Electronic structure of TNT and NTO adsorbed complexes
To understand the reactivity of TNT and NTO during the initial steps of reduction, it is convenient to analyze preliminary some electronic and geometrical parameters of adsorbed complexes. First, we would like to analyze the data related to the interaction of both TNT and NTO compounds with the Fe13O13. The interactions of the title compounds with Fe13O13 are characterized by the formation of several adsorbed complexes that differ by the orientation of adsorbed molecules regarding the TP and BP planes of nano-particle. The molecular structures of those complexes are presented in ESI (see Figure 1S and 2S and Table 1S). The complexes which are the most stable according to relative values of Gibbs free energy are presented in Figure 3. Table 1 collects the most important parameters of charge distribution, spin state and geometry which are necessary for future discussion.
As follows from the data presented in Table 1 both TNT and NTO compounds are predicted to be chemosorbed on the surface of Fe13O13, since intermolecular NO…Fe (R4 and R5) distances are virtually the same (sometimes they are even shorter) as the distances Fe-O in the isolated nano-particle (2.02 Å). Therefore, the case of very strong interaction between nano-particle and adsorption moieties is revealed
Figure 3 is here.
There are at least two reasons for such strong adsorption. First, all atoms of Fe13O13 are characterized by downgraded coordination numbers. Second, as follows from the data presented in Table 1, adsorption of both considered species is accompanied by barrierless electron transfer from Fe13O13 to TNT and NTO (actually, this is a typical case of chemical reduction). A similar effect has been observed in our early study (Zilberberg et al. 2004) devoted to the interaction of nitrobenzene and FeO. However, there are also some specifics due to the different nature of the two adsorbed species. Namely, since the electron affinity of TNT is much stronger than that of nitrobenzene (-2.2 eV and -1.04 eV respectively, at M06-2X level of theory), the former molecule can accept up to two electrons from Fe13O13. The indication of this ability are the values of electron charge transfer to TNT
Тable 1 is here.
which approaches -2 a.u. depending on the type of DFT approximation and values of spin density (which in the case of BLYP and B3LYP functionals even exceeds 2 a.u.). We also would like to mention that most of this spin-density is located on nitrogen and oxygen atoms of nitro-groups of TNT (see Figures 3S and 4S, where the shape of TNT LUMO, LUMO+1 and LUMO+2 orbitals along with spin difference density are presented). Additional evidence of the nature of redistribution of electron density after adsorption of TNT on Fe13O13 is provided by a mean value of <S2> operator. The data presented in Table 1 suggest that all three considered DFT approximations predict it as 870 a.u. This corresponds exactly to an eigenvalue of S(S+1), S=29 and means that more than six of d-electrons located on each Fe atom are unpaired. During the adsorption of TNT up to two unpaired electrons are transferred to TNT molecule. This is in full correspondence with our recently published results (Zilberberg et al. 2004) where we also established that more than six unpaired electrons per Fe atom participate in electron transfer to a molecule of nitrobenzene. Adsorbed NTO demonstrates similar TNT behaviour. However, since it has lower than TNT (1.84 eV, M06-2X) electron affinity, only up to one electron is transferred from Fe13O13. There is one more important conclusion. In the case of nitrobenzene - FeO interaction we stated that pure and hybrid functionals behave differently. Namely, pure functional (BLYP) does not reveal a profound effect regarding electron transfer, but it demonstrates quite good correspondence with the experiment related to the thermodynamics of the reaction: nitrobenzene + FeO. The calculations carried out using hybrid functional B3LYP demonstrated opposite trends which disagree with an experiment that approaches -2 a.u. depending on the type of DFT approximation and values of spin density (which in the case of BLYP and B3LYP functionals even exceeds 2 a.u.). We also would like to mention that most of this spin-density is located on nitrogen and oxygen atoms of nitro-groups of TNT (see Figures 3S and 4S, where the shape of TNT LUMO, LUMO+1 and LUMO+2 orbitals along with spin difference density are presented). Additional evidence of the nature of redistribution of electron density after adsorption of TNT on Fe13O13 is provided by the mean value of <S2> operator. The data presented in Table 1 suggest that all three considered Analyzing the data collected in Table 1 one can conclude that all three functionals qualitatively describe the similar trends in the electronic structure of TNT and NTO complexes adsorbed by Fe13O13. Namely, both molecules interact tightly with the surface of Fe13O13. The adsorption initiates the transfer of up to two electrons to the antibonding LUMOs of TNT and up to one electron to the antibonding LUMO of NTO.
Pathways of TNT decomposition
More complex and representative structures of both an iron-containing moiety and adsorbed on its surface species as considered in the current work could generate pathways that could be different than the ones found for oversimplified models (Zilberberg et al. 2004). The thermodynamical parameters related to the computationally generated pathway computed in this study are collected in Tables 2 – 4
Let us start from the analysis of pathways similar to those found in an earlier study (Zilberberg et al. 2004). As we already mentioned in the Introduction such model describes an intra-complex transfer of one of the oxygen of nitro-group to Fe(II) containing species. We found that TNT adsorbed on both considered planes of Fe13O13 is able to participate in such a type of decomposition. The corresponding thermodynamical parameters are collected in Table 2. However, the thermodynamical parameters which characterize the course of this reaction on the top plane could be considered ineffective from the practical point of view (high activation barrier and endergonicity). For the corresponding energetic profile see ESI Figure 5s.
Table 2 is here.
Table 3 is here.
Table 4 is here.
Much more interesting for the analysis are the results presented in Figure 4. Namely, BLYP-GD3BJ and M06-2X level calculations predict low enough values of activation barriers to consider that the transfer of oxygen to the Fe13O13 could be carried out with an observable chemical rate. Also, according to B3LYP-GD3BJ and BLYP-GD3BJ level data, braking of one of the nitrogen-oxygen bonds of the nitro-group is quite satisfactory compensated by the formation of two additional oxo ferrous iron bonds in Fe13O13. This results in significant exergonicity of this reaction. However, M06-2X level calculations predict this reaction to be just slightly endothermic. To explain this difference, we assumed that in the case of B3LYP-GD3BJ and BLYP-GD3BJ oxygen atom transfer includes a more significant transfer of charge and electron density than in the case of M06-2X. However, we did not find a convincing explanation since both electronic characteristics are virtually the same at B3LYP-GD3BJ, BLYP-GD3BJ and M06-2X levels of theory.
Figure 4 is here
As we already mentioned more complex structure of Fe13O13 considered in this study could in principle generate the pathways that are different from simple oxygen transfer from nitro-group to Fe(II)-containing mineral or species. The example of such a pathway is presented in Figure 5.
Figure 5 is here.
The main feature of this pathway is the participation of the hydrogen atom of the CH3-group in the decay of the ortho-nitro-group of TNT. The structures of located transition state and product are presented in Figure 5. Intuitively, one expects the appearance of an intermediate on the path from the transition state to the product. However, we have not located such a structure at any of the considered here DFT approximations
Pathways of NTO decomposition
In the case of NTO only the pathway that is very similar to the one which characterizes a decay of ortho-nitro-group of TNT has been predicted. As follows from the Gibbs energy profile presented in Figure 6, the first step, in this case, is a transfer of the closes NH proton in the direction of the nitro-group oxygen. Since the mobility of this hydrogen is greater than in the case of hydrogen mobility from the CH3-group (it is more acidic (Golius et al. 2015) the observable reaction barriers are smaller than the one that relates to TNT transformation. Similar to TNT, the reaction is characterized by high exothermicity effects.
Figure 6 is here.
One may see that predicted decay of ortho-nitro-group is kinetically less effective than simple oxygen transfer that characterizes decay of para-nitro-group of TNT. However, it is characterized by even more pronounced exothermic effect that is predicted in the case of the orto-nitro-group.
Finally, we would like to convey two details of the computational challenges of this project.
First: during performed calculations, for several initial geometries we faced multiple cases of SCF non-convergences. Due to such failures of calculation procedures, we cannot assure that there are no additional minima corresponding to reactive intermediates in addition to the structures presented in Figures 4 - 6 profiles. This relates especially to the cases of decay of ortho-nitro-group of TNT and to decay of NTO. For example, we have found an intermediate that corresponds to an intramolecular proton transfer in NTO. (see Figure 6s of ESI). This pathway starts from another adsorption state which is less effective than presented in Figure 6. However, it has approximately the same values of Gibbs free energy barriers as the profile presented in Figure 6.
Second: one may argue that the relaxation of the Fe13O13 nanoparticle could quantitatively change the results described above. We tried to perform full optimization of the Fe13O13 nano-particle with and without adsorbed NTO molecule. However, we met even more problems than already described. Therefore, we present here just two trustworthy results.
1. The geometry of the optimized cluster does relax significantly compared to considered in this work (compare the structures presented in Figure 1 and Figure 7s). The structure with the lowest energy corresponds to spin equal to 26 compared to the structure having S = 29 and studied in this work.
2. During the optimization, we were not able to locate a transition state of NTO decomposition. However, we have found that the product (see Figure 8s) of such decomposition is similar to the one presented in Figure 5. We concluded that most probably the relaxed structure has a similar electron donor ability to transfer barrierless electron(s) to adsorbed species
Therefore, we believe that the electron donor properties of relaxed structure Fe13O13 and its decomposition ability is similar to the one considered in this work.