STRUCTURE AND REDOX PROPERTIES OF HEXAHYDRO-1,3,5-TRINITRO-1,3,5- TRIAZINE (RDX) AND OCTAHYDRO-1,3,5,7-TETRANITRO-1,3,5,7-TETRAZOCINE (HMX) ADSORBED ON A SILICA SURFACE. A DFT M05 COMPUTATIONAL STUDY

Adsorption of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) on (001) surface of α-quartz was studied at the M05/tzvp level using cluster approximation. Hydrogen bonds between nitramines and silica surface were analyzed by atoms in molecules (AIM) method. Electron attachment causes significant change in geometry of adsorbed complexes. Redox properties of adsorbed RDX and HMX were compared with those of gas-phase and hydrated species by calculation of the ionization potential, electron affinity, oxidation and reduction Gibbs free energies, oxidation and reduction potentials. Calculations show that adsorbed RDX and HMX have lower ability to undergo redox transformations than hydrated ones.


Introduction
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are well-known water and soil contaminants [1]. Their extensive use causes serious environmental problems due to toxicity of the nitramines. In the past decade many efforts were direct to remove these contaminants from polluted environments. Alkaline hydrolysis, photolysis, reduction by nanoscale zero-valent iron, oxidation by Fenton reagent and permanganate were found to be the most efficient methods [2][3][4]. An important information for development of remediation strategies is persistence of contaminants in natural environments, depended from ability to form stable complexes during sorption by soil and/or good aqueous solubility. Adsorption limits the potential for these compounds to migrate to groundwater.
Several studies to elucidate the adsorption of RDX and HMX by different components of soil were conducted [5][6][7][8]. It was found that the nitramines has a higher tendency to adsorb on the mineral phases than on organic matter. Adsorbed compounds undergo subsequent decomposition mostly through redox processes, thus it is very important to predict their ability to undergo reduction and oxidation transformations in adsorbed form. The objective of the present study is prediction of redox properties for RDX and HMX adsorbed on a silica surface, taken here as the (001) surface of α-quartz and comparison them with the same properties for dissolved in water nitramines.

Computational Methodology
The Gaussian 09 program package was used for all of the calculations [12]. To simulate the hydroxylated (001) surface for α-quartz a cluster approach was utilized. The dangling bonds of the cluster were saturated by hydrogen atoms in order to keep the silica model electroneutral. The models obtained contain three oxygen-siliconoxygen layers, with a formula of Si21O62H40. The geometry of all species was optimized at the M05/tzvp level. The harmonic vibrational frequencies were calculated for all structures obtained to establish that a minimum was observed. Solvent effects were taking into account by single-point calculations using the PCM(Pauling) and SMD solvation models for ion-radical and neutral molecule computations, respectively. The Multiwfn program [13] was used to conduct a topological analysis of the distribution function of the electron density ρ(r) in the framework of the R. Bader atoms in molecules (AIM) theory.
The binding energy of interactions (E) was calculated with the Espinosa formula E = 0.5V, where V is the density of potential energy in a critical point. The energy of adsorption presents a difference between the total energy of the adsorbed complex and the energy of the separated silica and nitramine. Deformation energy of the silica surface Edef.(S) presents a difference in energies of the silica surface model at the geometry optimized with the adsorbate after removing the contribution brought by nitramine and optimized silica surface model. Deformation energy of nitramine was calculated as a difference in energy of nitramine frozen within the geometry of the adsorbed complex and the energy of optimized nitramine. Adiabatic electron affinities (EA) and ionization potentials (Ip) were computed as the total energy difference between the charged species and the neutral forms, corrected for zero point energy.
The Gibbs free energies of electron attachment and electron loss for adsorbed and dissolved in water nitramine were calculated as follows: i) α-quartz adsorption: ii) water hydration: where, R and O denote reduced and oxidized species, respectively. The values of the oxidation and reduction potentials are calculated as follows: The absolute potential of the normal hydrogen electrode EH was taken as -4.36 eV [14].

Adsorbed complexes
Nitramines RDX and HMX have different conformations. The most representative conformations, according to literature data [15,16], were checked for ability to form stable complexes with silica surface. It should be noted that on the base of the preliminary calculation two stable complexes for RDX and two for HMX were chosen for further analysis (Fig. 1, Table 1). For clarity only hydrogen atoms on the top of the silica surface models are shown in the Fig. 1. Complexes RDX(a) and RDX(b) contain AAA and AAE conformations of RDX, respectively. Complexes HMX(a) and HMX(b) corresponds to boat-chair and  conformations of HMX (Fig. S1, SI). Calculated adsorption energy of RDX and HMX is in a range of -13.64 --15.38 kcal/mol (Table 1). HMX is slightly better adsorbed on silica surface than RDX. Nitramines bind with silica surface by hydrogen bonds formed between the oxygen atoms of nitrogroups and the hydrogen atoms of the surface hydroxyl groups, and between the oxygen atoms of the surface and the hydrogens of the adsorbed nitramines ( Fig. 1, Table 2).
An analysis of the data presented in Fig. 1 shows that total orientation of nitramines relative to the silica surface changes slightly after losing an electron while the changes are significant in case of electron attaching. Ionization also affects the amounts and strength of hydrogen bonds. As expected, in anion-radicals electronegative nitrogroup orient close to the silica surface as compared with neutral ones, an amount of bonds between oxygens of nitrogroups and hydrogens of the surface is increase, the strength of  (Table 1). This means that ionized nitramines stronger bind to silica surface than the neutral ones. Calculated deformation energy for neutral complexes of silica surface is larger than that of nitramines (Table 1). This points a more significant modification of geometry of surface hydroxyl groups than geometry change of nitramines during adsorption (Fig. 1).
Electron attachment process causes increase of deformation energy for nitramines and for silica surface (Table 1) (Tables 3, 4). There is an exponential correlation between H-bonds lengths and electron density at BCPs with a correlation coefficient of 0.96 (Fig. S2, SI).  Good correlation with a correlation coefficient of 0.99 was also observed between an interaction energy and an electron density at BCPs (Fig. S3, SI).
The calculated electron density properties of studied complexes show that the interactions between nitroamines and surface have low electron density ρ and positive Laplacian (∇ 2 ρ > 0) ( Tables 3, 4

Redox properties
The calculated electron affinity, Gibbs free energy of reduction, and reduction potential characterize the ability of nitramines to be reduced (Table 5), while the ionization energy, Gibbs free energy of oxidation, and oxidation potential reflect the ability of nitramines to be oxidized (Table 6). Close values of Gibbs free energy of reduction and electron affinity, and, respectively, Gibbs free energies of oxidation and ionization energies indicate insignificant contribution of entropy TΔS term in Gibbs free energy of redox transformation. Analysis of Tables 5 and 6 shows that redox properties depend on conformation of RDX and HMX.
Gibbs free energies of reduction or oxidation for adsorbed and solvated forms differ from gasphase Gibbs free energy by contribution of adsorption and hydration, which are listed in Table 7. Calculated data show that hydration more decrease gas-phase energy than adsorption. This means that hydration more facilitates redox transformation of nitramines than adsorption. Indeed, as one can see from Tables 5 and 6, hydrated nitramines have larger reduction potentials and smaller oxidation potentials than adsorbed species.  According to topological analysis of electron density these H-bonds may be classified as noncovalent and partly covalent closed-shell interactions. Electron attachment leads to increase binding between nitramines and silica surface due to stronger hydrogen bonds formation. Calculated data show that hydration more decrease gas-phase Gibbs free energies of reduction and oxidation than adsorption. Calculation of ionization potential, electron affinity, oxidation and reduction potentials show that adsorbed RDX and HMX are more resistant to oxidation and reduction processes as compared with the hydrated species.