with a 4  4 supercell (16 Mg atoms and a total of 80 atoms in the layer), and a vacuum layer of 15 Å was added in the (0001) direction. The employed supercells had the same angles of the optimized hexagonal brucite cell (α = β = 90
, γ = 120
), and the new optimized lattice parameters a and c were 12.495 and 19.013 Å, respectively. For all the calculated structures, the supercell angles and the lattice parameters were kept fixed with the atomic positions being optimized. The equilibrium nuclei positions of all structures were found by minimizing the total energy gradient. The relative ion positions were relaxed until all full (i.e., Newtonian) force components were smaller than 0.025 eV/Å. All the molecular graphics have been generated by the XCRYSDEN graphical package.47,48 Vibrational calculations were carried out to check the nature of all the optimized structures. Imaginary frequencies were not found for the reactants or products of all reactions. One small imaginary frequency was found for each transition state. To analyze the thermodynamics of all reactions, we computed the Gibbs free energy differences ΔG including vibrational contributions using the harmonic approximation for brucite, a solid, and vibrational, translational, and rotation contributions for the gas-phase Sarin molecule, according to the well-known expression ΔG = ΔH
TΔS. 24938

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The enthalpy H and the entropy S of brucite were calculated using the expressions H(T) = Eelec + EZPE + Evib(T) and S(T) = Svib(T), where Eelec, EZPE, Evib(T), and Svib(T) are the electronic energy, the zero point energy, and the vibrational contributions to the enthalpy and entropy, respectively. Evib(T) is given by 3 2 Evib ðTÞ ¼

ZPE

and E

7 pω 7 6 7 6
i 5 4 pωi exp
1 kB T

3N
3 6

∑

i¼1

3N
3 pω

∑

i¼1

i

ð1Þ

ð2Þ

2

The vibrational entropy Svib(T) is computed according to "

 
1 3N
3 pω pωi i vib exp S ðTÞ ¼ kB
1 kB T kB T i¼1


pωi
ln 1
exp ð3Þ kB T

∑

The Sarin molecule in the gaseous state is treated in the following approximation HðTÞ ¼ Eelec þ EZPE þ Evib ðTÞ þ Etrans ðTÞ þ Erot ðTÞ þ pV

ð4Þ where Etrans(T) and Erot(T) are the translational and the rotational contributions to the enthalpy, respectively, both equal to (3/2)RT, R being the gas constant. The pV term equals RT. The entropy S(p,T) for a gas can be calculated by the expression Sðp, TÞ ¼ Strans ðp, TÞ þ Srot ðTÞ þ Svib ðTÞ trans

ð5Þ

rot

where S (p,T) and S (T) are the translational and the rotational contributions to the entropy. The rotational and the translational entropy, in the ideal-gas approximation, are given by 8 2 9 !3=2 3 < pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi = 2 πI I I 8π k T A B C B 5 þ 3=2 Srot ðTÞ ¼ kB ln4 : ; σ h2 ð6Þ S

( "
# )  2πMkB T 3=2 kB T ðp, TÞ ¼ kB ln þ 5=2 h2 p

ð7Þ

where IA, IB, and IC are the moments of inertia of the molecule; σ is the symmetry number of the molecule; T is the absolute temperature; kB and h are the Boltzmann and Planck’s constants, respectively; p is the pressure; and M is the molecular mass. The value of the pressure was 1 atm, and T was the room temperature (T = 298.15 K). Minimum energy paths (MEPs), in the presence of the brucite surface, were computed to obtain reaction barriers and the main structural modifications. The calculation of the MEP connecting the different minimum geometries employed the nudged elastic band (NEB) method. This is an efficient approach to find the MEP between the initial and final states of a reaction on a surface

present

theo.b

P
C11

1.79

1.78

P
O1 P
F

1.48 1.62

1.47 1.60

P
O2

1.60

1.59

O2
C2

1.48

1.46

H
C2

1.10

1.09

C3
C2

1.52

1.51

C4
C2

1.52

1.52

— P
O2
C2

120.38

120.28

— C1
P
O2
C2

162.55

161.27

structural parameter

by

EZPE ¼

trans

Table 1. Bond Lengths and Angles Calculated for the Sarin Moleculea

a

All values of bond lengths are in Å. b From Walker et al. using the MP2 method with a 6-311G** basis set.51

and to estimate its energy barrier. In the NEB method a sequence of replicas (or images) is created, and each one of them is connected to its neighbors by a virtual spring with constant k, to represent the reaction course from reagents to products. The path representation created by both the images and the springs simulates an elastic band. Initially, the images are generated along a straight line by linear interpolation. Afterward, an optimization algorithm is applied to relax the images so that the elastic band converges to the MEP.49,50 In this work, for all the calculated reaction paths, 11 images were used to find the MEP using the NEB method.

3. RESULTS AND DISCUSSION Before investigating the adsorption and the degradation of Sarin on the brucite surface, we calculated the free Sarin molecule to evaluate the accuracy of the properties of the free molecule using the same methodology. There are many conformers of the Sarin molecule, which differ by rotations about the P
O and Cisopropyl
O bonds.51,52 Walker and collaborators, using the HF, MP2, and DFT methods, calculated the energy of five conformers of Sarin. Among them, there are two lowest-energy conformers. The energy difference between the lowest-energy conformers and the highest-energy conformers is approximately 30 kJ/mol and between the two lowest-energy conformers is 0.2 kJ/mol. In this work, we considered only the lowest-energy isomer as the initial geometry for optimization because the conformation of the molecules, although it affects biological activity, is not expected to influence the adsorption process.51 The optimized structure of gas-phase Sarin is shown in Figure 1, and the main structural parameters are in Table 1. For the free Sarin molecule, we computed bond lengths, angles, and the vibrational spectrum. In particular, the gas-phase Sarin structural parameters are in good agreement with the MP2/6-311G** values of Walker et al.51 There are also available experimental infrared spectra for liquid Sarin and a calculated gas-phase infrared spectrum using the DFT/ B3LYP method.16,53 Figure 2 shows the calculated infrared Sarin gas-phase spectrum, and Table 2 presents the observed and calculated vibrational modes. Anharmonicity effects were not included. Although the observed vibrational modes of liquid Sarin may be affected by intermolecular interactions, the computed gas-phase Sarin vibrational modes are in good agreement with experiment (Table 2). The computed shifts for the major bonds vary from
25 to
45 cm
1 with respect to the liquid 24939

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Table 2. Selected Frequencies (cm
1) of the Free and Adsorbed Sarin mode νas(CH3) νs(CH3)

adsorbed

adsorbed

Sarin (liq.)a

Sarin (gas)

Sarin (F)

Sarin (O)

2985 2932

3033 2950

3033 2952

3034 2953

ν(CH)

2878

2962

2957

2958

δas(CH3, iPr)

1468

1452

1452

1453

δas(CH3, iPr)

1461

1438

1439

1439

δs(CH3, iPr)

1390

1400

1401

1401

δs(CH3, iPr)

1380

1398

1400

1400

δ(CH)

1351

1350

1352

1352

ν(PdO) ν(C
O)

1277 1014

1252 970

1255 972

1249 973

νs(C
C
C)

884

877

881

881

ν(P
F)

835

790

785

792

ν(P
C)

778

743

743

745

ν(P
O)

721

693

696

693

Experimental vibrational modes for liquid Sarin are from ref 53. ν = stretching; δ = deformations; s = symmetric; as = asymmetric; iPr = isopropyl. a

Figure 2. Simulated infrared spectrum of (A) gas-phase Sarin, (B) adsorbed Sarin molecule on brucite through the fluorine, and (C) adsorbed Sarin molecule on brucite through the phosphonyl oxygen atom. The spectrum shows bands between 640 and 3100 cm
1.

Sarin molecule. The calculated vibrational modes are also in good agreement with the ones calculated by Bermudez16 using the DFT method with the hybrid B3LYP functional. From the elementary reactions shown below, we propose a mechanism for the global degradation reaction of the Sarin molecule using brucite, Mg(OH)2 + xC 4 H 10 O2 PF f Mg(OH)2
xF x + xC 4 H11 O3 P, MgðOHÞ2 þ xC4 H10 O2 PF f MgðOHÞ2 3 3 3 xC4 H10 O2 PF ðPhysisorptionÞ

ðR1Þ

MgðOHÞ2 3 3 3 xC4 H10 O2 PF f ½MgðOHÞ2
x þ Fx
3 3 3 xC4 H11 O3 P ðChemisorptionÞ

ðR2Þ

½MgðOHÞ2
x þ Fx
3 3 3 xC4 H11 O3 P f MgðOHÞ2
x F 3 3 3 xC4 H11 O3 P ðIonic RecombinationÞ

ðR3Þ

MgðOHÞ2
x F 3 3 3 xC4 H11 O3 P f MgðOHÞ2
x Fx þ xC4 H11 O3 P ðDesorptionÞ

ðR4Þ

Reaction R1 is the adsorption of the Sarin molecule on the brucite layer; R2 is the attack of the hydroxyl oxygen atom OLHa onto the Sarin molecule adsorbed on the brucite; R3 corresponds to the migration of the adsorbed fluoride into the OLHa hydroxyl vacant position forming the Mg(OH)2
xF 3 3 3 xC4H11O3P compound; and the R4 products are both the isopropyl methylphosphonate molecule and the free Mg(OH)2
xFx compound. The first interaction of the Sarin molecule with the brucite surface occurs through adsorption. Experimental and theoretical studies concerning the adsorption of organophosphorus compounds on surface materials show that the main stabilization effect results from the formation of a chemical bond between the

Figure 3. Adsorbed Sarin molecule on the brucite layer. (A) Adsorbed Sarin through the fluorine atom and (B) adsorbed Sarin through the phosphonyl oxygen atom.

surface and phosphonyl oxygen atoms. Therefore, we analyzed the interactions of fluorine and phosphonyl oxygen atoms (PdO) with the brucite layer. The optimized structures of Sarin adsorbed on brucite, Mg(OH)2 3 3 3 xC4H10O2PF, through the fluorine and phosphonyl oxygen atoms, are shown in Figure 3, and the main geometric parameters are presented in Table 3. The results show a weak interaction between Sarin and the brucite surface, as indicated by the distances F
Ha (2.41 Å) and O
Ha (2.27 Å). The distances P
F (1.62 Å) and PdO (1.48 Å) in the adsorbed Sarin are equal to the free Sarin molecule values, and the distances OL
Mg1 = OL
Mg2 = OL
Mg3 (2.07 Å) remain the same as the isolated brucite. These results indicate a physisorption of Sarin molecule on brucite. The infrared spectra for the adsorbed Sarin on the brucite through the fluorine and the phosphonyl oxygen atoms were simulated for comparison with the gas-phase Sarin infrared spectrum. The calculated infrared spectra for the adsorbed Sarin are depicted in Figure 2, and the calculated vibrational modes are presented in Table 2. The weak interaction between the Sarin molecule and brucite is confirmed by a small shift in the bands 24940

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Table 3. Some Geometry Parameters Associated with Adsorption of the Sarin Molecule on the Brucite Surfacea d(P
F)

structure

d(P
O)

d(F
Ha)

d(O
Ha)

d(O
Ha)

d(OL
Mg)

C4H10FO2P

1.62

1.48

-

-

-

-

Mg(OH)2 3 3 3 xC4H10O2PFb Mg(OH)2 3 3 3 xC4H10O2PFc Mg(OH)2

1.62

1.48

2.41

3.78

0.97

2.07

1.62

1.48

3.03

2.27

0.97

2.07

-

-

-

-

0.97

2.08

a The letters a and L denote the nearest brucite hydrogen and oxygen atoms, respectively, from the Sarin molecule. All values are in Å. b Sarin adsorbed by the fluorine atom. c Sarin adsorbed by the phosphonyl oxygen atom.

Table 4. Calculated Values of ΔE, ΔZPE,
ΔTS, ΔH, ΔU, ΔA, and ΔG (kcal/mol) for the Four Elementary Reactions (T = 298.15 K) ΔE

ΔZPE


ΔTS

ΔH

ΔU

a

R1


0.9


4.4

11


5.1


4.5

6.5

R1b


1.2


4.5

10.3


5.3


4.8

5.5

4.9

R2


2.9

2.8

0.8


1.3


1.3


0.5


0.5

R3


25.4


1.3


1.2


25.9


25.9

R4

14.5


1.7


11.6

13.5

12.9

reaction

ΔA


27 1.3

ΔG 5.9


27 1.9

Sarin adsorbed by a fluorine atom. b Sarin adsorbed by a phosphonyl oxygen atom. a

corresponding to the vibrational modes of the major bonds involved in the adsorption in comparison to the free molecule. The bands related to the P
F and PdO bond stretching in the adsorbed Sarin through the fluorine atom and the phosphonyl oxygen atom are 785 and 1249 cm
1, respectively; they are shifted to lower wavenumbers in the adsorbed molecule compared to the free molecule. The computed Δν(P
F) shift with respect to the free molecule wavenumbers is
5 cm
1 and for Δν(PdO) is
3 cm
1. The other bands shift a little to higher wavenumbers. The calculated adsorption Gibbs free energy variation (ΔGR1) is 5.9 kcal/mol for Sarin adsorbed through the fluorine atom and 4.9 kcal/mol for Sarin adsorbed on the phosphonyloxygen atom. We calculated the enthalpy variations (ΔH), Gibbs free energy variations (ΔG), and the TΔS term at room temperature (Table 4). The values of ΔH, at room temperature, are negative, and the values of ΔG are positive. Thus, the destabilizing entropy contribution is larger than the stabilizing contribution arising from the enthalpy at room temperature. This is a general phenomenon that occurs for reactions when one reagent is a gas due to the larger contribution to the TS value for the gas phase. Such significant negative contribution is provided by the Strans, Srot, and Svib terms. In this work, we consider the Mg(OH)2 3 3 3 xC4H10O2PF compound an intermediate of the global reaction. The DFT method has been successfully applied to a variety of atoms, molecules, and solids both to forward knowledge and to analyze experimental results. However, it has been reported that DFT with local density approximation functionals has deficiencies with respect to weak and long-range interactions, such as the van der Waals. Therefore, calculations with dispersion corrections would be needed.54,55 On the other hand, it is known that ab initio second-order Møller
Plesset perturbation theory (MP2) includes dispersion forces. For instance, Sponer et al.56 calculated interaction energies for more than 80 geometries of stacked cytosine dimers using MP2 and DFT methods and concluded that none of the DFT methods can fully reproduce the MP2 results; thus, the DFT method needs further improvement to describe the base stacking energy satisfactorily. Physisorption

Figure 4. Computed scheme of the four elementary reactions. (A) Adsorbed Sarin on the brucite through the fluorine atom. (B) Optimized structure of the Mg(OH)2
x+Fx
3 3 3 C4H11O3P compound formed from the Sarin attack by the brucite hydroxyl. (C) Optimized structure of the Mg(OH)2
xF 3 3 3 xC4H11O3P compound formed by an iso propyl methylphosphonate molecule adsorbed on the surface of the Mg(OH)2
xF compound. (D) Desorption of the isopropyl methylphosphonate molecule adsorbed on the surface of the Mg(OH)2
xF compound.

processes occur through dispersion interactions; nevertheless, we consider that inclusion of dispersion corrections does not change considerably the low physisorption binding energy in this system based on the fact that the value of the computed adsorption Gibbs free energy variation agrees with the values, for similar systems, calculated by Michalkova et al.38 using the B3LYP/6-31G(d) and MP2/6-31G(d) methods. Using both approaches, those authors found positive values of the Gibbs energy differences (≈ 4 kcal/mol) for the adsorption of Sarin and Soman on the edge of tetrahedral fragments of clay minerals containing Si4+ and Al3+ as central cations. The oxygen atom of the hydroxyl OLHa attacks the Sarin molecule adsorbed on the brucite by breaking the P
F bond and forming a P
OL bond, resulting in an isopropyl methylphosphonate molecule and an adsorbed fluoride on the positively charged brucite layer (R2). The structure of the formed compound, Mg(OH)2
x+Fx
3 3 3 xC4H11O3P, is shown in Figure 4. To propose a mechanism for the degradation process of Sarin, 24941

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Table 5. Some Geometry Parameters Associated with the Dissociation of the Sarin Molecule on the Brucitea structural parameter

Mg(OH)2 3 3 3 xC4H10O2PF

ET1

[Mg(OH)2
x]+Fx
3 3 3 xC4H11O3P 3.34

d(P
F)

1.62

1.74

d(F
Ha)

2.41

1.92

1.24

d(OL
P)

4.50

1.94

1.58

d(OL
Ha)

0.97

0.98

1.15

d(OL
Mg1)

2.07

2.43

3.84

d(OL
Mg2)

2.07

2.81

4.29

d(OL
Mg3)

2.08

3.06

4.01

ET1 corresponds to the transition state of the formation reaction of the [Mg(OH)2
x]+Fx
3 3 3 xC4H11O3P intermediate R2. The letters a and L denote the nearest brucite hydrogen and oxygen atoms, respectively, from the Sarin. The numbers 1
3 denote the nearest brucite magnesium atom from the OL. All values are in Å. a

Figure 5. Calculated minimum-energy path for the second reaction step (R2: degradation of the Sarin molecule by brucite), then formation of the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P compound (A) and, for the third reaction step (R3: formation of the Mg(OH)2
xFx 3 3 3 xC4H11O3P compound) (B). The relative energy corresponds to the energy difference between the image step and the reactants (in each elementary reaction, the energy of the reactants was taken as zero).

Table 6. Calculated Geometry Parameters Associated with the Formation of the Mg(OH)2
xFx 3 3 3 xC4H11O3P Intermediate from the [Mg(OH)2
x]+Fx
3 3 3 xC4H11O3P Intermediate R3 and to Changes on the Surface of the Structure during the Incorporation of Fluoride in the Lamellar Structurea structural parameter

[Mg(OH)2
x]+Fx
3 3 3 xC4H11O3P

ET2

Mg(OH)2
xFx 3 3 3 xC4H11O3P

d(P
F)

3.34

3.17

3.58

d(F
Ha)

1.24

1.20

1.50

d(OL
P)

1.58

1.58

1.60

d(OL
Ha) d(F
Mg1)

1.15 5.22

1.15 4.04

1.03 2.15

d(F
Mg2)

5.25

4.06

2.14

d(F
Mg3)

3.56

2.79

2.13

a

ET2 corresponds to the transition state of the formation reaction of the Mg(OH)2
xF 3 3 3 xC4H11O3P intermediate. The labeling is the same as in Table 5. All values are in Å.

we used the structure of the adsorbed Sarin through the fluorine atom; this procedure can be adopted because the difference of the adsorption Gibbs free energy between the two modes of adsorption is small, and because our goal was to break the P
F bond; the structure of the adsorbed Sarin in this situation favored this break. The P
F, OL
P, and F
Ha distances were originally 1.62, 4.50, and 2.41 Å for the

adsorbed Sarin and changed to 3.34, 1.58, and 1.24 Å in the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P compound (Table 5). The increase of the P
F distance shows the break of this bond; the decreased OL
P distance shows the formation of this bond; and the decrease of the F
Ha distance indicates a strongly adsorbed fluoride on the positively charged brucite layer. This result agrees with the experimental work that identified the 24942

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The Journal of Physical Chemistry C isopropyl methylphosphonate compound as the product of hydrolysis of the Sarin by brucite.9 The calculated Gibbs free energy variation for the Mg(OH)2
x+ Fx
3 3 3 xC 4 H11 O 3 P compound (ΔG R2 ) is
0.5 kcal/mol. This result indicates that the P
F bond breaking and the subsequent formation of the isopropyl methylphosphonate molecule can be a favorable process. Figure 5 shows the minimum energy path calculated for formation of the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P compound. The activation energy is 32.6 kcal/mol and was calculated using the energy difference between the saddle point structure and the Mg(OH)2 3 3 3 xC4H10O2PF structure. The barrier of the reverse path is 33.1 kcal/mol, calculated from the energy difference between the saddle point and the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P compound. We obtained only one imaginary frequency for the transition state (wavenumber = 111.6i cm
1 ) corresponding to an O LH a hydroxyl anion motion toward the phosphorus atom and a fluorine moving toward its position in the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P compound. The distance P
F increases from 1.62 Å in the Mg(OH)2 3 3 3 xC4H10O2PF compound to 1.74 Å in the transition state, and the distance P
OL decreases from 4.50 to 1.94 Å (Table 5). In this transient structure, where the OL
Mg1, OL
Mg2, and OL
Mg3 bonds are breaking, these distances are 2.43, 2.81, and 3.06 Å, respectively. In the third reaction step R3, the adsorbed fluoride on the brucite surface in the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P compound migrates into the OLHa hydroxyl vacant position forming the Mg(OH)2
xFx 3 3 3 xC4H11O3P compound depicted in Figure 4. The structure of this compound is formed by an isopropyl methylphosphonate molecule adsorbed on the surface of the Mg(OH)2
xFx compound. The distances F
Ha, F
Mg1, F
Mg2, and F
Mg3 in the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P were 1.24, 5.22, 5.25, and 3.56 Å, respectively, and in the Mg(OH)2
xFx 3 3 3 xC4H11O3P became 1.50, 2.15, 2.14, and 2.13 Å (Table 6). For the R3 step, the calculated formation Gibbs free energy variation (ΔGR3) is
27.0 kcal/mol. This value shows that the formation of the Mg(OH)2
xFx compound with an isopropyl methylphosphonate molecule adsorbed on its surface is a spontaneous process. The minimum energy path for this reaction was also calculated, and it is shown in Figure 5. The activation energy for the formation of the Mg(OH)2
xF 3 3 3 xC4H11O3P compound from the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P is calculated using the energy difference between the saddle point structure and the structure of the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P. The computed value is 5.6 kcal/mol for the direct reaction. The reverse path has a barrier of 32.6 kcal/mol, and it was calculated by the energy difference between the saddle point and the Mg(OH)2
xFx 3 3 3 xC4H11O3P compound. The calculated transition state has only one imaginary frequency with a wavenumber of 108.7i cm
1, corresponding to a fluoride moving toward the OLHa hydroxyl vacant position. The distances F
Ha, F
Mg1, F
Mg2, and F
Mg3 decrease from 1.24, 5.22, 5.25, and 3.56 Å in the Mg(OH)2
x+Fx
3 3 3 xC4H11O3P compound to 1.20, 4.04, 4.06, and 2.79 Å in the transition state (Table 6). Removing the adsorbed isopropyl methylphosphonate molecule in the Mg(OH)2
xFx 3 3 3 xC4H11O3P compound, we obtain the isopropyl methylphosphonate molecule and the free Mg(OH)2
xF compound (R4) (Figure 4). The computed desorption Gibbs free energy variation (ΔGR4) is 1.9 kcal/mol.

ARTICLE

The Gibbs free energy variation for the Sarin reaction with brucite can be calculated through the expression ΔGR1 + ΔGR2 + ΔGR3 + ΔGR4 and equals
19.7 kcal/mol. The results of the deactivation process of the Sarin using brucite show the potential of this layered hydroxide to degrade organophosphorus compound inhibitors of the enzyme acetylcholinesterase, as verified experimentally by Ewing and Lerner.9

4. CONCLUSIONS We studied the degradation reaction of Sarin using brucite. This process was investigated by calculations based on DFT. We computed the physisorption and the dissociative chemisorption of the Sarin molecule on brucite, the infrared vibrational spectrum of the gas and adsorbed Sarin, reaction paths, activation energies, and Gibbs free energy variations. We analyzed the adsorption of the Sarin on the brucite layer through the fluorine and the phosphonyl oxygen atoms (PdO). The weak interaction between the Sarin molecule and the brucite surface indicates that the Sarin molecule physisorbs on brucite. The bands corresponding to the vibrational modes of the major bonds involved in the adsorption, namely, P
F and PdO, are shifted to lower wavenumbers in the adsorbed molecule compared to the free molecule. The computed Δν(P
F) shift with respect to the free molecule wavenumbers is
5 cm
1 and Δν(PdO) is
3 cm
1. The adsorption Gibbs free energy difference of the two adsorption modes is small, being 5.9 kcal/mol for Sarin adsorbed through the fluorine atom and 4.9 kcal/mol for Sarin adsorbed on the phosphonyloxygen atom. Two intermediates and transition states were found. The two transition states correspond to a hydroxyl anion motion toward the phosphorus atom and to a fluoride moving toward the hydroxyl vacant position; the activation barrier for the ratelimiting step, among the calculated ones, is 32.6 kcal/mol, corresponding to the adsorbed Sarin attacked by the brucite hydroxyl. The result is an isopropyl methylphosphonate molecule and an adsorbed fluoride on the positively charged brucite layer. The magnitude of this reaction barrier is high enough to consider that the degradation reaction of the Sarin on brucite is slow. The products of the global reaction were an isopropyl methylphosphonate molecule and the Mg(OH)2
xFx compound. The entire process has ΔG =
19.7 kcal/mol. A set of four elementary reactions for the global mechanism were proposed. The results of the deactivation process of Sarin using a brucite surface show the potential of layered hydroxides to degrade organophosphorus compound inhibitors of the enzyme acetylcholinesterase. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been supported through a CAPES-Brazilian Ministry of Defense grant. The authors also thank CNPq, Vale S.A. (CEX-RDP-00138-10) FAPEMIG and FAPERJ, Brazilian agencies, for support. 24943

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The Journal of Physical Chemistry C

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