Mechanistic Insight into the Molecular TiO2-Mediated Gas Phase

Article pubs.acs.org/crt

Mechanistic Insight into the Molecular TiO2‑Mediated Gas Phase Detoxication of DMMP: A Theoretical Approach Tamalika Ash, Tanay Debnath, Avik Ghosh, and Abhijit Kumar Das* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India S Supporting Information *

ABSTRACT: The detoxication of DMMP (dimethyl methylphosphonate) mediated by molecular TiO2 has been investigated computationally using density functional theory (DFT). From our previous studies, it is evident that the unimolecular detoxication of OPCs (organophosphorus compounds) is kinetically unfeasible at room temperature due to the significantly high activation barrier. Thus, the aim of our work is to find out whether molecular TiO2 can make any significant impact on the kinetic feasibility of the detoxication processes or not. Here, we have identified a total of three detoxication pathways, where in the first step the detoxication occurs through H-abstraction with the assistance of TiO2, and in the second step, the titanium complex is separated from the respective phospho-titanium complexes. The outcomes reveal that the TiO2-mediated detoxication pathways are at least 20.0 kcal/mol more favorable than their respective unimolecular pathways and that among them, the α-H-mediated isomerization is found to be the most feasible pathway. When the separation of a titanium complex is under consideration, the double H2O-assisted mechanism is found to be the favored pathway. Overall, the entire work provides a widespread idea about the efficiency of molecular TiO2-assisted detoxication of DMMP, which can be well applicable to other OPCs also. example, brucite, 11 MgO(001), 12 TiO 2 , 13−21 SiO 2 , 22,23 Rh(100),24 Mo(111),25 Pt(111),26 Ni(111),27 Pd(111),27 Al2O3,28−31 Fe2O3,32 and Y2O333 have already been used for degradation purposes. Among all of these decomposing reagents, degradation using the TiO2 surface and TiO2 powder are very well known. Rusu et al.13 studied the adsorption and decomposition phenomena of DMMP on TiO2 surfaces. They showed that DMMP is adsorbed on powdered TiO2 through the interaction of the electron rich PO end with the Ti+n (n = 3,4) center. By using a transmission FTIR study, they observed that DMMP gets decomposed above 214 K through the breakage of the P−O bond. Moss et al.14 investigated the adsorption and degradation of DMMP over the UV-irradiated TiO2 powder. They characterized the adsorption of DMMP on the TiO2 surface by DRIFTS and revealed that adsorbed DMMP is photodegraded in a stepwise manner to remove PO4−3, H2O, and CO2 as products. Panayotov et al.15 also investigated the degradation of DMMP on nanoparticles Au/ TiO2 and the pure TiO2 surface. They showed that the Au nanoparticles on the TiO2 surface activate the DMMP toward oxidation compared to pure TiO 2. Trubitsyn et al.16 investigated the decomposition of DMMP experimentally using the FTIR technique on the high surface area of anatase TiO2. They showed that at first DMMP undergoes reactive adsorption and then decomposes catalytically. Hence, it is

1. INTRODUCTION Organophosphorus compounds1−34 (OPCs), one of the most nefarious of synthetic chemical reagents, have motivated researchers to investigate their convenient detection and detoxication processes. OPCs are mainly characterized by the presence of a phosphorus(V) center with a terminal oxide and three single-bonded substituents (alkyl, alkoxyl, sulfhanyl, etc.). OPCs such as sarin, soman, VX, etc. are extremely toxic nerve and blistering agents and are often known as chemical warfare agents (CWAs).3,4 OPCs can be differentiated from other CWAs because of the presence of their phosphorylating mode of action.3 Some of OPCs are also used as pesticides for controlling pests (for example, paraoxon, parathion, etc.), which possess the same mode of action as nerve agents but are less hazardous. The nerve agents act as potent inhibitors of the enzyme acetylcholinesterase (AChE)35−37 which carries out the hydrolysis of the neurotransmitter acetylcholine. The inhibition of the activity of acetylcholinesterase by OPC leads to excess accumulation of acetylcholine which is very dangerous to humans. Because of the severe toxicity of CWAs, the use of such CWAs in the laboratory is quite unsafe. Thus, various innocuous and inexpensive simulants having similar chemical and physical properties such as those of CWAs are frequently used in the laboratory, and dimethyl methylphosphonate (DMMP) is one of the well-known simulants.25,30,32 The decomposition of OPCs using different types of metal oxide surfaces, powdered metal oxides, etc. is of great interest at present. Solid surfaces as well as powder metal oxides, for © XXXX American Chemical Society

Received: January 25, 2017 Published: April 12, 2017 A

DOI: 10.1021/acs.chemrestox.7b00019 Chem. Res. Toxicol. XXXX, XXX, XXX−XXX

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Chemical Research in Toxicology

Table 1. Activation Barrier Heights (ΔEa) and Reaction Enthalpy (ΔHrxn) Values Related to the IMα Pathway at the M06-L/ genECP Levela ΔEa

IMαuni

ΔHrxn

61.2

44.8

1st step IMαTiO2

channel

ΔEaddct

ΔEa

ΔHrxn

TiO2

−46.7

19.9

17.8

IMα1st

a

2nd step ΔEaddct

ΔEa

ΔHrxn

2 IMα2nd_unassist TiO2 IMα2nd_H2O

−37.7

49.8 23.8

40.5 14.6

TiO2 IMα2nd_2H 2O

−49.2

12.3

10.4

channels TiO

The values are reported in kcal/mol.

Table 2. Activation Barrier Heights (ΔEa) and Reaction Enthalpy (ΔHrxn) Values Related to the IMβ Pathway at the M06-L/ genECP Levela ΔEa

IMβuni

ΔHrxn

62.9

46.2

1st step IMβTiO2

a

2nd step

channel

ΔEaddct

ΔEa

ΔHrxn

2 IMβTiO 1st

−46.7

35.9

21.4

ΔEaddct

ΔEa

ΔHrxn

2 IMβTiO 2nd_unassist TiO2 IMβ2nd_H2O

−39.5

47.8 25.2

33.1 13.3

TiO2 IMβ2nd_2H 2O

−51.8

15.8

10.3

channel


processes instead of catalytic processes. Overall, the entire work is thus intended to provide a better understanding of the detoxication of the OPCs mediated by molecular TiO2, which endorses the detoxication in an easier way.

observed that until now a lot of work has been performed regarding the decomposition of OPCs mediated by TiO2 surfaces. However, as far our knowledge goes the degradation of OPCs assisted by molecular TiO2 is completely unveiled. In our present study, we have investigated whether similar to the TiO2 surface, molecular TiO2 can act as an alternative assisting reagent for facilitating the degradation of OPCs or not. DMMP has two types of hydrogens, namely, α- and β-H via which the OPC can be detoxified. Yang et al.38 investigated the gas phase unimolecular decomposition of DMMP using different DFT functionals. The α-H-mediated detoxication of DMMP can occur only via an isomerization process, whereas for β-H assistance, the detoxication can be achieved in two ways, isomerization and decomposition. All three TiO2mediated detoxication pathways are three-step processes, where the pathways are initiated by the formation of a strong 1:1 adduct between DMMP and TiO2. In the first step, unlike the unimolecular process, the H-abstraction takes place by the TiO of TiO2 instead of PO, which facilitates detoxication by reducing the activation barrier heights as well as decreasing the reaction enthalpy values. The second step deals with the separation of the titanium complex from the phospho-titanium complex, which can be obtained in three ways; among them, one is an unassisted mechanism, and rest are water-assisted pathways. The separation of the titanium complex from the phospho-titanium complex using two H2O molecules makes the reaction kinetically more feasible than the unassisted and single H2O-assisted one. The unassisted pathway directly separates TiO2 from the phospho-titanium complex, whereas for water-assisted processes, OTi(OH)2 is obtained as a side product along with the phosphorus containing nontoxic compounds. The recovery of TiO2 from OTi(OH)2 can also be achieved in three ways; among which, the double H2Oassisted pathway is the kinetically most feasible way to regain molecular TiO2. However, due to the very high endothermic nature, molecular TiO2 is hard to reproduce, and thus, it is better to denote the detoxication processes as TiO2-assisted

2. COMPUTATIONAL DETAILS All geometry optimizations and electronic structure calculations have been performed using Gaussian 09, revision D.01,39 suite of the quantum chemistry program. Density functional theory (DFT)40,41 has been adopted for optimizing the geometries of all of the reactants, transition states (TSs), and products involved in the unimolecular and TiO2-mediated detoxication pathways of DMMP, where we have employed the M06-L42,43 functional in conjunction with the 6-311+ +G(d,p) 44−46 basis set for main group elements and the LANL2DZ47−49 basis set for the Ti atom. Altogether, the basis set is designated as the genECP basis set. The M06-L, a local functional belonging to the M06 family, was developed by Zhao and Truhlar.42 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 (such as MP250). The metaGGA M06-L functional is highly recommended for transition metal chemistry because of its good performance during the energetics calculation. Medvedev et al.41 demonstrated that the meta-GGA functionals are built by the constraint−satisfaction approach, which produce much better electron densities, and thus, almost accurate energetics can be achieved. Even though M06-L is a local functional, it is not only important for theoretical reasons but also essential for practical calculations of large complex systems employing specialized algorithms that are tens or hundreds of times faster for local functionals than nonlocal ones. 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 the right minima, a parallel intrinsic reaction coordinate (IRC)51,52 calculation has been performed with all TSs. All energies reported in the article are zero-point-corrected electronic energies obtained at 0 K temperature and 1 atm pressure. B


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Table 3. Activation Barrier Heights (ΔEa) and Reaction Enthalpy (ΔHrxn) Values Related to the Dβ Pathway at the M06-L/ genECP Levela ΔEa

Dβuni

ΔHrxn

75.6

40.2

1st step DβTiO2

channel TiO2

Dβ1st

a

2nd step

ΔEaddct

ΔEa

ΔHrxn

−46.7

53.3

13.6

ΔEaddct

ΔEa

ΔHrxn

2 Dβ2nd_unassist TiO2 Dβ2nd_H2O

−32.4

48.7 19.0

34.3 9.2

TiO2 Dβ2nd_2H 2O

−45.6

11.4

7.0

channel TiO


Scheme 1. Predicted Reaction Pathways for TiO2-Mediated Detoxication of DMMP

The kinetics study for both the unimolecular as well as TiO2mediated detoxication pathways of DMMP is performed using transition state theory (TST).53−56 All of the kinetics calculations are performed for a temperature range of 300 K−1500 K keeping the pressure fixed at 1 atm. The entire kinetic study is performed with The Rate program,57 where the rotations are treated classically, and vibrations are treated quantum mechanically within the harmonic approximation.

3. RESULTS AND DISCUSSION DMMP has two types of hydrogens (α- and β-) that can take part in the detoxication processes. In our previous article,58 we have explored all possible unimolecular detoxication pathways of both sarin and soman via intramolecular H-transfer. For DMMP also, the unimolecular detoxication associated with αand β-H transfers will follow similar mechanisms. However, to detoxify the OPCs in a more efficient way we have C


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Chemical Research in Toxicology mechanistically explored the molecular TiO2-mediated detoxication pathways of DMMP. To interpret the assisting efficiency of molecular TiO2, we have compared the feasibility of the explored pathways with the respective unimolecular pathways from the perspective of activation barrier heights. The activation barrier heights and reaction enthalpy values for all three detoxication pathways are tabulated in Tables 1, 2, and 3. The predicted TiO2-mediated detoxication pathways of DMMP are shown in Scheme 1. The optimized geometries of the TSs for both the isomerization and decomposition pathways are given in Figures 1, 2, and 3, and the associated PESs (Potential energy surfaces) are depicted in Figures 4, 5, and 6.

Figure 2. Optimized geometries of the TSs associated with the IMβ pathway at the M06-L/genECP level. Bond lengths are given in angstroms (Å).

Figure 1. Optimized geometries of the TSs associated with the IMα pathway at the M06-L/genECP level. Bond lengths are given in angstroms (Å).

3.1. α-H Assisted Isomerization Pathway: IMα. 3.1.1. Unimolecular Pathway: IMαuni. As evident from our previous article,58 the α-H-assisted unimolecular isomerization pathways of sarin and soman are mechanistically quite similar to the keto-enol type tautomerization. In the case of DMMP also, the α-H-facilitated isomerization (IMαuni) occurs through the transfer of an α-H from -CαH3 to PO through the formation of a four-membered ring in the TS (TSuni IMα), and a phospho-enol type complex (P1) is obtained as the final product, where the P-center remains in +5 oxidation state. The activation barrier associated with this pathway is calculated to be 61.2 kcal/mol, which is too high to make the reaction kinetically feasible. Moreover, the highly positive reaction enthalpy value (44.8 kcal/mol) shows its endothermic character. So, both the kinetic as well as thermodynamic parameters indicate that the unimolecular isomerization via α-H transfer is unfeasible in nature. 3.1.2. TiO2-Mediated Pathway: IMαTiO2. In order to increase the feasibility of the isomerization pathway, we have performed the aforesaid reaction in the presence of molecular TiO2. The TiO2-mediated isomerization of DMMP via internal α-H

Figure 3. Optimized geometries of the TSs associated with the Dβ pathway at the M06-L/genECP level. Bond lengths are given in angstroms (Å).

transfer (IMαTiO2) is a three-step reaction, where in the first 2 step a phospho-enol-titanium complex (intTiO IMα_1st) is obtained as the product. The separation of the titanium complex from 2 intTiO IMα_1st occurs in the second step of the reaction, which can be achieved in three ways. Among them, one is an unassisted pathway, where TiO2 gets eliminated directly, and for the other two cases, OTi(OH)2 is separated out through the assistance of single and double H2O molecules. The recovery of TiO2 from OTi(OH)2 is the third step of the reaction, which can also be carried out in three ways. In the following section, we have discussed all these steps elaborately. TiO 3.1.2.1. First Step: IMα1st 2. In the presence of molecular TiO TiO2, at first DMMP forms a 1:1 adduct (add1st 2) with an adduct formation energy of −46.7 kcal/mol. In the adduct state, D


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the oxygen of -PO interacts with the Ti(+4) center, and the Ti···O distance is found to be 2.05 Å. As evident from Figure 4, after the formation of an adduct, one of the oxygens of TiO2 abstracts an α-H from -CαH3 to produce a phospho-enol2 titanium complex (intTiO IMα_1st), where Ti becomes bonded with the oxygen of PO having a Ti−O distance of 1.85 Å. The simultaneous conversion of -TiO into -Ti−OH and -P−CH3 into PCH2 occur through the formation of a well-defined six2 membered TS (TSTiO IMα_1st) (see Figure 1) of an activation barrier of 19.9 kcal/mol. The important aspect of the TiO2mediated process is the reduction of an activation barrier by an amount of 41.3 kcal/mol compared to that of the IMαuni pathway. The abstraction of α-H by TiO instead of PO leads to the production of a stable six-membered TS, which increases the feasibility of the reaction by lowering the 2 activation barrier. Though the IMαTiO 1st pathway is endothermic in nature (17.8 kcal/mol), the calculated reaction enthalpy for this pathway is 27.0 kcal/mol lower than the respective unimolecular process. Hence, not only from the perspective of an activation barrier but also from the thermodynamic viewpoint, it is well understood that the assistance of molecular TiO2 increases the feasibility of the isomerization pathway both kinetically and thermodynamically. TiO 3.1.2.2. Second Step: IMα2nd2. The second step, which is associated with the separation of the titanium complex from 2 intTiO IMα_1st, can be obtained in three ways. Among them, the

Figure 4. Potential energy surface for the IMα pathway at the M06-L/ genECP level. Relative energy values are given in kcal/mol. The green line indicates the most feasible path of detoxication.

Figure 5. Potential energy surface for the IMβ pathway at the M06-L/genECP level. Relative energy values are given in kcal/mol. The green line indicates the most feasible path of detoxication. E


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Figure 6. Potential energy surface for the Dβ pathway at the M06-L/genECP level. Relative energy values are given in kcal/mol. The green line indicates the most feasible path of detoxication.

type complex (P1) through the separation of OTi(OH)2. 2 The reaction occurs via a four-membered TS (TSTiO IMα_2nd_H2O) with an activation barrier of 23.8 kcal/mol, which is about 26.0 TiO2 ). When kcal/mol lower than the unassisted one (IMα2nd_unassist the reaction enthalpy is considered, it is noticed that although the products P1 and OTi(OH)2 lie 14.6 kcal/mol above 2 addTiO IMα_2nd_H2O, the corresponding value is 25.9 kcal/mol lower TiO2 pathway. Thus, both the kinetic than that of the IMα2nd_unassist and thermodynamic feasibilities increase due to the participation of H2O. (3) Double H2O-assisted channel for the separation of the 2 titanium complex: IMαTiO 2nd_2H2O. Similar to the single H2OTiO2 assisted channel (IMα2nd_H2O), here also one of the H2O molecules is weakly attached to the Ti(+4) center, where the H2O···Ti distance is 2.09 Å. The second H2O molecule is attached to the first H2O through a hydrogen bonding interaction, and the distance between two H2O molecules is TiO2 ), where 1.60 Å. The adduct is designated as (addIMα_2nd_2H 2O the adduct formation energy is observed to be −49.2 kcal/mol. As depicted in Figure 4, in this pathway, at first a H2O molecule attacks the Ti(+4) center, facilitating the breakage of the Ti−O bond, and simultaneously, P−O− abstracts a proton from the second H2O to produce OH− which abstracts a proton from TiO2 ) associated the first H2O molecule. The TS (TSIMα_2nd_2H 2O

unassisted process directly separates TiO2, whereas for the rest 2 of the cases, OTi(OH)2 is separated out from intTiO IMα_1st. (1) Unassisted channel for the removal of TiO 2 : TiO2 TiO2 IMα2nd_unassist . The direct removal of TiO2 from intIMα_1st occurs via simultaneous transfer of proton from -Ti−OH to -P−O, and consequently, the Ti−O bond breaks through the TiO2 ). The formation of a four-membered TS (TSIMα_2nd_unassist barrier height associated with this conversion is calculated to be 49.8 kcal/mol, which is too high to make the reaction kinetically plausible. At the end of the reaction, a phosphoenol type complex (P1) is obtained as the final product, and TiO2 is released in its molecular form. The endothermicity value calculated for this step is 40.5 kcal/mol. Hence, the high activation barrier along with the highly positive reaction enthalpy indicate the difficulty to release TiO2 via an unassisted mechanism. (2) Single H2O-assisted channel for the separation of the TiO2 titanium complex: IMα2nd_H . The reaction commences with 2O TiO2 ) between the formation of a 1:1 adduct (addIMα_2nd_H 2O TiO2 intIMα_1st and H2O, where the H2O molecule gets weakly bonded with the Ti(+4) center having a H2O···Ti distance of 2.10 Å. The adduct formation energy is calculated to be −37.7 kcal/mol. The attack of the H2O molecule to the Ti(+4) center results in the breakage of the Ti−O bond, and consequently, P−O− abstracts a proton from H2O to form a phospho-enol F


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3.2. β-H-Assisted Isomerization Pathway: IMβ. 3.2.1. Unimolecular Pathway: IMβuni. The isomerization via intramolecular β-H transfer is already explored for sarin and soman. In the case of DMMP also, the reaction follows a similar mechanism, where simultaneous transfer of β-H to PO and bond formation between the P(+5) center and Cβ occur through a four-membered TS (TSuni IMβ) with an activation barrier of 62.9 kcal/mol. Finally, the three-membered phospho-epoxy alcohol (P2) is obtained as a product. The oxidation state of phosphorus remains unchanged in the isomerized product. The reaction enthalpy value (46.2 kcal/mol) tabulated in Table 2 suggests the reaction to be highly endothermic. Thus, from both kinetic and thermodynamic aspects, it can be manifested that the pathway is unfeasible at normal conditions. 3.2.2. TiO2-Mediated Pathway: IMβTiO2. Like the IMαTiO2 pathway, here also we have investigated the effect of TiO2 assistance on this pathway which is a three-step process 2 initiated by the formation of the same adduct (addTiO 1st ). The phospho-epoxy-titanium complex is obtained in the first step of the reaction, which further undergoes dissociation to release the titanium complex in the second step. 2 3.2.2.1. 1st Step: (IMβTiO 1st ). As shown in Figure 5, the adduct TiO2 formation (add1st ) followed by the isomerization via the β-H transfer from -CβH3 to TiO leads to the generation of a 2 three-membered phospho-epoxy-titanium complex (intTiO IMβ_1st). The simultaneous abstraction of β-H by TiO and bonding between the Cβ and P(+5) center occur through a well-defined TiO2 ) (see Figure 2), where the six-membered TS (TSIMβ_1st activation barrier is 35.9 kcal/mol. In contrast to the IMβuni 2 pathway, here, the formation of stable six-membered TSTiO IMβ_1st increases the feasibility of the TiO2-mediated isomerization pathway by lowering the barrier height about 27.0 kcal/mol. Similar to IMβuni, in this case also, the oxidation state of P(+5) remains unchanged in the isomerized product. The endothermic nature of this pathway is also noticeable from its reaction enthalpy value of 21.4 kcal/mol. Therefore, both the kinetic as well as thermodynamic parameters indicate the greater feasibility of the assisted process over the unimolecular one. TiO2 . In accordance with the 3.2.2.2. Second Step: IMβ2nd TiO2 pathway, here also the separation of the titanium IMα 2 complex from intTiO IMβ_1st can be achieved in the following three ways: (1) Unassisted channel for the removal of TiO 2 : 2 IMβTiO 2nd_unassist. In the unassisted pathway, TiO2 is directly TiO2 detached from the intermediate, intIMβ_1st via a four-membered TiO2 TS (TSIMβ_2nd_unassist) with an activation barrier of 47.8 kcal/ mol, where the H of Ti−OH is transferred to P−O. As a result, the Ti−O bond breaks to eliminate TiO2, and a threemembered phospho-epoxy alcohol (P2) is obtained as a final product. The highly positive reaction enthalpy value suggests the endothermic nature of this step. Hence, both the activation barrier as well as the reaction enthalpy values are not in favor of the reaction feasibility. (2) Single H2O-assisted channel for the separation of the TiO2 titanium complex: IMβ2nd_H . The single H2O-assisted channel 2O TiO2 is initiated by the formation of a 1:1 adduct (addIMβ_2nd_H ) 2O TiO2 between intIMβ_1st and H2O, where the H2O molecule is weakly bound to the Ti(+4) center having a Ti···O distance of 2.10 Å, and the calculated adduct formation energy is −39.5 kcal/mol. The attack of the H2O molecule at the Ti(+4) center leads to the destruction of the Ti−O bond, and simultaneously, P−O− abstracts a proton from the H2O via a four-membered TS

with this reaction is characterized by a six-membered ring with an activation barrier of 12.3 kcal/mol, which is 37.5 and 11.5 TiO2 2 kcal/mol lower than those of IMαTiO 2nd_unassist and IMα2nd_H2O, TiO2 respectively. Like IMα2nd_H2O, in this case also, the products are situated at about 10.4 kcal/mol higher than that of 2 addTiO IMα_2nd_2H2O, which is 30.1 and 4.2 kcal/mol lower than that of the unassisted and single H 2 O-assisted ones, respectively. Therefore, in terms of kinetic feasibility, it can 2 be stated that the IMαTiO 2nd_2H2O pathway is the most suitable for 2 separating the titanium complex from intTiO IMα_1st. dehyd 3.1.2.3. Third Step: TiO2 . The dehydrolysis of O Ti(OH)2 for releasing TiO2 can be carried out in three ways, where the greater kinetic feasibility of the double H2O-assisted mechanism over the other two is noticeable. (1) Unassisted channel for TiO2 elimination: (TiO2)dehyd unassist. While analyzing the unassisted TiO2 elimination pathway from OTi(OH)2, it is observed that one H2O molecule is detached from the complex through the formation of a four-membered TS (TSdehyd unassist), and finally, molecular TiO2 is regained. The corresponding barrier height for this process is 45.9 kcal/mol, which indicates the unfeasible nature of the process. (2) Single H2O-assisted channel for TiO2 elimination: . Further, we have explored the single H2O-assisted (TiO2)Hdehyd 2O pathway to get back molecular TiO2. In doing so, it is observed that initially the H2O molecule interacts with the Ti(+4) center ), where the Ti···OH2 distance is 2.15 Å, and the (addHdehyd 2O adduct formation energy is −25.3 kcal/mol. The entire reaction passes through the formation of a six-membered ring in the TS (TSHdehyd ), where one of the Ti−OH abstracts a proton from the 2O H2O molecule to generate OH−, and simultaneously, the OH− abstracts a proton from the other Ti−OH. As a result, TiO2 gets separated out with the formation of two H2O molecules. The barrier height associated with this pathway is found to be 46.1 kcal/mol, which is almost identical to the unassisted one ((TiO2)dehyd unassist). Thus, it is observed that the assistance of a single H2O molecule does not play any significant role for lowering the activation barrier in comparison with that of the (TiO2)dehyd unassist pathway. (3) Double H2O-assisted channel for TiO2 elimination: (TiO2)dehyd 2H2O . The double H2O-assisted pathway begins with the formation of a 1:2 adduct (adddehyd 2H2O ) between the OTi(OH)2 complex and H2O molecules, where the adduct formation energy is −12.2 kcal/mol. Here, the dehydrolysis process occurs via simultaneous abstraction of three protons to produce TiO2 and three molecules of H2O. The entire reaction passes through the formation of an eight-membered TS (TSdehyd 2H2O ) of activation barrier 20.8 kcal/mol, which is ∼25.0 kcal/mol lower dehyd than that of both (TiO2)dehyd unassist and (TiO2)H2O pathways. Therefore, it can be articulated that in terms of kinetic feasibility, the TiO2 elimination via the participation of double H2O molecules is the exclusive among all. Looking at the energetics associated with the TiO 2 elimination pathways, it is inferred that in spite of the kinetic feasibility of the double H2O-assisted channel, the regeneration of TiO2 from OTi(OH)2 is very difficult due to the very high endothermicity value. Thus, though the assistance of TiO2 makes a significant impact to increase the feasibility of the detoxication processes, due to the difficulty arising during the recovery process, it is better to designate the processes as TiO2mediated detoxication processes rather than catalytic processes. G


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Chemical Research in Toxicology 2 (TSTiO IMβ_2nd_H2O) to generate three-membered phospho-epoxy alcohol (P2) and OTi(OH)2 as products. As shown in Table 2, the activation barrier associated with this reaction is 25.2 kcal/mol, which is about 22.6 kcal/mol lower than that of the TiO2 IMβ2nd_unassist channel. During the calculation of thermodynamic parameter also, the endothermicity value is found to be 19.8 kcal/mol lower than that of the unassisted channel. Thus, it is apparent that the participation of H2O not only increases the kinetic feasibility but also makes the reaction thermodynamically more feasible by decreasing the endothermicity value. (3) Double H2O-assisted channel for the separation of the 2 titanium complex: IMβTiO 2nd_2H2O. In the case of the double H2O2 assisted pathway, 1:2 adduct (addTiO IMβ_2nd_2H2O) formation takes TiO2 place between intIMβ_1st and H2O molecules, where the first H2O molecule is weakly attached to the Ti(+4) center with a H2O···Ti distance of 2.08 Å and the second H2O molecule interacts with the first H2O through a H-bonding interaction distance of 1.58 Å. The adduct formation energy calculated for this step is −51.8 kcal/mol. The reaction is initiated with the attack of the first H2O molecule to the Ti(+4) center, facilitating the breakage of the Ti−O bond, and simultaneously, P−O− abstracts a proton from the second H2O to produce OH−, which abstracts a proton from the first H2O molecule. TiO2 From Figure 2, it is apparent that the TS (TSIMβ_2nd_2H ) 2O associated with this reaction is characterized by a six-membered ring with an activation barrier of 15.8 kcal/mol, which is 32.0 TiO2 and 9.4 kcal/mol lower than that of IMβ2nd_unassist and TiO2 IMβ2nd_H2O, respectively. As in the case of the single H2Oassisted reaction, here also, the endothermicity value decreases and becomes 10.3 kcal/mol. Therefore, in terms of kinetic feasibility, it can be inferred that the double H2O-assisted mechanism is the most suitable pathway for separating the titanium complex. 3.3. β-H-Assisted Decomposition of DMMP: Dβ. 3.3.1. Unimolecular Pathway: Dβuni. The unimolecular decomposition of DMMP with the participation of β-H proceeds with the transfer of a β-H from −CβH3 to PO, and simultaneously, the P−O bond breaks through the formation of a five-membered ring in the TS (TSuni Dβ) to produce nontoxic phospho-alcohol (P3) and formaldehyde as products. The activation barrier experienced during this decomposition process is calculated to be 75.6 kcal/mol. As a result of decomposition, the P-center gets reduced from +5 to +3 in the product, P3. Therefore, it is apparent that with such a high activation barrier that DMMP cannot be decomposed via the Dβuni pathway at room temperature. Along with the kinetic unfeasibility, the process is also thermodynamically unfavorable as the associated reaction enthalpy value is 40.2 kcal/mol. 3.3.2. TiO2-Mediated Pathway: DβTiO2. In harmony with the previous two TiO2-mediated detoxication processes, here also the TiO2-mediated decomposition of DMMP via β-H transfer is found to be a three-step reaction, where the phospho-alcohol and formaldehyde are obtained as final products. The detailed mechanism of the TiO2-mediated decomposition is discussed below. TiO 3.3.2.1. First Step: Dβ1st 2. Similar to the previous two TiO2mediated detoxication pathways, here also the formation of a similar 1:1 adduct between DMMP and molecular TiO2 2 (addctTiO 1st ) at the beginning of the reaction is noticed. The abstraction of a β-H by one of the oxygens of TiO2 followed by the breakage of Cβ-H bond leads to the removal of

formaldehyde as a side product through the rupture of the P−O bond; as a result, the −PO bond is converted to −P− O, and the bond between −P−O and Ti is strengthened with a Ti···O distance of 1.84 Å. As shown in Figure 3, the entire process takes place though the formation of a well-defined 2 seven-membered TS (TSTiO Dβ_1st) with an activation barrier of 53.3 kcal/mol, where the phospho-alcohol-titanium complex is TiO 2 obtained as an intermediate (intDβ_1st ). Similar to the unimolecular process, here also the P-center gets reduced from the +5 oxidation state to +3. Analyzing the mechanism of the assisted pathway, it is observed that the pathway is mechanistically quite similar to the unimolecular one as both the processes occur via β-H transfer; but unlike the unimolecular one, here, the β-H is transferred to TiO. The important aspect of this pathway is the reduction of activation barrier by an amount of 22.3 kcal/mol compared to that of the Dβuni pathway. When the thermodynamic parameter is considered, it is observed that the reaction is still endothermic in nature but that the reaction enthalpy value is 26.6 kcal/mol lower than that of the Dβuni pathway. Therefore, from the above discussion, it can be articulated that the participation of TiO2 has a significant impact on the reaction energetics. TiO 3.3.2.2. Second Step: Dβ2nd2. Like the previous two detoxication pathways, here also the titanium complex can be 2 separated from intTiO Dβ_1st in three ways, among which the double H2O-assisted pathway is the most feasible one. The detailed mechanism for the separation of the titanium complex is discussed below. 2 (1) Unassisted channel for the removal of TiO2: DβTiO 2nd_unassist. TiO2 The direct removal of TiO2 from the intDβ_1st complex occurs through the breakage of the O−Ti bond followed by the abstraction of a proton from Ti−OH, where a four-membered 2 TS (TSTiO Dβ_2nd_unassist) with an activation barrier of 48.7 kcal/mol is detected. The high activation barrier of the unassisted pathway clearly represents the kinetic unfeasibility of the TiO2 removal pathway via an unassisted mechanism. The reaction enthalpy calculated for this pathway is 34.3 kcal/mol, which implies that the pathway is also thermodynamically unfeasible. (2) Single H2O-assisted channel for the separation of the TiO2 titanium complex: Dβ2nd_H . The separation of the titanium 2O complex via a single H2O-assisted mechanism begins with the TiO2 2 formation of a 1:1 adduct (addTiO Dβ_2nd_H2O) between intDβ_1st and H2O, where the H2O interacts with the Ti(+4) center with a Ti···O distance of 2.14 Å and the adduct formation energy is calculated to be −32.4 kcal/mol. The attack of the H2O molecule on the Ti(+4) center followed by the breakage of the O−Ti bond facilitates the abstraction of a proton from the H2O 2 molecule through a four-membered TS (TSTiO Dβ_2nd_H2O) of barrier height 19.0 kcal/mol. At the end of the reaction, phospho-alcohol (P3) and OTi(OH)2 are obtained as products, which are situated about 9.2 kcal/mol above 2 intTiO Dβ_1st. Hence, the participation of a single H2O molecule increases the feasibility of the reaction by decreasing the activation barrier and reaction enthalpy value of 29.7 and 25.1 2 kcal/mol, respectively, compared to that of the DβTiO 2nd_unassist process. (3) Double H2O-assisted channel for the separation of the TiO2 TiO2 titanium complex: Dβ2nd_2H . The hydrolysis of intDβ_1st via the 2O assistance of double H2O molecules leads to the production of TiO2 TiO2 1:2 adduct (addDβ_2nd_2H ) between intDβ_1st and H2O 2O molecules, where the first H2O molecule is weakly bonded H


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metal oxides for facilitating the detoxication of different OPCs in the near future.

with the Ti(+4) center with a H2O···Ti distance of 2.11 Å, and the second H2O forms H-bond with the first H2O molecule with a distance of 1.62 Å. The adduct formation energy calculated for this step is −45.6 kcal/mol. At the beginning of the reaction, the first H2O molecule attacks the Ti(+4) center, which leads to the breakage of the O−Ti bond producing P− O−. The abstraction of a proton from the second H2O molecule by P−O− produces OH−, and concurrent uptake of another proton from the first H2O molecule by OH− occur through a TiO2 well-predicted six-membered TS (TSDβ_2nd_2H ) with an 2O activation barrier of 11.4 kcal/mol, which is 37.3 and 7.6 TiO2 2 kcal/mol lower than those of the DβTiO 2nd_unassist and Dβ2nd_H2O pathways, respectively. The endothermicity value calculated for this step is almost similar to that obtained for the single H2Oassisted one. Similar to the previous two TiO2-mediated detoxication pathways, here also the double H2O-assisted mechanism is proved to be the most feasible way for separating OTi(OH)2. 3.4. TST Calculation. From the TST calculation, it is evident that at a particular temperature the rate constant values associated with the TiO2-mediated detoxication pathways are higher than those obtained for unimolecular pathways. Among all three TiO2-mediated detoxication pathways, the rate 2 constants calculated for the IMαTiO 1st pathway are higher than those of the other two at any temperature. Moreover, the rate 2 constant for the IMαTiO 1st pathway at 300 K is calculated to be −2 −1 1.31 × 10 s , suggesting the reaction to be kinetically feasible TiO2 2 at room temperature. In the case of the IMβTiO 1st and Dβ1st pathways, the respective rate constant values imply that the reactions can occur at higher temperature (≥600 K). Hence, from the TST calculation also, it is confirmed that at room temperature the detoxication of DMMP can occur exclusively TiO through the IMα1st 2 pathway.

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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.7b00019. Optimized geometries of the intermediates and adducts associated with all three TiO2-mediated detoxication pathways, of the final products associated with all the three detoxication pathways, and of the TSs associated with the TiO2dehyd pathway; potential energy surface for the TiO2dehyd pathway; activation barrier heights (ΔEa) and reaction enthalpy (ΔHrxn) values related to the TiO2dehyd pathway, and values of rate constants for all three detoxication pathways obtained from the TST calculation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A & 2B Raja S C Mullick Road, Jadavpur, Kolkata-700032, India. Phone: +91 33 2473 4971, ext. 1257. E-mail: [email protected]. ORCID

Abhijit Kumar Das: 0000-0003-3295-0281 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. T.A. and T.D. are thankful to CSIR, and A.G. is thankful to UGC for providing research fellowships. Notes

4. CONCLUSION In summary, we have conducted a DFT study to explore the molecular TiO2-mediated detoxication pathways of DMMP. A detailed mechanism of detoxication reveals that there are three pathways by which DMMP can be detoxified; among them, two are isomerization processes, and one is a decomposition process. The calculated results infer that the first step of all TiO2-mediated detoxication pathways, i.e., the H-abstraction by TiO2, is at least 20 kcal/mol more favorable than the respective unimolecular processes. Among the three detoxication pathways, the α-H-mediated isomerization pathway is proved to be the most feasible one as the activation barrier associated with this pathway is only 19.9 kcal/mol, which is 41.3 kcal/mol lower than its unimolecular pathway. Therefore, molecular TiO2 can be proposed as a promising assisting reagent for facilitating the detoxication of DMMP. Moreover, the separation of titanium complexes from the respective phospho-titanium complexes can be achieved with the assistance of double H2O molecules. In the last step, in spite of kinetic feasibility, the regeneration of TiO2 from O Ti(OH)2 is inhibited due to the high endothermic nature of the pathway, and thus, TiO2 acts as an assisting reagent instead of catalyst. The interesting aspect obtained from the TST calculation is that the detoxication of DMMP, which is not feasible at room temperature via the unimolecular process, can now be achievable through the TiO2-mediated α-H assisted pathway. Overall, the present study will motivate the researchers to investigate the efficiency of other molecular

The authors declare no competing financial interest.

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ABBREVIATIONS OPC, organophosphorus compound; CWA, chemical warfare agent; AChE, acetylcholinesterase; DMMP, dimethyl methylphosphonate; DFT, density functional theory; TS, transition state; STQN, synchronous transit-guided quasi-Newton; IRC, intrinsic reaction coordinate; TST, transition state theory

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REFERENCES

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