Comparative Study of Decomposition adsorption of Sarin on ZnnOnand CdnOn ( n = 1 , 4 ) , by Theoretical Method

The calculations of the electronic and structural properties for interactions of Sarin with ZnnOnand CdnOn(n=1,4) have been conducted by B3LYP/6-31++G(d,p) level of DFT method. The ZnOaffects on disconnection of propyl bond of Sarin and Cd4O4 interacts with Fluorine atom of Sarin. The results of Radial Distribution Function (RDF) show that the interaction of Sarin is probable at 380 K with ZnO as covalent bond and at 308 K with Cd4O4 by Van der waals forces. keywords: Sarin, ZnnOn, CdnOn, Decomposition, DFT, RDF


INTRODUCTION
Chemical warfare agents (CWAs) have been used in the World Wars that to cause killing and injuring a large number of people 1 .The stable complexes of CWAs with acetylcholine esterase enzyme at cholinergic synapses of nervous systems (in humans) leads to a variety of effects such as hypotension, muscle tremors and convulsions 2 .Sarin is the nerve agent of CWA.It is a derivative of methyl phosphono fluoridate, Fig1.CWAs are far too dangerous for experimental study.Hence, researchers prefer to use theoretical methods to investigate their decomposition possibility.The nano metal oxides with low coordination number 3 have unusual electronic properties and adsorption behavior due to their defect sites (Frenkel&Schottky), Lewis acid (metal cations) sites, Lewis base (oxide anions) sites and, high surface area 4,5 .Decomposition adsorptionof Chemical warfareagents (CWAs) on nano metal oxides yields non-toxic products.Dimethyl methylphosphonate, [DMMP, CH 3 PO(OCH 3 ) 2 ] is a nontoxic organophosphorus compound and it used as a simulant for CWAs chemical warfare agent.Both experimental and theoretical studies have established that decomposition of DMMP is facilitated on the small cluster of nano metal oxides Al 2 O 3 6,7 ,MgO 8 , SiO 2 9 , TiO 2 10 , ZnO 11 and Mg 4 O 4 12 due to their electronic and surface properties.Metal oxides are used as adsorbents, catalysts and catalyst supports and they are environment friendly for decontamination applications, such as decontamination on the battlefield, filtration systems, and decomposition of CWAs 13 .One type of nano metal structure as form of M 4 O 4 , is similar to nanocone.The nanocones are observed as caps on the ends of nanotubes, and also as free standing nanostructures 15 .More recently, a theoretical study by Alfieri and Kimoto has indicated that nanocones with disclination angles 60º ,120º, 240ºand 300ºare stable 16 .The cone is entirely characterized by its cone angle 17 .
Jin Chang and Eric R. Waclawik at 2012 18 and A. Bagheri Ghomi at 2016 19 have reported the synthesis of Zn 4 O 4 nanocone.In the current paper, we investigate the M 4 O 4 type of nano metals asapotential candidate for adsorption of Sarin.

Model and simulation details
Studies by Harrison have shown that DFT method with Lee-yang-Parr's correlation Functional (B3LYP) provides better agreements with experimentally derived band gaps for a wide class of zinc-blend and wurtzite-structured III-V materials 20 .Therefore, in this work, the B3LYP level of DFT has been used for all of the calculations and B3LYP values have been scaled by a factor of 0.96 21 .But one concern lies in choice of basis sets on the accuracy of the final results.The experimentally rotational constants are related to molecular geometry 22,23 .Therefore, the rotational constants of the optimized Sarin at B3LYP/6-31G (d), B3LYP/6-31+G(d) and B3LYP/6-31++G(d,p) have been calculated and compared with the experimental values for choice the suitable basis set.The error is being defined as ((Cal-Exp)/Exp) and summed over all three rotational constants.
A small cluster of nanocone containing four Zn atoms (or Cd atoms) and four O atoms with disclination angle equal to 240º and height equal to 4 have been created by Nanotube Modeler 2014 software 25 .
In this article, the geometry of the Sarin and MO have been fully optimized while the M 4 O 4 (M=Zn or Cd) has been fully frozen.
We have considered two position of Sarinconnections on MO and M 4 O 4 (M=Zn or Cd).
First position (P1) is the connection of oxygen from phosphonylgroup (P=O) of Sarin to M of metal oxide.
Second position (P2) is the connection of Fluorine from Sarin to M of metal oxide.
The geometry of connected systems of P1 and P2 complexes has been fully optimized.
For evaluation of basis sets difference for adsorbed systems, the interaction energies (∆E int ) of studied adsorption systems have been corrected by the Basis Set Superposition Error (BSSE), Eq.1.
that the first term in ∆E int is the energy of the adsorbed system and the next two terms are the energies of the bare M n O n (M= Zn, Cd and n= 1,4) and the free Sarin molecule, respectively,and The harmonic vibrational frequencies of the fully optimized structures have been calculated to  (d,p) confirm the stationary point as a local minima with all positive frequencies.The electronic properties of nanostructures have been described by lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) 26 .

RESULTS and DISCUSSIONS
In this work, we have focused on the rotational constants of Sarin for choosing the suitable basis set.The Sarin structure has been optimized at B3LYP level of DFT with 6-31G(d),6-31G+(d) and 6-31++G(d,p) basis sets.The results for the rotational constants in Table 1 shows that the average error of calculated rotational constants are about 8.4% for B3LYP/6-31G(d), 11% for B3LYP/6-31G+(d) and 3.5%for B3LYP/6-31++G(d,p).Therefore, the calculated rotational constants by B3LYP/6-31++G(d,p) with the smallest error are in more agreement with the experimental values withrespect to other.Thus, we are focused on the B3LYP/6-31++G (d,p) results for the next calculations.
The structural stability of nanostructures can be described by calculated energy.In this work, the interaction between Sarin and M n O n (M=Zn or Cd and n=1,4) has been studied for two position of connection.The P1 Position indicates the interaction of oxygen from P=O group of SarinM n O n .TheP2 position is related to interaction of Fluorine of Sarin with metal from M n O n .Table 2 presents the corrected interaction energies, dipole moments, bond lengths and natural charge of oxygen and metalfor all the interacting systems.
Since the increase of electron transfer is occurred with the decrease of the energetic difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the selection of metal oxide with for adsorption of types of gases becomes possible.
The electron transition is a factor of strength of interaction.Thus, the narrow band gap between the HOMO and LUMO levels results in easy transition of electrons from HOMO level to LUMO.Table 3 shows HOMO and LUMO energy gaps (E gap ) for complexes.The calculated gap of energy for the bare ZnO is 2.39 eV and this decrease to 2.23 eV for bare Zn 4 O 4 .The E gap of Sarin, ZnO complexes It is important to pay attention that the gap of HOMO-LUMO energy for CdO and Cd 4 O 4 are same and equal to 2.04 eV.Also, a small change can be seen in E gap of Sarin, CdOcomplexes, about 29.41% in P1 and 30.39% in P2 complexes.By comparing E gap of MO (M= Zn or Cd) complexes, it is clear that change of E gap for P1 complex of ZnO is more than CdO complex and P2 complexes of MO.
The obtained results show that by varying size of metal oxide mentioned in Table 3 As pointed in Table 4, the charge transfer (Q T ) of P1 complex of ZnO (0.0194) is more than CdO (0.0142).This result is related to strong interaction of ZnO with Sarin at P1 position.As seen in the figure 1, the bond of propyl is fractured due to interaction of ZnO with Sarin, and this nerve agent is decomposed.

Figure 11
Figure 11 shows that Sarin is decomposed at 380 K due to stocky connection with ZnO.In other words, ZnO connects with Sarin by formation of covalent bond.The Van der waals energies of Zn 4 O 4 , CdO and Cd 4 O 4 complexes are weaker, respectively.It can be seen in chart 1 that optimum temperature for interaction of these complexes are 320, 380 and 308 K, respectively.

Table 4 :CONCLUSIONS
Charge transfer of Donor and Acceptor bonds for complexes at connected In this research, the study of decomposition adsorption of Sarin on Zn n O n and Cd n O n (n=1,4) by B3lyp/6-31++g(d,p)quantum calculations shows that the ZnO can decompose Sarin by disconnection of propyl group of Sarin and formation of covalent bond with Sarin.Also, interaction of Cd 4 O 4 with Fluorine of Sarin is important.The calculations of Radial Distribution Function support the quantum analysis results.The results of RDF show that the interaction of Sarin is probable at 380 K with ZnO and at 308 K with Cd 4 O 4 .

Table 2 : Calculated ∆E int (kcal/mol), bond lengths(Å), Dipole moment(Debye), Natural Charge of oxygen (Oxygen in phosphonyl group) and NaturalCharge of metal (M) at b3LYP/6-31++G (d,p) structure ∆E int r (O=P)/ r(O…M) a r
19.73,-9.31,-35.06,-21.23, -15.03, -8.81, -37.11and -24.50 Kcal/mol, respectively.The values of ∆E int show that the connection between the phosphonyl O atom of Sarin with M n O n (n=1 or 4) (all of P1 positions) is energetically favored over Fluorine connecting (P2 positions).Increasing of bond length of O=P of Sarin at P1 complexes of ZnO (1.61 Å) and Zn 4 O 4 (1.68 Å) in comparison to a single Sarin (1.57Å) is related to interaction of oxygen (in P=O group) with Zn of Zn n O n (n=1, 4) of these complexes and approximately no changes have been observed for this bond length at Zn 4 O 4 , Sarin P2 and P1 and P2 complexes of Cd 4 O 4 , Sarin.The value of bond length in evaluation of interaction strength is not enough.Therefore, we must investigate other factors.Dipole moment gives clear information about the arrangement of charges in nanostructures.The result of dipole moment 4nanocones can further be explained toachieve different electronic properties.At all of the structures, the dipole moment of connected structures are increased and dipole moment of P1 complexes are more than P2.We have investigated the electronic properties by natural bond orbital (NBO) analysis.

Table 3 : Calculated HOMO energies (EHOMO), LUMO energies(ELUMO),HOMO-LUMO energy gap (E gap ) of pristine and Sarin adsorbed Structure E HOMO E LUMO E gap %∆E gap eV eV
, the interaction abilities changes.By evaluating HOMO/ LUMO energy gaps, it is obvious that the energy gap of M 4 O 4 complexes is lower than MO complexes.Therefore, electron transfer is more probable in the M 4 O 4 complexes.%∆E gap of Zn 4 O 4 complexes at both of the P1 (4.93%) and P1(2.24%) positions are lower than Cd 4 O 4 (18.62% for P1 and 52.82% for P2) complexes.Therefore it seems like that nano metal oxide of Cd 4 O 4 has more ability for interaction to Sarinthan Zn 4 O 4 .Also, by comparing P1 and P2 complexes of Cd 4 O 4 it is seen that adsorption of Sarin on the Cd 4 O 4 at P2 position is more probable.
In this work, Q T of MO metal oxides of Zn and Cd at P2 position is approximately equal.The charge transfer of P1 and P2 complexes of Cd 4 O 4 with 0.071 and 0.0039, respectively, show that in M 4 O 4 form of nano metal, the interaction of Cd 4 O 4 with Sarin is more probable than Zn 4 O 4 .These results have compatibility with %∆E gap .The results of quantum calculations show that for decomposition of Sarin, the ZnO and Cd 4 O 4 are better structures, that ZnOaffects on fraction of propyl bond (P1 position) and Cd 4 O 4 interacts with Fluorine atom of Sarin (P2 position).