Article pubs.acs.org/JPCC
Dissociative Adsorption of 5‑Nitro-2,4-dihydro‑3H‑1,2,4-triazol-3-one (NTO) on Aluminum-Terminated (0001) Surface of α‑Alumina As Predicted from Plane-Wave Density Functional Theory Manoj K. Shukla* and Frances Hill Environmental Laboratory, US Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180, United States S Supporting Information *
ABSTRACT: Plane wave density functional theory (DFT) was used to study the adsorption of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO), an insensitive high performance explosive material, on Al-terminated (0001) surface of (4 × 4) α-Al2O3 using the PBE, PBEsol, BLYP, and recently developed van der Waals functional (vdWDF2) within the Generalized Gradient Approximation. The interactions of valence electrons with atomic cores were approximated using the ultrasoft pseudopotentials. Various orientations of NTO with respect to the alumina surface were considered. It was revealed that the carbonyl site of NTO binds more strongly than the nitro group site with the surface aluminum ion. Important information revealed from the present investigation was that NTO undergoes dissociative adsorption on the Al-terminated αalumina surface. During this process, both the carbonyl oxygen and the nitro oxygen are involved in binding with surface Al ions and a proton from the N4 site of the adsorbate (NTO) is dissociated and migrated to nearby surface oxygen site forming an OH bond. Consequently, the NTO adsorbed on the alumina surface is in the anionic form. Further, consequent to the adsorption, the interacting surface Al-atoms are pulled up toward the NTO with respect to the plane containing the rest of the Al atoms. Moreover, an analysis of charge density difference maps suggested a buildup of charge density in the NTO-alumina bonding region, indicating covalent nature of adsorption on the Al-terminated (0001) surface of α-alumina.
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INTRODUCTION 5-Nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) is classified as an insensitive high performance explosive material which was developed in 1983 at the Los Alamos national Laboratory.1−5 It is chemically stable (less sensitive) against impact, shock, temperature, and radiation induced ignition than traditional energetic materials such as 2,4,6-trinitotoluene (TNT), 1,3,5trinitrohexahydro-s-triazine (RDX), and octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine (HMX).1−4 NTO has undergone significant experimental and theoretical investigations.2,5−10 The majority of these investigations have been devoted to thermal and photochemical decomposition of solid NTO,2,9,10 while some investigations were also devoted to determination of its structural details.5−9 Several possibilities have been proposed for decomposition pathways of NTO, but a universal mechanism for such reactions is still elusive.5 Theoretical calculations have suggested that homolysis of the C-NO2 bond is a possible initial step in the unimolecular decomposition of NTO particularly at high temperature, or under shock or impact conditions.5,6 Based upon electron paramagnetic resonance (EPR) spectroscopy and high performance liquid chromatography (HPLC) experimental investigations, rupture of the N−H bond has been suggested as the main dissociation path of NTO under both photochemical and thermochemical conditions.11 The activation energy of the decomposition of NTO has been found to be in the range of 40.7−88.0 kcal/mol This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
depending upon the experimental methods used in such evaluation. For example, isothermally determined activation energy of NTO was found to be 88.0 kcal/mol,11 while 40.7 kcal/mol was revealed based upon the chemiluminescence detection of NO in the 110−140 °C temperature range12 and 44.5 kcal/mol based upon the nitro group stretching vibration within the 195−210 °C range of temperature.9 The structure of NTO is characterized by the presence of a triazole ring with a keto carbon (C3) and a nitro group at the fifth position (see Figure 1 for the structure and atomic numbering schemes). Due to the presence of the labile NH bond NTO behaves as a weak acid with a pKa of 3.76.13 This property of NTO was utilized in the development of an assay based on aqueous acid−base titration that compares favorably with results obtained by HPLC methods.13 NTO forms salts with a number of metals and aromatic and aliphatic amines through deprotonation of the N4 site, yielding salts that also are high energetic and insensitive materials. 14 Harris and Lammertsma6 investigated the tautomerization, ionization, and bond dissociation properties of NTO at the Hartree− Fock (HF), B3LYP-DFT, and MP2 levels using the 6-31+G(d) and 6-311+G(d,p) basis sets and found that the ketoReceived: August 29, 2013 Revised: December 10, 2013
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adsorption, the interacting Al atoms were shown to be pulled in the upward direction toward TNT and such interaction was shown to have covalent character.19 In the present work, we investigated adsorption of NTO on an Al-terminated (0001) surface of (4 × 4) α-Al2O3 at the plane wave DFT level, within the GGA approximation, using different functionals including a recently developed van der Waals density functional (vdW-DF2). Both the parallel and vertical orientations of NTO with respect to the alumina surface were considered. We found that NTO on the Alterminated α-alumina surface is adsorbed in a dissociative manner where a proton from the N4 site of NTO is dissociated and is moved toward a surface oxygen site in a barrierless process. We believe that our predicted results provide fundamental information about adsorption and dissociation of NTO on the pristine alumina surface.
Figure 1. Structure and atomic numbering schemes of 5-nitro-2,4dihydro-3H-1,2,4-triazol-3-one (NTO).
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COMPUTATIONAL DETAILS In the present investigation we used the Quantum Espresso program based upon plane-wave density functional theory (DFT).39 All calculations were carried out under threedimensional periodic boundary conditions under the generalized-gradient approximation (GGA) using the following functionals: the Perdew−Burke−Ernzerhof (PBE),40 a revised form of PBE (PBEsol) which has been suggested to be more suitable for solids,41 BLYP,42 and a recently developed, highly accurate van der Waals density functional (vdW-DF2)43−46 as implemented in the Quantum Espresso program package.39 The interaction of electron−ion core was approximated by ultrasoft pseudopotentials as available in the Quantum Espresso program package. The ultrasoft pseudopotentials available for PBE functional were also used for the vdW-DF2 functional. The wave function cutoff for PBE, BLYP, and vdW-DF2 functionals was 25 Ry, and 50 Ry was used for the PBEsol functional. The kinetic energy cutoff for charge density and potential equal to 10 times the wave function cutoff was used. Structural degrees of freedom of investigated system were completely relaxed in this study. The system under investigation is large, containing 491 atoms (among them 489 heavy atoms), therefore, geometry optimizations were performed at the gamma point. We also computed electronic density of states (DOS) and in this calculation 2 × 2 × 2 uniform k-point grids (12 k-points) were used. We have considered an Al-terminated (0001) surface of (4 × 4) α-Al2O3 in the present investigation. The supercell contains 480 atoms with the top layer consisting of 16 Al atoms. The supercell was constructed from the X-ray crystallographic geometry of α-Al2O3 which shows Al-terminated (0001) surface where Al and oxygen atoms are arranged in the AlOAlAlOAlAlO... type of layers totaling 18 layers stacked in cdirection; the lattice parameters of the hexagonal unit cell were experimentally determined to be a = 4.7602 Å and c = 12.9933 Å.47 The consideration of such a large (4 × 4) unit cell in the present calculation was necessary to avoid self-interaction of adsorbate (NTO) with its periodic image along the “a” and “b” directions. The nearest neighbor distance of NTO with its periodic image was found to be more than 13 Å in the present calculation for both orientations of NTO with respect to the adsorbent surface. The adsorption energy or binding (ΔEad) energy of NTO on the α-Al2O3 surface was computed using the formula:
tautomeric form of NTO was the most stable neutral species while the anion obtained by deprotonation of the N4 site was the most stable anion. The anionic structure was found to be in agreement with experimental data where the X-ray crystallographic investigation of both the 1,3-diaminoguanidinium and ethylenediammonium salts of NTO exhibit the predicted deprotonation at the N4 site.15,16 These authors have also computed the C-NO2 and N−H bond dissociation energies of the most stable tautomer and these energies were estimated to be 70 and 93 kcal/mol, respectively. It is known that live-fire training ranges and weapon testing sites and production and storage facilities of munitions are contaminated with residues of high energy compounds.17,18 Adsorption of high energy compounds on different clay surfaces is an important area of research which can provide important information on how these high energy contaminants are transported in soil and provide motivation for different strategies that might be needed for their decontamination.19−22 For example, due to the stronger adsorption of TNT relative to RDX in soil, TNT is of more concern as a sediment contaminant while RDX is of more concern as a groundwater contaminant.23 Aluminum is the third most abundant element in the earth’s crust and alumina is one of the most important materials in the earth’s surface, so interactions of alumina with possible contaminants can have many environmental ramifications.24,25 Alumina is an important ceramic material and has been used in many applications.25,26 It shows remarkable insulating property and structural stability over a large temperature range.26,27 It is well-known that the α-phase is the most stable form of alumina (α-Al2O3).28,29 Various experimental and theoretical investigations have suggested that the most stable (1 × 1) α-Al2O3 slab has an Al-terminated surface.30−34 Al-terminated α-Al2O3 surfaces have been used as a substrate in several investigations of interactions with different molecules, but most of these investigations are devoted to interactions with water molecules.35−38 In our recent investigation of adsorption of TNT on Al-terminated (4 × 4) α-Al2O3 (0001) surface employing the plane-wave DFT method using PBE variant of the GGA approximation, we found that the parallel orientation of the adsorbate with respect to the adsorbent surface was preferred over the perpendicular orientation.19 Further, the oxygen atom of each of the nitro groups of TNT in the parallel orientation was found to be involved in the adsorption interaction with separate surface Al atoms. Moreover, consequent to the
ΔEad = −(Ecomplex − E NTO − Ealumina) B
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Table 1. Computed Bond Lengths of Isolated NTO, NTO-Anion, and NTO within the Adsorbed Complex Geometry Obtained Using Different DFT Functionalsa bond N1−N2 N2−C3 C3−N4 N4−C5 C5−N1 C3−O C5−N6 N6−O61 N6−O62 N4−H N2−H
B3LYP/MP2 1.360 1.400 1.403 1.366 1.296 1.210 1.445 1.236 1.222 1.009 1.009
(1.362)/1.366 (1.364) (1.420)/1.393 (1.412) (1.376)/1.405 (1.381) (1.342)/1.358 (1.342) (1.328)/1.310 (1.343) (1.237)/1.219 (1.245) (1.452)/1.441 (1.445) (1.236)/1.247 (1.248) (1.240)/1.238 (1.250) (−)/1.008 (−) (1.008)/1.007 (1.006)
PBE 1.365 1.410 1.413 1.370 1.311 1.220 1.452 1.254 1.237 1.017 1.018
BLYP
(1.356) (1.391) (1.350) (1.351) (1.338) (1.286) (1.436) (1.289) (1.224) (1.713) (1.020)
1.380 1.415 1.418 1.377 1.310 1.219 1.460 1.262 1.245 1.017 1.017
PBEsol
(1.372) (1.397) (1.351) (1.354) (1.341) (1.289) (1.443) (1.299) (1.233) (1.799) (1.020)
1.346 1.401 1.401 1.360 1.302 1.211 1.438 1.238 1.222 1.018 1.017
(1.336) (1.380) (1.342) (1.342) (1.326) (1.273) (1.422) (1.269) (1.209) (1.653) (1.020)
vdW-DF2 1.387 1.417 1.422 1.379 1.312 1.221 1.462 1.268 1.250 1.014 1.014
(1.376) (1.400) (1.353) (1.356) (1.343) (1.293) (1.444) (1.308) (1.236) (1.819) (1.016)
a
Quantities in parentheses for B3LYP/MP2 functionals correspond to NTO anion obtained by the deprotonation of the N4 site and for other functionals correspond to NTO in the NTO-alumina complex geometry (Figure 4c).
Figure 2. Optimized geometry and interlayer spacing (in Å) of (4 × 4) α-Al2O3 in (a) bulk structure and (b) surface relaxed structure using different DFT functionals. In these figures, PBsol and Vdw refer to PBEsol and vdW-DF2 functional ,respectively, while Exp represents experimental values of bulk (1 × 1) α-Al2O3.47 The (0001) surface boundary has been removed in the bottom figure.
where Ecomplex represents the total energy of the complex and ENTO and Ealumina represent the total energy of the NTO and αAl2O3, respectively, within the complex geometry. Further, molecular geometries of gas phase NTO in both neutral and deprotonated forms were also optimized using the B3LYP functional and the 6-31G(d,p) basis set. Harmonic vibrational frequency analysis of NTO at the B3LYP/6-31G(d,p) level suggested computed molecular geometries as minima at the respective potential energy surfaces. Moreover, the MP2/6-
31G(d,p) level of geometry optimization of both the neutral and anionic forms of NTO was also performed using the respective B3LYP/6-31G(d,p) optimized geometry. This part of the calculations were performed using the Gaussian 09 suite of programs.48
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RESULTS AND DISCUSSION Molecular Structure of NTO. A high level theoretical calculation at the MP2 level of theory with a 6-311G(d,p) basis
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For example, in the bulk alumina using the PBE functional, the Al−O bond distance was found to be about 1.859 Å while the corresponding distance between atoms lying in the first (top) and second layers was reduced to about 1.693 Å. On the other hand, the Al−O bond distance for second and third layer atoms was computed to be 1.811 Å. Similarly, the Al−O bond distance between fourth and fifth and between fifth and sixth layer atoms was computed to be 1.938 and 1.872 Å, respectively, while that between seventh and eighth layer atoms was 1.868 Å. In comparing these results with the bulk structure, it is clear that surface relaxation is limited to only a few atomic layers from the surface and interior layers are generally not significantly influenced. The computed surface relaxation behavior is in agreement with other investigations where small size alumina surfaces were considered.51,52 Density of states (DOS) of bulk and surface relaxed Alterminated (4 × 4) α-Al2O3(0001) obtained using the PBE functional are shown in Figure 3. As a result of surface
set has predicted that the keto form of NTO is the most stable among all possible tautomeric forms.6 Therefore, the keto form of NTO was selected for the present investigation. Initially, we evaluated the reliability of the present level of calculations using a plane-wave DFT approach in predicting the molecular geometry of isolated NTO in the gas phase. Geometrical parameters computed using different DFT functionals under a plane-wave approach are presented in Table 1. It is well-known that MP2 and B3LYP level of DFT calculation with a reasonably good basis set provides reliable molecular geometry6,7,49 and therefore gas phase geometry of NTO was also optimized at the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels and computed geometrical parameters are also presented in Table 1 for comparison. These methods suggested that the planar form of NTO is the most stable in the gas phase. Further, it is evident from the data presented in Table 1 that plane-wave DFT predicted geometrical parameters of NTO are in good agreement to those predicted at the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels. Bulk and Relaxed (4 × 4) α-Al2O3(0001). Computed lattice parameters of Al-terminated (4 × 4) supercell of αAl2O3(0001) and the corresponding experimental values are presented in Table S1 in the Supporting Information. These computed lattice parameters were used in the further calculations such as bulk structure geometry, surface relaxation and surface adsorption. Further, the experimental value of the lattice parameter “a” (for (4 × 4) supercell) was obtained by multiplying four times the corresponding experimental lattice constant of (1 × 1) unit cell.47 We found that computed lattice parameters are generally similar and very close to the respective experimental values (Table S1). Moreover, the slight increase in the computed parameters are in agreement with the wellknown fact that GGA approximation slightly overestimates the lattice parameters compared to the corresponding experimental values.50 Figure 2a shows the optimized bulk geometry and interlayer spacing obtained using various functionals. It is clear that all methods predict similar interlayer spacing agreeing with the experimental values (0.84 Å of interlayer spacing between successive Al and oxygen layers and 0.49 Å between successive Al-layers) obtained from the (1 × 1) α-Al2O3 crystallographic data.47 The (0001) surface relaxed geometry of (4 × 4) α-Al2O3 is depicted in Figure 2b along with the interlayer spacing. In the surface relaxation calculations, 20 Å of vacuum space along the c-direction was also added to avoid the interaction of top layer of the considered structure with the bottom layer of the periodic image. Surfaces undergo significant inward relaxation, and maximum relaxation is revealed for the top layer of the (0001) surface. Therefore, the top layer (Al-containing plane) is very close to the oxygen-containing second layer, and the predicted distance between these two layers has been computed to be about 0.12, 0.11, 0.19, and 0.24 Å using the PBE, PBEsol, BLYP, and vdW-DF2 functionals, respectively. Thus, PBEsol and PBE functionals reveal the largest relaxation with interlayer spacing of about 0.11 Å, while the van der Waals corrected density functional vdW-DF2 predicted the least relaxation with the corresponding interlayer spacing of about 0.24 Å. Although the vdW-DF2 predicted relaxation of the top layer is comparatively smaller than the relaxation predicted using the other functionals, such relaxation is still very significant compared to the corresponding unrelaxed value of about 0.83 Å. Consequently, the Al−O bond length between successive layers was also significantly modified due to surface relaxation.
Figure 3. Density of states (DOS) of bulk alumina (in orange color), surface relaxed alumina (in blue color), and NTO-alumina complex (in green color) using the PBE functional.
relaxation the valence band is slightly modified as evidenced by the disappearance of peak near the Fermi level while the lower valence band does not show appreciable change. Significant change is shown in the conduction band which shows appearance of new peaks and that band has been shifted toward Fermi energy level indicating a decrease in the energy band gap due to the surface relaxation. We also performed DOS and projected DOS (pDOS) calculations on bulk and surface relaxed Al-terminated (1 × 1) α-Al2O3(0001) at the PBE level using 9 × 9 × 9 uniform k-grids (365 k-points) and they are shown in Figure S1 of the Supporting Information. Analysis of pDOS suggested that the conduction band mainly arises due to the 3s and 3p orbitals of Al atoms while the upper valence band has major contribution from the 2p orbitals of the oxygen atoms and lower valence band is due to the 2s orbital of the oxygen atom. This analysis is generally in agreement with other theoretical and experimental results.26−28 Moreover, it is clear that the new peak in the lower region of the conduction band is mainly contributed from the orbitals of surface Al atom which has undergone inward relaxation (Figure S1). Thus, the appearance of the new peak in the lower region of the conduction band of surface relaxed (4 × 4) α-alumina has main contribution from the surface Al-layer which has undergone significant inward relaxation. Moreover, several k-points are recommended for DOS calculation, but given the size of the D
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Figure 4. Optimized geometry of the NTO-alumina complex using the PBE functional where NTO interact with surface through (a) nitro group, (b) carbonyl group, and (c) both the carbonyl and nitro groups. In (a) and (b) though the third surface Al−O bond is not visible its bond length is also depicted there. In (c) the indices 1−6 represents different surface Al−O bonds whose lengths are also depicted in the figure. The dashed line indicates that proton from the N4 site of NTO is dissociated. Further, the (0001) surface boundaries are removed in these figures.
system ((4 × 4) super cell) we used only 12 k-points in the calculation. On the other hand, computed DOS of (1 × 1) and (4 × 4) α-alumina as shown in the Figure S2 of the Supporting Information are generally similar, and it suggests that computed DOS of (4 × 4) α-Al2O3 using small number of k-points in the present investigation deemed to be reliable. Adsorption of NTO on (4 × 4) Al-Terminated αAl2O3(0001) Surface. Both the perpendicular and parallel orientations of NTO with respect to the (0001) plane of alumina in the initial configuration of the complex were considered for the adsorption investigation. Two possible sites are available in NTO to bind with an Al atom on the surface of alumina: the two oxygen atoms comprising the nitro group (labeled O61 and O62 in Figure 1) and the oxygen atom of the carbonyl group (labeled O3 in Figure 1). To avoid any interaction between the adsorbate and the bottom of the adsorbent in its periodic image, more than 15 Å of vacuum space was added along the c-direction in these calculations. Geometry of the complex of NTO adsorbed perpendicularly through nitro group to Al-terminated (0001) α-alumina surface was optimized using PBE and BLYP functionals. The optimized geometry using the PBE functional is shown in Figure 4a. It is evident from this figure that NTO is adsorbed through only one nitro oxygen atom which is bonded with a single surface Al
atom. The length of the Al−O61 bond consequent to adsorption, where O61 represents the nitro group oxygen, was found to be 1.963 Å (using a PBE functional) and 1.960 Å (using a BLYP functional). Similar geometries were reported for the adsorption of TNT19 and nitromethane (NM)21 on αalumina in a perpendicular orientation to the surface. For comparison, the Al−O bond formed through TNT adsorption, where O represents the nitro group oxygen atom of TNT, was computed to be 1.958 Å using the PBE functional,19 while the corresponding bond in the NM-alumina system was found to be in the range of 1.938−1.942 Å using the PW91 DFT functional.21 The computed adsorption energy of NTO on the alumina surface was found to be 26.4 kcal/mol using the PBE functional and 22.0 kcal/mol using the BLYP functional. This computed adsorption energy is similar to the value (25.2 kcal/ mol) obtained for TNT adsorbed perpendicularly on alumina surface obtained using the PBE-DFT functional.19 Thus, it is clear that NTO is adsorbed strongly on the alumina surface. Further evidence for strong adsorption in this system is the deformation of the surface Al-atom position involved in direct interaction with NTO which is pulled out of the plane by about 0.34 Å as predicted by the PBE functional and 0.3 Å using the BLYP functional. E
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Figure 5. Graph (in center) shows the total energy (−11878.4 Ry as a reference along the y-axis) of the complex with respect to the optimization cycle (along x-axis) as predicted by the PBE-DFT level of geometry optimization of NTO adsorbed on α-alumina surface. The points (a−h) on the graph show various stages of geometry optimization and corresponding figures represent the snapshots of the corresponding geometry, where (a) and (h) represent the initial and final geometry of the NTO-alumina complex.
For the Al atom interacting with NTO, the surface Al−O bond (hereafter designated as Al−Obulk) increases from 1.69 Å in the surface relaxed structure to about 1.72 Å in the NTOalumina complex using the PBE functional (Figure 4a). Such an increase within the α-alumina structure is understandable since the binding Al-cation is pulled upward out of the surface plane due to the adsorption of NTO. The increase in the Al−Obulk bond length is found to be localized and other parts of the surface are not noticeably affected. Similar results were also found using the BLYP functional with the corresponding Al− Obulk bonds found to be in the range of 1.719−1.723 Å. The interlayer spacing of the alumina structure does not show noticeable change consequent to perpendicular adsorption of NTO other than the slight elongation of the Al−Obulk bond mentioned above. Similar results were obtained for TNT adsorption on the alumina surface.19 Comparison of geometrical parameters of NTO adsorbed perpendicularly through the nitro group on the alumina surface to the isolated NTO at the PBE-DFT level revealed that the appreciable changes are localized at the nitro group where the N6O61 bond involved in the adsorption interaction is elongated by about 0.047 Å after adsorption. While the N6O62 (not involved in the adsorption interaction) and the N6C5 bonds are compressed by about 0.017 and 0.026 Å, respectively, consequent to the adsorption. This trend among nitro group bond lengths is in agreement with that obtained for the TNT adsorption on the alumina surface.19 Moreover, the N1N2 bond of NTO was compressed and the N2C3 bond was stretched by about 0.013 and 0.010 Å, respectively, due to adsorption on the alumina surface. Other bonds of NTO did not change appreciably consequent to adsorption on the alumina surface.
We also investigated the perpendicular adsorption of NTO through the carbonyl group on the Al-terminated alumina surface using the PBE functional. The optimized geometry of the complex is shown in Figure 4b. The adsorbed Al−O3 bond distance was computed to be 1.872 Å and it is quite smaller than the adsorbed distance of 1.963 Å observed for the Al−O61 bond (Figure 4a). Thus, it is apparent that the Al−O adsorption bond is stronger in the complex where carbonyl oxygen of NTO is involved in the adsorption than was predicted for the complex involving nitro group oxygen of NTO in the adsorption on the alumina surface. The adsorption energy was predicted to be 34.2 kcal/mol, which is about 8 kcal/mol more than that observed when nitro group oxygen was involved in the adsorption on alumina surface. This result agrees with a previous study that shows that the carbonyl group is a much stronger hydrogen bond acceptor than the nitro group.53 Consequent to the adsorption, the interacting Al-atom is pulled out of the surface plane by about 0.36 Å. As was seen previously, the Al−Obulk bond length is elongated in the range of 1.721−1.726 Å compared to 1.693 Å in the pristine relaxed surface (Figure 4b). The interlayer spacing showed no noticeable change as compared to the surface relaxed αalumina. These results are similar to those obtained for perpendicular adsorption on alumina surface involving the nitro group oxygen. Compared to the isolated NTO calculated using the PBE functional, the adsorption of NTO on the alumina surface results in an increase in the C3O bond length by 0.018 Å accompanied by decrease in the N2C3 and C3N4 bond lengths by 0.03 and 0.029 Å, respectively. The optimized geometry of the NTO-alumina complex formed when NTO was initially placed parallel to the alumina surface is shown in Figure 4c. In the optimized complex, the NTO molecule has rotated almost perpendicular to the alumina F
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Figure 6. Isosurface depicting change in electron density after adsorption of NTO on Al-terminated (4 × 4) α-Al2O3(0001) surface through (a) nitro group oxygen, (b) carbonyl group, and (c) both the nitro and carbonyl groups using the PBE functional. Yellow region shows isosurface for gain and green region shows the isosurface for the loss of charge density. Isosurface corresponds to 0.0025 e/Å3. The (0001) surface boundaries have been removed in these figures.
alumina surface is related to the known property that NTO behaves as a weak acid and forms salts with metals and aromatic and aliphatic amines.13,14 Our results also favor the dissociation of an NH bond as predicted by EPR spectroscopy and HPLC experiments as the main dissociation path of NTO under both photochemical and thermochemical conditions.11 The computed Al1−O and Al2−O adsorption bond distances, where Al1 and Al2 represent surface Al atoms involved in the direct adsorption interactions with carbonyl and nitro groups, respectively (Figure 4c), depending upon the DFT functionals used in the present investigation, were predicted to be in the range of 1.825−1.842 and 1.926− 1.970 Å, respectively (see Table S2 in the Supporting Information for details). Based upon the Al−O adsorption bond distance, it is evident that the carbonyl group interacts more strongly than the nitro group with the alumina surface. The surface Al atoms involved in the direct adsorption are pulled out of the surface plane of Al atoms with a predicted vertical displacement of the Al1 atom of about 0.1 Å more than the Al2 atom. The relatively larger vertical displacement of the Al1 atom is related to the much stronger hydrogen bond acceptor ability of the carbonyl group as compared to the nitro group.53 Depending upon the DFT functionals used in the current investigation, the vertical displacement of the Al1 atom was found to be in the range of 0.37−0.49 Å while for the Al2 atom vertical displacement was predicted to be in the range of 0.27−0.39 Å from the alumina surface containing the remaining Al atoms. The vdW-DF2 functional predicted the lowest and PBE/PBEsol predicted the largest displacement. Moreover, the oxygen atom which received the proton due to migration of NTO proton is also predicted to be vertically displaced in the range of 0.04−0.06 Å depending upon the DFT functional used in the calculation. However, the comparison of interlayer spacing of alumina in the adsorbed complex to the surface relaxed structure did not reveal any noticeable differences. Thus, NTO adsorption leads to a localized region of deformation on the alumina surface, but the interlayer spacing does not show appreciable change. For example, in the adsorption region, the surface Al−Obulk bond distance was found to be in the range of 1.708−1.737 Å, except the Al−OH bond (represented as bond 1 in Figure 4c) which was predicted to be 1.845 Å due to the protonation of oxygen, the surface Al− Obulk bond distance in the surface relaxed α-alumina was predicted to be 1.693 Å using the PBE functional. Other functionals predicted similar results and details of these bonds are presented in the Table S2 of the Supporting Information.
surface such that both the nitro group and the carbonyl oxygen atoms of the adsorbate are able to interact with the alumina surface. Thus, it is clear that the orientation of NTO parallel to the alumina surface is not a stable configuration. An important feature revealed in the adsorption process is that the proton from the N4 site of NTO is cleaved and migrates to form a bond with the nearest available oxygen atom on the alumina surface (in fact this oxygen is from the second layer which becomes very close to the first layer due to the surface relaxation) showing the dissociative adsorption of NTO on the alumina surface. Figure 5 shows the graph of energy minimization of the complex due to the adsorption of NTO on alumina surface in the parallel initial configuration. It reveals the migration of proton from the N4 site of the adsorbate toward the alumina surface and proceeds with no energy barrier. Geometries of the complex at different energy minimization stages as predicted using the PBE functional are also shown in this Figure 5. It is evident that during the initial energy minimization process, the total energy of the complex decreases sharply as the NTO molecule orients parallel (approximate) to the alumina surface (point b on the graph in Figure 5). As the geometry optimization progresses, the NTO molecule reorients so that both the nitro group oxygen atom and carbonyl oxygen atom are involved in the interaction with the nearest surface Al-atoms (point d on the graph). This region (b to d on the graph) is relatively flat and energy does not show significant change. However, after point “d” in the curve, the energy again decreases relatively sharply (d to e on the graph) as a proton from the N4 site of NTO detaches and reattaches to the surface oxygen as depicted in Figure 5. Energy convergence is obtained when NTO is adsorbed in a nearly perpendicular orientation with respect to the alumina surface through the nitro group oxygen atom and a carbonyl oxygen atom and proton from the N4 site of adsorbate is migrated to a surface oxygen atom. Further, we found that this complex (Figure 4c) is about 44.4 kcal/mol lower in energy than the complex formed due to the interaction of carbonyl oxygen atom of NTO (Figure 4b) and about 50.5 kcal/mol lower in energy than the complex formed due to the interaction of nitro group oxygen of NTO (Figure 4a) on the alumina surface. Thus, it is clear that NTO adsorption on Al-terminated alumina surface takes place in a dissociative fashion where a proton from the N4 site of NTO is detached and is attached to the nearby oxygen site located near the surface and such dissociative adsorption is achieved without any energy barrier. The dissociative adsorption of NTO on the G
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investigated. It was revealed that carbonyl group of NTO interacts more strongly than the nitro group with the Alterminated alumina surface. Optimal adsorption was obtained when both the carbonyl oxygen and nitro group oxygen are involved in the adsorption process on the alumina surface. This adsorption is accomplished in a dissociative fashion where a proton from the N4 site of NTO dissociates and forms a new OH bond with the alumina surface oxygen atom. Thus, NTO on an Al-terminated α-alumina shows dissociative adsorption. Consequent to the adsorption the interacting surface Al-atoms show significant upward displacement from the plane containing the rest of the Al-atoms. The surface Al−Obulk bonds, where Al atom is in direct interaction with NTO, showed significant increase due to adsorption compared to the pristine relaxed alumina surface. However, such an increase in the surface Al−Obulk bonds was found to be localized and such an increase was not revealed farther from the interacting region. Moreover, apart from the upward shift of interacting Al atoms, the interlayer spacing of alumina did not reveal noticeable change compared to the relaxed alumina. Moreover, the DOS of the NTO-alumina complex is also found to be similar to that of the surface relaxed alumina.
The surface Al−Obulk bond distances farther from the adsorption region were generally found to be similar to the unperturbed surface relaxed α-alumina and thus offer further evidence for the localized nature of deformation following adsorption. The DOS of the adsorbed complex (Figure 4c) were also computed using the PBE functional and is shown in Figure 3. It is clear that DOS of the NTO-alumina complex is similar to that of the surface relaxed alumina, except that the larger energy peak of the conduction band is intensified in the complex. The computed adsorption energies of the NTO-alumina complex were predicted to be 64.7, 65.9, 51.3, and 65.2 kcal/ mol using the PBE, PBEsol, BLYP and vdW-DF2 functionals, respectively. Since, in the adsorbed complex NTO is in an anionic form, monomers (NTO and α-alumina) optimized separately were used in the adsorption energy calculation. It is clear that the computed adsorption energy for this complex is much larger (2−3 times) than those obtained for complexes when either of the nitro and carbonyl oxygen atoms is involved in adsorption on the alumina surface. However, such results are expected since both the nitro and carbonyl groups are involved in the adsorption process and that NTO has donated a proton to surface oxygen atoms possibly enhancing the adsorption process on the Al-terminated (0001) surface of α-Al2O3. The electron density difference maps of NTO adsorption on Al-terminated (4 × 4) α-Al2O3(0001) surface using the PBE functional are shown in Figure 6. These maps were obtained by subtracting the charge densities of monomers (NTO and αalumina) in the adsorbed complex geometry from the charge density of the adsorbed complex and plotted with the VESTA program.54 In these plots, the isosurface with yellow color indicates regions of electron gain and the green color indicates regions of electron loss. Figure 6 shows a significant amount of electron density buildup in the bonding region between the NTO and alumina surface in all orientations of the adsorbate. The loss of electron density appears from (1) the oxygen atom of the nitro group not involved in the adsorption interaction, (ii) the NO and or CO bond involved in the adsorption interaction and (iii) the surface Al atom involved directly in the adsorption process. As depicted in Figure 6c for the case of adsorption of NTO through nitro and carbonyl oxygen atoms on the alumina surface, a substantial decrease in charge density from the N4H bond of adsorbate is evident. Such a decrease in the charge density is due to the dissociative adsorption of NTO where proton from N4 site is dissociated and moved toward oxygen site of alumina surface forming an OH bond. It has been suggested that the buildup of electron density in the bonding region is indicative of a covalent type of interaction22,37 as is seen in Figure 6 for the adsorption of NTO on an alumina surface. Similar results were also revealed for the adsorption of TNT on the Al-terminated (4 × 4) α-Al2O3(0001) surface.19
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ASSOCIATED CONTENT
S Supporting Information *
Complete authors list for refs 39 and 48; Tables S1, S2; Figures S1, S2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 601-634-5431. E-mail:
[email protected]. mil. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government. The tests described and the resulting data presented herein, unless otherwise noted, were obtained from research conducted under the Environmental Quality Technology Program of the United States Army Corps of Engineers and the Environmental Security Technology Certification Program of the Department of Defense by the USAERDC. Permission was granted by the Chief of Engineers to publish this information. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. The authors thank Drs. Aimee Poda and Michael Cuddy of USACE for their editorial comments.
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CONCLUSIONS We have investigated the adsorption of an insensitive high performance explosive material 5-nitro-2,4-dihydro-3H-1,2,4triazol-3-one (NTO) on an Al-terminated (0001) surface of (4 × 4) α-Al2O3 using plane-wave DFT method under the GGA approximation. Various functionals such as PBE, PBEsol, BLYP and the recently developed van der Waals functional (vdWDF2) were used. Ultrasoft pseudopotentials were used to approximate electron−ion core interaction. Various orientations of NTO with respect to the alumina surface were
REFERENCES
(1) Rothgery, E. F.; Audette, D. E.; Wedlich, R. C.; Csejka, D. A. The Study of the Thermal Decomposition of 3-Nitro-1,2,4-triazol-5-one (NTO) by DSC, TGA-MS, and ARC. Thermochim. Acta 1991, 185, 235−243. (2) Beard, B. C.; Sharma, J. Early Decomposition Chemistry of NTO (3-Nitro-1,2,4-triazol-5-one). J. Energ. Mater. 1993, 11, 325−343. (3) Xie, Y.; Hu, R.; Wang, X.; Fu, X.; Zhu, C. Thermal Behaviour of 3-Nitro-1,2,4-triazol-5-one and Its Salts. Thermochim. Acta 1991, 189, 283−296.
H
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The Journal of Physical Chemistry C
Article
(4) Smith, M. W.; Cliff, M. D. NTO-Based Explosive Formulations: A Technology Review, DSTO-TR-0796; DSTO Aeronautical and Maritime Research Laboratory: Australia, 1999. (5) Meredith, C.; Russell, T. P.; Mowrey, R. C.; McDonald, J. R. Decomposition of 5-Nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO): Energetics Associated with Several Proposed Initiation Routes. J. Phys. Chem. A 1998, 102, 471−477. (6) Harris, N. J.; Lammertsma, K. Tautomerism, Ionization, and Bond Dissociations of 5-Nitro-2,4-dihydro-3H-1,2,4-triazolone. J. Am. Chem. Soc. 1996, 118, 8048−8055. (7) Sorescu, D.C.; Bennett, C. M.; Thompson, D. L. Theoretical Studies of the Structure, Tautomerism, and Vibrational Spectra of 3Amino-5-nitro-1,2,4-triazole. J. Phys. Chem. A 1998, 102, 10348− 10357. (8) Ritchie, J. P. Structures and Energies of the Tautomers and Conjugate Bases of Some 1,2,4-Triazolones. J. Org. Chem. 1989, 54, 3553−3560. (9) Prabhakaran, K. V.; Naidu, S. R.; Kurian, E. M. XRD, Spectroscopic and Thermal Analysis Studies on 3-Nitro-1,2,4triazole-5-one (NTO). Thermochim. Acta 1994, 241, 199−212. (10) Garland, N. L.; Ladouceur, H. D.; Nelson, H. H. Laser-Induced Decomposition of NTO. J. Phys. Chem. A 1997, 101, 8508−8512. (11) Menapace, J. A.; Marlin, J. E.; Bruss, D. R.; Dascher, R. V. Photochemical and Thermochemical Decomposition of 3-Nitro-1,2,4triazol-5-one and Perdeuterio-3-nitro-1,2,4-triazol-5-one in Neat and Mixed Systems. J. Phys. Chem. 1991, 95, 5509−5517. (12) Ostmark, H. Thermal Decomposition of NTO; FOA Report D201782.3, November 1991. (13) Nandi, A. K.; Singh, S. K.; Kunjir, G. M.; Singh, J.; Mandal, A. K.; Pandey, R. K. Assay of the Insensitive High Explosive 3-Nitro1,2,4-triazol-5-one (NTO) by Acid-Base Titration. Cent. Eur. J. Energ. Mater. 2013, 10, 113−122. (14) Singh, G.; Felix, S. P. Studies on energetic compounds 25. An Overview of Preparation, Thermolysis and Applications of the Salts of 5-Nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO). J. Hazard. Material 2002, A90, 1−17. (15) Cromer, D. T.; Hall, J. H.; Lee, K. Y.; Ryan, R. R. The Structure of the Ethylenediammonium Salt of 3-Nitro-1,2,4-triazol-5-one, C2H4(NH3)2·2C2N4O3H. Acta Crystallogr., Sect. C 1988, 44, 1144− 1147. (16) Cromer, D. T.; Hall, J. H.; Lee, K. Y.; Ryan, R. R. Structure of the 1,3-Diaminoguanidinium Salt of 3-Nitro-1,2,4-triazol-5-one, CH8N5+·C2HN4O3−. Acta Crystallogr., Sect. C 1988, 44, 2206−2208. (17) Dontsova, K. M.; Hayes, C.; Pennington, J. C.; Porter, B. Sorption of High Explosives to Water-Dispersible Clay: Influence of Organic Carbon, Aluminosilicate Clay, and Extractable Iron. J. Environ. Qual. 2009, 38, 1458−1465. (18) Clausen, J.; Robb, J.; Curry, D.; Korte, N. A Case Study of Contaminants on Military Rranges: Camp Edwards, Massachusetts, USA. Environ. Pollut. 2004, 129, 13−21. (19) Shukla, M. K.; Hill, F. Computational Investigation of Adsorption of 2,4,6-Trinitrotoluene (TNT) on (0001) Surface of (4 × 4) α-Alumina. J. Phys. Chem. C 2013, 117, 13136−13142. (20) Kunz, R. R.; Gregory, K. E.; Aernecke, M. J.; Clark, M. L.; Ostrinskaya, A.; Fountain, A. W., III. Fate Dynamics of Environmentally Exposed Explosive Traces. J. Phys. Chem. A 2012, 116, 3611− 3624. (21) Sorescu, D. C.; Boatz, J. A.; Thompson, D. L. First-Principles Calculations of the Adsorption of Nitromethane and 1,1-Diamino-2,2dinitroethylene (FOX-7) Molecules on the α-Al2O3(0001) Surface. J. Phys. Chem. B 2005, 109, 1451−1463. (22) Borck, O.; Hyldgaard, P.; Schrö der, E. Adsorption of Methylamine on α-Al2O3(0001) and α-Cr2O3(0001): Density Functional Theory. Phys. Rev. B 2007, 75, 035403-1−7. (23) Pennington, J. C.; Brannon, J. M. Environmental Fate of Explosives. Thermochim. Acta 2002, 384, 163−172. (24) Fauer, G. Principles and Applications of Inorganic Geochemistry; MacMillan: New York, 1991.
(25) Gitzen, W. H. Alumina as a Ceramic Material; American Ceramic Society: Columbus, OH, 1970; Special Publication No. 4. (26) French, R. H. Electronic Band Structure of Al2O3, with Comparison to AlON and AIN. J. Am. Ceram. Soc. 1990, 73, 477−489. (27) Ching, W. Y.; Xu, Y.-N. First-Principles Calculation of Electronic, Optical, and Structural Properties of α-Al2O3. J. Am. Ceram. Soc. 1994, 77, 404−411. (28) Levin, I.; Brandon, D. Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences. J. Am. Ceram. Soc. 1998, 81, 1995−2012. (29) Lee, C.-K.; Cho, E.; Lee, H.-S.; Seol, K. S.; Han, S. Comparative Study of Electronic Structures and Dielectric Properties of Alumina Polymorphs by First-Principles Methods. Phys. Rev. B 2007, 76, 245110-1−7. (30) Guénard, P.; Renaud, G.; Barbier, A.; Gautier-Soyer, M. Determination of the α-Al2O3(0001) Surface Relaxation and Termination by Measurements of Crystal Truncation Rods. Surf. Rev. Lett. 1998, 5, 321−324. (31) Ahn, J.; Rabalais, J. W. Composition and Structure of the Al2O3{0001}-(1 × 1) Surface. Surf. Sci. 1997, 388, 121−131. (32) Suzuki, T.; Hishita, S.; Oyoshi, K.; Souda, R. Structure of αAl2O3(0001) Surface and Ti Deposited on α-Al2O3(0001) Substrate: CAICISS and RHEED Study. Surf. Sci. 1999, 437, 289−298. (33) Soares, E. A.; Van Hove, M. A.; Walters, C. F.; McCarty, K. F. Structure of the α-Al2O3 (0001) Surface From Low-Energy Electron Diffraction: Al Termination and Evidence for Anomalously Large Thermal Vibrations. Phys. Rev. B 2002, 65, 195405-1−13. (34) Marmier, A.; Parker, S. C. Ab Initio Morphology and Surface Thermodynamics of α-Al2O3. Phys. Rev. B 2004, 69, 115409-1−9. (35) Thissen, P.; Grundmeier, G.; Wippermann, S.; Schmidt, W. G. Water Adsorption on the α-Al2O3(0001) Surface. Phys. Rev. B 2009, 80, 245403-1−6. (36) Nelson, C. E.; Elam, J. W.; Cameron, M. A.; Tolbert, M. A.; George, S. M. Desorption of H2O From a Hydroxylated Single-Crystal α-Al2O3(0001) Surface. Surf. Sci. 1998, 416, 341−353. (37) Chakarova-Käck, S. D.; Borck, O.; Schröder, E.; Lundqvist, B. I. Adsorption of Phenol on Graphite(0001) and α-Al2O3(0001): Nature of Van Der Waals Bonds From First-Principles Calculations. Phys. Rev. B 2006, 74, 155402-1−7. (38) Blomqvist, J.; Salo, P. First-Principles Study for the Adsorption of Segments of BPA-PC on α-Al2O3(0001). Phys. Rev. B 2011, 84, 153410-1−4. (39) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. Quantum ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502-1−19, http://arxiv.org/abs/0906.2569. (40) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (41) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 2008, 100, 136406-1−4. (42) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (43) Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.; Lundqvist, B. I. Van Der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401-1−4. (44) Thonhauser, T.; Cooper, V. R.; Li, S.; Puzder, A.; Hyldgaard, P.; Langreth, D. C. Van Der Waals Density Functional: Self-Consistent Potential and the Nature of the van der Waals Bond. Phys. Rev. B 2007, 76, 125112-1−11. (45) Roman-Perez, G.; Soler, J. M. Efficient Implementation of a van der Waals Density Functional: Application to Double-Wall Carbon Nanotubes. Phys. Rev. Lett. 2009, 103, 096102-1−4. (46) Lee, K.; Murray, E. D.; Kong, L.; Lundqvist, B. I.; Langreth, D. C. Higher-Accuracy van der Waals Density Functional. Phys. Rev. B 2010, 82, 081101(R)-1−4. I
dx.doi.org/10.1021/jp408661h | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
(47) Lewis, J.; Schwarzenbach, D.; Flack, H. D. Electric Field Gradients and Charge Density in Corundum, α-Al2O3. Acta Crystallogr., Sect. A 1982, 38, 733−739. (48) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford CT, 2009. (49) Riley, K. E.; Holt, B. T. O.; Merz, K. M., Jr. Critical Assessment of the Performance of Density Functional Methods for Several Atomic and Molecular Properties. J. Chem. Theory Comput. 2007, 3, 407−433. (50) Hass, P.; Tran, F.; Blaha, P.; Schwarz, K. Construction of an Optimal GGA Functional for Molecules and Solids. Phys. Rev. B 2011, 83, 205117-1−6. (51) Kurita, T.; Uchida, K.; Oshiyama, A. Atomic and Electronic Structures of α-Al2O3 Surfaces. Phys. Rev. B 2010, 82, 155319-1−14. (52) Manassidis, I.; Gillan, M. J. Structure and Energetics of Alumina Surfaces Calculated from First Principles. J. Am. Ceram. Soc. 1994, 77, 335−338. (53) Allen, F. H.; Baalham, C. A.; Lommerse, J. P. M.; Raithby, P. R.; Sparr, E. Hydrogen-Bond Acceptor Properties of Nitro-O Atoms: A Combined Crystallographic Database and Ab Initio Molecular Orbital Study. Acta Crystallogr. 1997, B53, 1017−1024. (54) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272−1276.
J
dx.doi.org/10.1021/jp408661h | J. Phys. Chem. C XXXX, XXX, XXX−XXX
