Aluminum

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J. Phys. Chem. A 2010, 114, 2284–2292

Theoretical Infrared and Terahertz Spectra of an RDX/Aluminum Complex Carlos Guadarrama-Pe´rez, Julibeth M. Martı´nez de La Hoz, and Perla B. Balbuena* Department of Chemical Engineering, Texas A&M UniVersity, College Station, Texas 77843 ReceiVed: October 18, 2009; ReVised Manuscript ReceiVed: December 13, 2009

Density functional theory is employed to characterize the infrared and terahertz spectra of an explosive molecular species, RDX, deposited over an aluminum surface, modeled as a planar cluster of Al16. Changes in the inter- and intramolecular vibrational modes are systematically analyzed starting from the isolated monomer, dimer, and tetramer and then considering the interactions of the monomer with an Al plate. The results are compared to available experimental information for RDX films on Al surfaces. It is found that the RDX molecule changes conformation because of the interaction with the model Al surface, becoming closer to an AAA conformation with the three NO2 groups in nearly axial positions. The calculated spectra serve as an initial guideline to interpret the main peaks of previously reported RDX films on Al. 1. Introduction 1,3,5-Hexahydro-1,3,5-trinitro-1,3,5-triazine, a nitramine compound used in energetic materials, also known as hexogen, cyclotrimethylenetrinitramine, cyclonite, or simply as RDX, is a well-known explosive compound. The RDX molecule can be found in several conformations; two of them are shown in Figure 1: one with two nitro groups in axial positions and one in the equatorial position (AAE) and the other with the three nitro groups in axial positions (AAA). Various RDX crystalline forms exist: R, β, δ, γ, and ε,1 from which R is the most stable at room temperature2 where crystal packing is stabilized by CH · · · O intermolecular interactions between a methylene hydrogen of one molecule and oxygen atoms belonging to nitro groups of a neighbor molecule.3 The β form can be observed at room temperature and pressure, when carefully crystallized after cooling a very concentrated hot nitrobenzene solution.1 The δ form has also been observed at ambient temperature but at high pressures.4 The γ form has also been identified at high pressures, with a crystalline structure characterized by the presence of two molecules with different conformations, the first molecule with two NO2 axial groups and the last NO2 group in an intermediate position between equatorial and axial whereas the second molecule has three nearly axial NO2 groups.5 Barriers for interconversion among different RDX conformers range between 1.5 and 5 kcal/mol.6 Karpowicz and Brill7 measured Fourier transform infrared spectra (FTIR) in vapor phase and in solutions, and in R- and β-forms. They discussed the existence of intermolecular forces being responsible for the R form (Cs symmetry), stable in crystalline state, while interactions with solvent molecules may relax RDX molecules to a conformation where all three NO2 groups are located in equivalent positions (C3V symmetry). Easy and fast identification of explosive materials, mainly in airports and other public places, is crucial for implementing efficient preventive security measurements. Present methods used to identify the presence of explosive substances include X-ray and γ-ray that pose some health risks to operators. On the other hand, trace detection techniques are not infallible when used with sealed containers. A relatively new technique, * To whom correspondence should be addressed. E-mail: balbuena@ tamu.edu. Telephone: 979-845-3375. Fax: 979-845-6446.

terahertz (THz) spectroscopy, holds some promise for effective explosives detection. It is based on the measurement of the frequency-dependent response of a material to photons with energies of about 1 meV. This technique is based on the same principles used in FTIR, taking advantage of the light spectrum in the range from 100 GHz to 4 THz (1 THz ) 100/3 cm-1). In this region, located below the far infrared region and above microwave radiation, beside others, intermolecular interactions (weak bonds), skeletal bending modes through the entire molecule, and intermolecular vibration modes of hydrogenbonded molecules can be detected. Lattice vibrations of molecules (also called external modes) consisting of translational and rotational (librational) motions can be seen almost exclusively in this region. Recent molecular dynamics simulations by Boyd et al.8 using a new force field developed by the same authors9 reported the RDX vibrational spectrum aiming to explain the origin of the initial stages of detonation through the spread of lattice energy to internal degrees of freedom, seeking doorway modes for this energy. They compared the normal modes in two RDX conformers with those of their crystal model, concluding that motions involving nitro groups that correlate with significant molecular center of mass motion may be involved in the energy transfer from lattice to intramolecular vibrations as in shockwaves. Melinger et al.10 deposited a RDX polycrystalline thin film over an aluminum plate and, using waveguide terahertz timedomain spectroscopy, measured near 10 K a spectrum consisting of approximately 20 vibrational modes between 0.5 and 3.5 THz. The spectrum of the film that was described as having planar ordering on the inner surface of a metal parallel plate waveguide, showing bands sharper than those reported in previous studies.10 It was reported that prior to the measurements the metal surface was washed with solvent and then the plasma cleaned; however,

Figure 1. Most stable conformations of 1,3,5-hexahydro-1,3,5-trinitro1,3,5-triazine (RDX). The AAE form has one equatorial group while AAA does not.

10.1021/jp909976d  2010 American Chemical Society Published on Web 01/26/2010

Spectra of an RDX/Aluminum Complex

Figure 2. DFT calculated RDX structures at B3LYP/6-311G(d,p) level: (a) AAE and (b) AAA conformers.

the authors did not provide details about the surface structure or identify the corresponding modes. DFT calculations are important tools to understand and characterize spectroscopic information. Rice and Chabalowski11 have calculated, at a variety of theory levels (MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311+G**), three RDX conformers and their IR spectra from 700 to 1700 cm-1 and compared them with experimental results, in order to explain previous experimental results pointing to a gas-phase conformation where all NO2 groups are in axial positions and to a β-form solid RDX. Allis and co-workers12 simulated and analyzed the solid-state terahertz spectrum of R-RDX using periodic DFT calculations at a BP/DNP level of theory, reaching good agreement with their measured polycrystalline RDX terahertz spectra at 298 and 7 K. An interesting extension of these studies is the analysis of adsorbed RDX whether on surfaces or porous materials. In this work we analyze changes in the molecular spectrum when the RDX molecule is attached to a model aluminum surface using first-principles DFT calculations, and we compare the results to those observed by Melinger et al.10 We first compare IR calculated spectra of AAE and AAA RDX molecular forms with that calculated for a RDX molecule coordinated to an Al layer and then with the published experimental spectrum for RDX directly crystallized over an Al plate.10 2. Methods The initial RDX molecular geometry taken from Cambridge Crystallographic Data Base3 was optimized without restrictions using the B3LYP/6-311G(d,p) model chemistry.13-16 In monomers, dimer, and tetramer no negative frequencies were found,

J. Phys. Chem. A, Vol. 114, No. 6, 2010 2285 indicating that the optimized geometries are local minima. Tetramer and octamer geometries were taken from the published experimental X-ray structural data3 and subsequently optimized. To model RDX over Al, a single layer of Al16 was used to simulate the Al surface. Al16 and Al16-RDX were optimized using the same B3LYP/6-311G(d,p) method while restricting Al displacements in the z direction perpendicular to the plane of the surface. The calculated frequencies were not scaled, and spectra taken from literature were digitized to compare experimental and calculated data. In these comparisons, calculated spectra were taken from the representation obtained with Gaussview.17 All calculations were done with Gaussian03.18 Intermolecular interactions were interpreted using the NBO population analysis.19,20 3. Results Comparisons were made between calculated spectra for AAE and AAA monomers, a tetramer, the Al16 layer, Al16AAA-RDX, experimental crystalline RDX, and RDX crystallized over an Al plate, in order to assign vibrations to individual peaks and interpret experimental spectra. 3.1. Comparison between AAE and AAA Conformers. Our calculations in gas phase at the B3LYP/6-311G(d,p) level indicate that AAE-RDX is 0.55 kcal/mol more stable than the AAA conformer (Figure 2). Rice et al. had reported 0.19 kcal/ mol calculated at B3LYP/6-31G(d)6 and 0.64 kcal/mol at B3LYP/6-311+G(d,p),11 in reasonable agreement with our results. Differences in bond distances and angles are not perceptible between the AAE and AAA conformers, but the dihedrals N2-C2-N3-N6 and N3-C2-N2-N5 are larger in AAA than in AAE because of 1,3-diaxial repulsive interactions between NO2 groups. A comparison of the AAE and AAA geometries is included later in Table 3. Comparison between the calculated AAE and AAA spectra is shown in Figure 3. These spectra correlate very well with the experimental ones reported previously.7 Near 3000 cm-1, two bands corresponding to C-H stretching are observed for AAA and three for AAE. This is because of symmetry differences in both conformations. For AAE, two of these bands (those closest to 3000 cm-1) correspond to a CH2 axial stretching, and the most displaced corresponds to a CH2

Figure 3. Calculated IR spectra of AAE (black) and AAA (red) RDX conformers at B3LYP/6-311G(d,p) level of theory.

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Figure 4. Calculated IR spectrum section (0-500 cm-1) for the AAE (black) and AAA (red) conformers.

TABLE 1: Bond Distances (Å), Angles (deg), and Dihedrals (deg) for Dimer, Isolated AAE Molecule, and Octamer, Calculated at B3LYP/6-311G(d,p)a

Figure 5. RDX dimer minimal energy structure calculated at B3LYP/ 6-311G(d,p) theory level.

equatorial stretching while in AAA one band is present due to CH2 axial stretching and the other to CH2 equatorial stretching. Three main groups of bands can be observed in the central region of both spectra. One group around 1680 cm-1 is due to NO2 asymmetric stretching. The second group between 1200 and 1550 cm-1 is related to CH2 wagging, scissoring, and twisting vibrations, and the group between 750 and 1050 cm-1 is due to CH2 rocking and NC2 stretching. A significant difference between both spectra is the higher intensity of a band in the IR spectrum of the AAE conformer at 1300.5 cm-1 compared to that at 1296.4 in the AAA spectrum. Both bands are due to CH2 wagging plus N-N bond stretching. Also, while just one band can be seen at 1677.8 cm-1 in the AAA spectrum, accounting for asymmetric O-N-O stretching, up to three bands are due to the same kind of vibration in the AAE spectrum at 1645.9, 1670.2, and 1688.4 cm-1. Other differences can be observed near 500 cm-1, Figure 4, where the most notorious is the shift of a band calculated at 326.3 cm-1 in the AAE conformer (accounting for all CH2 rocking) which is equivalent to the one at 360.75 cm-1 in the AAA conformer spectrum. 3.2. Building Intermolecular Interactions: Monomer, Dimer, Tetramer, and Octamer. The calculated minimal energy geometry of an RDX dimer in gas phase corresponds to a system where both molecules are in AAE conformation, as displayed in Figure 5. Distances O1-H3′eq and O1′-H3eq are 2.33 Å, which is below the van der Waals radii sum (2.72 Å). Also a NBO population analysis20 included as Supporting Information (Table

parameter

dimer

AAE monomer

octamer (average)

RDX crystal (average)

N1-N4 N2-N5 N3-N6 N4-O1 N4-O2 N5-O3 N5-O4 N6-O5 N6-O6 C3-H3eq C3-H3ax C1-H1eq C1-H1ax C3-N1-N4 C2-N3-N6 C2-N2-N5 N3-C3-N1-N4 N2-C2-N3-N6 N3-C2-N2-N5

1.39 1.43 1.43 1.22 1.22 1.21 1.21 1.21 1.21 1.08 1.10 1.08 1.10 117.34 116.82 117.06 162.00 96.08 -97.17

1.40 1.43 1.43 1.22 1.22 1.21 1.21 1.21 1.21 1.08 1.10 1.08 1.10 115.69 116.85 116.85 166.23 95.94 -95.94

1.39 1.41 1.42 1.22 1.22 1.22 1.22 1.22 1.22 1.08 1.09 1.08 1.09 117.45 117.29 117.19 140.36 109.83 -96.34

1.36 1.41 1.41 1.23 1.23 1.22 1.22 1.22 1.22 0.93 0.96 0.93 0.96 118.74 117.11 116.89 147.49 91.12 -93.08

a Subscripts eq and ax indicate equatorial and axial distances respectively. The RDX crystal values are from published data.3

S1) shows that there are six interactions of the kind nO1′fσ*C3-H3eq (each one corresponding to a pair of electrons from the oxygen atom interacting with a CH antibonding orbital of the neighbor molecule) responsible for formation of the dimer. A detailed comparison of the geometric parameters between the AAE monomer and dimer is displayed in Table 1, showing some structural differences. Average parameters for the geometries of the calculated octamer and the experimental crystal are included for reference. In the dimer, N has less tetrahedral character (is more planar) probably because of the intermolecular electronic interaction between NO2 and CH, revealed by the smaller N1-N4 dimer distance and larger C3-N1-N4 angle (Table 1) and by the NBO population analysis (Supporting Information, Table S1). A tetramer and an octamer were also calculated starting with geometry obtained from a crystal containing eight molecules from the Cambridge Crystallographic Database and, in the case of the tetramer, randomly selecting four of those molecules, initially all in AAE conformation. The tetramer optimized at


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Figure 6. Calculated spectra of AAE monomer (black), dimer (red), and tetramer (green) using B3LYP/6-311G(d,p) model chemistry.

Figure 7. (a) Amplification (0 to 150 cm-1) of AAE-RDX monomer spectrum (black), dimer (red), and tetramer (green) calculated with B3LYP/ 6-311G(d,p) model chemistry. (b) Visualization of the calculated symmetric optical rotation at 17.3 cm-1 in RDX dimer.

B3LYP/6-311G(d,p) level resulted in a system of two monomers in AAE and two in AAA conformations. The geometry of the calculated octamer correlates fairly well with that of the experimental crystal, except for the dihedrals, some of which are reported in Table 1, where the values were averaged among the eight molecules in order to simplify the analysis. With respect to the monomer and dimer, the most pronounced difference is found in the N3-C3-N1-N4 dihedral angle, related to an axial NO2 group in the calculated octamer. Compared to the value in the crystal, in the AAE monomer and the dimer this angle is higher by around 20° whereas in the octamer it is below the crystal value just by 7°. Also, in the three molecules the C-H bonds are longer than the solid-state values. In Figure 6 we compare the calculated dimer spectrum with those of the AAE monomer and tetramer in order to understand the role of intermolecular interactions on the various modes. The main modes of monomer, dimer, and tetramer spectra coincide in position, over all the range (0-3500 cm-1), but some enhancements in intensities are observed in the dimer and

tetramer, respectively. Also, in the tetramer spectrum there are two bands near 3200 cm-1 instead of three found in the monomer and dimer; this difference may be due to the AAA contributions to the tetramer. Additional differences emerge when the 0-150 cm-1 region is amplified, with the most remarkable peak at 17.3 cm-1 which is attributed to an external vibrational mode (symmetric optical rotation (OR) through an axis between both molecules), as shown in Figure 7. In the tetramer, an extra band arises at 43.3 cm-1 which is assigned to an OR of two AAA-RDX molecules in the tetramer. 3.3. Calculated RDX Dimer and Tetramer versus Polycrystalline Experimental Terahertz Spectra. Comparison between experimental crystal12 and the calculated dimer and tetramer spectra (Figure 8) shows some correlation between bands. The experimental spectrum shows no bands equivalent to those of the calculated dimer at 10.9 and 17.3 cm-1 assigned to optical rotation of molecules, and on the basis of the results for the tetramer it could be suggested that as the intermolecular

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Figure 8. RDX terahertz experimental spectrum at 7 K12 (blue), calculated dimer (red), and tetramer (green) spectra.

TABLE 2: Spectral Modes and Assignments for Calculated RDX Dimer and Tetramer (in italics) and Experimental Crystalline RDX (in parentheses)12 calculated (experimental) 33.9, 20.5 (33), 28.7 (41), 43.3 (50) 40.7, 54.4 (59), 55.9, 65.3 (73) 49.9, 49.1 (54), 64.5, 65.4 (74), 112.1, 107.5 (116) 98.5, 94.4 (100), 106.8, 94.9 (103), 120.7, 107.13 (121) 67.9, 76.3 (83) 69.5, 78.8 (85)

type of motion OR OR + NO2 bending OR + NO2 libration OR + all NO2 bending OT + NO2 libration + NO2 bending

interactions increase, the bands in this range tend to blue-shift toward the experimental bands (as shown by the arrow). In the calculated spectra, bands at 33.9 and 51.9 cm-1 (dimer) and 20.5 and 43.3 cm-1 (tetramer) correspond to optical rotations in the molecule equivalent to those at 33 and 54 cm-1 in the experimental spectra. Another band at 59.6 cm-1 in the dimer, 65.3 cm-1 in the tetramer (73 cm-1 in experimental), accounts for ring optical rotation and NO2 bending vibrations. Several bands are due to optical rotations plus NO2 libration, as shown in Table 2. 3.4. Comparison of AAA Conformer and AAA-Al16 Complex. Figure 9 displays the calculated geometry of RDX attached to the model Al surface. Although AAE is the most stable configuration in gas and solid phases, the calculations indicate that when attached to a layer of Al16, RDX changes to a more stable geometry where all three NO2 groups locate in axial-like positions similar to those in the AAA conformer. Table 3 illustrates that while there is little difference in C-H distances, a remarkable distinction between the AAA conformer and AAA-Al16 systems is the geometry of N4. In the complex this N is in a tetrahedral geometry (N1-N4-O average angle ) 104.4°), and in the free molecule it is involved in a more planar geometry (N1-N4-O ) 116.4° for the AAA conformer). All N-N bond distances are approximately the same in the AAE and AAA monomers while in the complex the N1-N4 bond distance is longer because of electron transfer between N4 and the Al layer, and the other two NN distances are shorter.

Also, all the N-O bond distances are the same in the AAE and AAA monomers while in the complex the N4-O1 and N4-O2 bonds are longer because of O coordination to Al. N6-O6 also is elongated because of coordination. The geometry related to the N atoms in the cycle (N1, N2, and N3) provides information about the NO2 positions. Dihedrals and angles around N1, N2, and N3 are good indicators of the variation of the NO2 groups from an axial position. Similarly, the angle between three consecutive atoms in the cycle C1, C2, C3 and that of an axial substituent bonded to the central atom (i.e., C2-H) may also illustrate the deviation of the substituents from an axial position. In the AAA-Al16 complex, the NO2 group attached to Al is nearer to an axial position compared to the free molecule, as can be seen by a smaller value of the N2-C1-N1-N4 dihedral in AAA-Al16 than both RDX free conformers but closer to that of the AAA conformer. The NO2 attached to N3 also interacts with Al16, changing the molecular geometry, and the C2-N3-N6 angle is 117.16°, intermediate between those of the AAE and AAA monomers. Resonance plays an important role in N and NO2 geometries. When NO2 is free, electrons are in resonance through the N-NO2 moiety making the group R-N1-R (Figure 2) close to coplanar with the NO2 group. The N-NO2 moiety has three resonating structures:

Since there are two resonance structures with a free electron pair, the N atom from the nitro group (N4, Figure 5) is expected to be involved in a trigonal planar structure and the other nitrogen (N1), to form a pyramidal moiety. When RDX attaches to the Al surface, N4-Al bond formation breaks the resonance and nonbonding N1 electrons become available causing the C1-N1-N4 to be smaller (114.67°). This small change in N1 geometry moves its NO2



Figure 9. Side (a) and top (b) views of RDX molecule attached to Al16. Blue, red, purple, brown, and pink, correspond to Al, O, N, C, and H atoms, respectively. In the side view, Al-O interactions are shown as dashed lines.

TABLE 3: Geometric Parameters, Bond Distances (Å), Angles (deg), and Dihedrals (deg) for AAA-Al16, AAE, and AAA Conformers, Calculated at B3LYP/6-311G(d,p) Level of Theory parameter N1-N4 N2-N5 N3-N6 N4-O1 N4-O2 N5-O3 N5-O4 N6-O5 N6-O6 O1-Al1 O1-Al2 O2-Al2 O2-Al5 O6-Al3 C1-H1ec C1-H1ax C3-H3ec C1-N1-N4 C2-N2-N5 C2-N3-N6 N1-N4-O1 N1-N4-O2 N2-C1-N1-N4 N2-C2-N3-N6 N3-C2-N2-N5

AAE 1.40 1.43 1.43 1.22 1.22 1.21 1.21 1.21 1.21

1.08 1.10 1.08 115.69 116.85 116.85 116.6 116.6 166.23 95.94 -95.94

AAA-Al16 1.47 1.38 1.38 1.46 1.38 1.22 1.23 1.23 1.28 2.39 1.97 2.67 1.97 2.11 1.08 1.09 1.08 114.67 120.51 117.16 103.9 104.9 89.08 125.78 -99.61

AAA 1.42 1.42 1.42 1.22 1.22 1.22 1.22 1.22 1.22

1.08 1.09 1.08 118.03 118.13 118.06 116.4 116.4 -102.90 102.86 -103.24

group toward the center of the cycle and makes 1,3-diaxial interactions more important, pushing the other two nitro groups away from the center of the cycle, which is reflected in AAA-Al16: new, larger C2-N2-N5 and C2-N3-N6 angle values than in the AAE conformer. In AAA-Al16, the most planar fragment involving a nitrogen is C2-N2-N5 (bonded to a non Al interacting NO2, Figure 9), shown by a C2-N2-N5 angle value near 120° and a N2-C2-N3-N6 dihedral value of about 125.78°. O1 and O2 seem to be in bridge positions over the Al layer. N4 is also close enough to Al4 to be able to develop an interaction and become tetrahedral. In order to understand Al16-RDX behavior and to find if there is any correlation between bands of the monomer and the adsorbed molecule, the IR spectra of the calculated AAA monomer (red) and Al16-RDX (black) are plotted simultaneously in Figures 10 and 11. Although there are many

similarities, marked differences are shown in the 0 to 500 cm-1 zone; these are displayed in Figure 11, together with the corresponding modes of Al16. All modes calculated for Al16 correspond to in-plane vibrations (Al atoms were arranged in a plane, emulating the top layer of an fcc (111) surface) while in Al16-RDX in-plane vibrations can only be seen for peaks at 311.5 and 343.4 cm-1. However, the band at 343.4 cm-1 for Al16-RDX results from in-plane Al atoms vibration-coupled to RDX optical rotation. This band shows a blue shift with respect to the one at 327.3 cm-1 in Al16, due to Al in-plane vibrations. Also, in Al16-RDX, pure ring twisting is present in the last three vibrations (421.1, 440.7, and 461.4 cm-1), and ring twisting and other kinds of motion (for example NO2 libration) are present in several bands excluding those at 311.5 and 343.4 cm-1. Contrastingly, while the peak at 169.5 cm-1 for Al16 corresponds to an in-plane vibration of Al atoms, in AAA-Al16 the peak at 163.3 cm-1 involves out-of-plane vibration (in the z-direction) of Al atoms plus ring torsion. Correlation was found for bands at 203.4 cm-1 (Al out-ofplane vibration-coupled to RDX ring twisting and to a NO2 librational mode) and 223.9 cm-1 (Al out-of-plane vibrationcoupled to ring twisting) in AAA-Al16, with the band at 216.5 cm-1 in the AAA monomer (ring twisting). As seen previously, in Al16 no peaks were the result of out-of-plane vibrations. For AAA-Al16, NO2 librational modes are implied in those vibrations in the range from 67.4 to 203.4 cm-1, except for the band at 163.3 cm-1. Unexpectedly, the strongest vibration calculated in Al16 corresponding to an in-plane vibration at 268.6 cm-1 does not exactly match the same motion found in the strongest band for the Al16-RDX complex located at 272.2 cm-1 which is due to Al in-plane vibrations with some contribution in the z direction plus RDX optical rotation. As it can be seen in Figure 12, displacement vectors corresponding to such modes are randomly oriented. Very interesting changes are also observed in the mid-IR region, which are summarized in Table 4. The main differences observed between the two spectra are as follows: (a) Greater intensity of the AAA-Al16 peak at 563.03 cm-1. This peak corresponds to an inversion angle of the N6 (Figure 9) in the NO2 group. In the AAA conformer spectrum this peak is not observed; instead there is one peak at 592.52 cm-1 corresponding to CH2 rocking.

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Figure 10. IR spectra of AAA-RDX monomer (red) and RDX-Al16 (black) calculated at B3LYP/6-311G(d,p) level.

Figure 11. AAA monomer (red), RDX-Al16 (black), and Al16 (blue) IR calculated spectra (0-500 cm-1).

Figure 12. Displacement vectors for vibrations of aluminum atoms in (a) Al16 and (b) AAA-Al16.

(b) The AAA monomer does not have peaks between 784.44 and 890.92 cm-1 whereas four peaks at 794.62, 826.19, 852.23, and 885.50-1 cm are observed in the AAA-Al16 system. The only difference in the modes corresponding to these four peaks

with respect to the free AAA monomer is an asymmetrical stretching in the NO2 group bonded to the Al surface. (c) There is a big peak at 1677.5 cm-1 for the AAA monomer, corresponding to NO2 asymmetrical stretching, whereas a red-



TABLE 4: Calculated Vibrational Modes in the Mid-IR Range for the AAA Conformer and the AAA-Al16 Complex AAA frequency (cm-1)

AAA conformer mode

AAA-Al16 frequency (cm-1)

AAA-Al16 mode inversion angle NO2 (N6, Figure 9) NO2 scissoring CH2 rocking CN2 and NO2 scissoring NO2 asymmetrical stretching (N4, Figure 9) NO2 scissoring CH2 rocking NO2 asymmetrical stretching (N4, Figure 9) NO2 scissoring CN2 asymmetrical stretching NO2 scissoring (N4, Figure 9) CN2 asymmetrical stretching NO2 asymmetrical stretching (N4, Figure 9) CH2 rocking NC2 and NO2 symmetrical stretching CH2 rocking CN2 asymmetrical stretching NO2 symmetrical stretching CH2 rocking NO2 symmetrical stretching CH2 rocking

592.52 663.14

CH2 rocking CN2 scissoring

563.03 578.02

784.44

CN2 and NO2 scissoring CH2 rocking NC2 scissoring NO2 symmetrical stretching CH2 rocking CN2 asymmetrical stretching CH2 rocking CN2 symmetrical stretching

767.29

890.92 912.62 939.54 1009.52 1255.66

CH2 rocking CH2 twisting NC2 asymmetrical stretching

1482.32

CH2 wagging NO2 symmetrical stretching CH2 scissoring NO2 symmetrical stretching CH2 wagging NO2 symmetrical stretching CH2 scissoring

1677.50

NO2 asymmetrical stretching

1296.36 1351.85 1400.96

794.62 826.19 852.23 885.50 926.97 946.66 1013.17 1252.07 1308.71 1356.64 1406.75 1482.55 1603.47

shifted peak at 1603.47 is observed for the AAA-Al16 system related to the NO2 asymmetrical stretching. (d) The last peak in this region is observed at 1677.5 cm-1 for the AAA conformer (corresponding to NO2 asymmetrical stretching). In the AAA-Al16 complex this same peak is again red-shifted at 1603.47 cm-1. 3.5. AAA-Al16 Complex versus Experimental RDX over Aluminum Surface. A comparison between the terahertz spectra of RDX experimentally crystallized over an Al plate and that calculated for the AAA-Al16 model complex is shown in Figure 13. In order to simulate terahertz experimental observations, the full width at half-maximum of each calculated peak was set to 3 cm-1.

CH2 rocking NC2 asymmetrical stretching CH2 wagging NO2 symmetrical stretching CH2 wagging and twisting CH2 wagging NO2 symmetrical stretching CH2 scissoring NO2 asymmetrical stretching

The assignments and correspondence among modes are reported in Table 5. The group of bands near 20 cm-1 in the experimental spectrum may be assigned to intermolecular RDX-RDX interactions, where optical translations and rotations should be observed as inferred from the calculated dimer modes discussed in relation to Figure 8. Other assignments reveal coupling of the molecular motions to those of the atoms of the substrate. We note that this work is only a first attempt to interpret this complex spectrum. In the experiments of ref 10 a polycrystalline RDX thin film is deposited on a metal surface; thus, our simulated system lacks the RDX-RDX intermolecular interactions. In addition, our simplified model for the Al surface may

Figure 13. Overlap of terahertz and IR spectra of AAA-Al16 (red) and that of experimental RDX crystallized over an Al plate (blue).10

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TABLE 5: Spectral Modes and Assignments for Calculated AAA-Al16 and Experimental RDX Crystallized over an Aluminum Plate10 calculated (experimental) 31.8 (30.6) 38.5 (41.7), 43.2 (45.9), 51.5 (52.7), 108.8 (112.9) 64.5 (71.5) 67.4 (74.9), 78.9 (81.1), 85.4 (85.3), 92.3 (96.4), 105.9 (108.1), 114.3 (118.1) a

type of motion out-of-plane Al vibration + ORa normal to Al layer plane + NO2 rocking out-of-plane Al vibration + ORa parallel to Al layer plane in-plane Al vibration out-of-plane Al vibration + NO2 libration

RDX optical rotation.

introduce some artifacts because of its small size and thickness and the presence of edge effects. However, we consider that developing a stepwise understanding of the way intermolecular or lattice modes may be built is a useful approach. We are currently carrying out density functional theory calculations in periodic cells, to investigate both molecular and thin film interactions with the substrate and vibrational modes. We have also performed classical molecular dynamics simulations of RDX crystals and thin films. Those will be reported elsewhere. 4. Conclusions DFT calculations illustrate that differences between the vibrational spectra of the RDX AAE and AAA monomers in gas phase are mainly given by higher AAE monomer intensities for CH2 wagging and N-N bond stretching whereas the modes corresponding to CH2 rocking are blue-shifted in the AAA monomer. As intermolecular interactions are built (from monomer to dimer and tetramer), the largest differences are found in the terahertz and low IR (0-140 cm-1) region of the spectrum, and they are attributed to symmetric optical rotations between pair of molecules. Low frequency peaks, found in the range 10-20 cm-1 in the dimer and tetramer, as well as optical rotations and NO2 bending appear shifted toward higher intensities in the experimental crystal spectrum. Optical rotations in the range 30-55 cm-1 for dimer and tetramer agree fairly well with those of the crystal. In the RDX-Al16 complex, the RDX molecular geometry is closer to that of the AAA conformation, with the three NO2 groups in nearly axial positions. However, comparison of the spectra of the gas-phase AAA monomer, the Al16 surface and that of the complex yield significant differences. In-plane modes of Al16 are shifted to higher frequencies in the complex, and several new coupled modes (Al16-RDX) appear between 200 and 400 cm-1 in the complex. Other differences are revealed in the mid-IR region of the spectra, where the modes of the molecule attached to the model aluminum surface are red-shifted with respect to those in the free molecule because of the interactions of the NO2 group with the metal atoms. The comparison of the AAA-Al16 complex with the experimental results of ref 10 yields some similarities, and preliminary assignments are performed based on them; however, we note that more elaborate models are needed to achieve a better interpretation of the complex interactions that may exist between a polycrystalline RDX film and the metal surface. Acknowledgment. We gratefully acknowledge financial support from the US Army Research Office and the US Defense Threat Reduction Agency (DTRA). Computational resources

Guadarrama-Pe´rez et al. from TAMU Supercomputer Facility, University of Texas TACC, and NERSC are also acknowledged. Supporting Information Available: NBO population analysis of the RDX dimer. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Millar, D. I. A.; Oswald, I. D. H.; Francis, D. J.; Marshall, W. G.; Pulham, C. R.; Cumming, A. S. The crystal structure of beta-RDXsan elusive form of an explosive revealed. ChemComm 2009, 473, 562–564. (2) McCrone, W. C. Crystallographic Data. 32. RDX (Cyclotrimethylenetrinitramine). Anal. Chem. 2002, 22, 954–955. (3) Hakey, P.; Ouellette, W.; Zubieta, J.; Korter, T. Redetermination of cyclo-trimethylenetrinitramine. Acta Crystallogr., Sect. E: Struct. Rep. Online 2008, 64, o1428. (4) Ciezak, J. A.; Jenkins, T. A. The Low-Temperature High-Pressure Phase Diagram of Energetic Materials: I. Hexahydro-1,3,5-Trinitro-sTriazine. Propellants, ExplosiVes, Pyrotechnics 2008, 33, 390–395. (5) Davidson, A. J.; Oswald, I. D. H.; Francis, D. J.; Lennie, A. R.; Marshall, W. G.; Millar, D. I. A.; Pulham, C. R.; Warren, J. E.; Cumming, A. S. Explosives under pressure-the crystal structure of [gamma]-RDX as determined by high-pressure X-ray and neutron diffraction. CrystEngComm 2008, 10, 162–165. (6) Vladimiroff, T.; Rice, B. M. Reinvestigation of the Gas-Phase Structure of RDX Using Density Functional Theory Predictions of ElectronScattering Intensities. J. Phys. Chem. A 2002, 106, 10437–10443. (7) Karpowicz, R. J.; Brill, T. B. Comparison of the molecular structure of hexahydro-1,3,5-trinitro-s-triazine in the vapor, solution and solid phases. J. Phys. Chem. 1984, 88, 348–352. (8) Boyd, S. G.; Boyd, K. J. A computational analysis of the interaction of lattice and intramolecular vibrational modes in crystalline alpha-RDX. J. Chem. Phys. 2008, 129, 134502. (9) Boyd, S.; Gravelle, M.; Politzer, P. Nonreactive molecular dynamics force field for crystalline hexahydro-1,3,5-trinitro-1,3,5 triazine. J. Chem. Phys. 2006, 124, 104508–104510. (10) Melinger, J. S.; Laman, N.; Grischkowsky, D. The underlying terahertz vibrational spectrum of explosives solids. Appl. Phys. Lett. 2008, 93, 011102. (11) Rice, B. M.; Chabalowski, C. F. Ab Initio and Nonlocal Density Functional Study of 1,3,5-Trinitro-s-triazine (RDX) Conformers. J. Phys. Chem. A 1997, 101, 8720–8726. (12) Allis, D. G.; Zeitler, J. A.; Taday, P. F.; Korter, T. M. Theoretical analysis of the solid-state terahertz spectrum of the high explosive RDX. Chem. Phys. Lett. 2008, 463, 84–89. (13) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. ReV. A 1988, 38, 3098. (14) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648–5652. (15) 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. (16) Hariharan, P. C.; Pople, J. A. The influence of polarization functions on molecular orbital hydrogenation energies. Theor. Chem. Acc. 1973, 28, 213–222. (17) Gaussian, Inc. GaussView; 3.0 ed., 2003. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.02, Gaussian, Inc., Wallingford, CT, 2003. (19) Foster, J. P.; Weinhold, F. Natural hybrid orbitals. J. Am. Chem. Soc. 2002, 102, 7211–7218. (20) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1.

JP909976D

Aluminum

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