ARTICLE pubs.acs.org/JPCA
Terahertz Spectroscopy and Solid-State Density Functional Theory Simulations of the Improvised Explosive Oxidizers Potassium Nitrate and Ammonium Nitrate Ewelina M. Witko, William D. Buchanan, and Timothy M. Korter* Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100, United States
bS Supporting Information ABSTRACT: Terahertz spectroscopy provides a noninvasive and nondestructive method for detecting and identifying concealed explosives. In this work, the room-temperature and cryogenic terahertz spectra of two common improvised explosive oxidizers, namely, potassium nitrate (KN) and ammonium nitrate (AN), are presented, along with detailed solid-state density functional theory (DFT) analyses of the crystalline structures and spectral features. At both 294 and 78 K, KN exhibits two terahertz absorption features below 100 cm1 that have been assigned through DFT simulations to arise from hindered nitrate rotations in the KN-II crystalline polymorph. The terahertz spectrum of AN exhibits a pronounced temperature dependence. The 294 K spectrum is free of any absorptions, whereas the 78 K spectrum consists of several narrow and intense peaks. The origin of this large difference is the polymorphic transition that occurs during cooling of AN, where room-temperature AN-IV is converted to AN-V at 255 K. The 78 K terahertz spectrum of AN is assigned here to various ion rotations and translations in the ANV polymorph lattice. The analysis of the room-temperature AN-IV terahertz spectrum proved to be more complicated. The solidstate DFT simulations predicted that the room-temperature crystal structure of AN is not very well described using the standard Pmmn space-group symmetry as previously believed. The AN-IV polymorph actually belongs to the Pmn21 space group, and the perceived Pmmn symmetry results from vibrational averaging through nitrate rotations. This newly observed Pmn21 crystal symmetry for room-temperature AN is the reason for the absence of absorption features in the 294 K terahertz spectrum of AN and provides new insight into the polymorphic transitions of this ionic solid.
1. INTRODUCTION Terahertz vibrational spectroscopy has attracted much attention as a means for the rapid detection and identification of solidstate explosives of various types, including military-grade,1 homemade,2,3 and related oxidizers.4 Terahertz frequency radiation (∼3200 cm1, 0.16 THz) is particularly attractive for security and defense applications because it is able to penetrate many common materials (paper, plastic, fabric) but still provide spectroscopic data with chemical accuracy. Terahertz spectroscopy probes low-frequency vibrational motions in crystalline samples such as optical rotations and translations that are strongly influenced by the three-dimensional arrangement of the molecules within the solid. Thus, molecular solids often exhibit unique characteristic terahertz spectra that can be used for the detection and identification of these materials. The sensitivity of terahertz spectroscopy to crystal structure can be utilized beyond chemical identification because this technique is also able to readily differentiate between crystalline polymorphs of the same compound. This particular aspect of terahertz spectroscopy is employed in the pharmaceutical industry for rapid nondestructive screening of drug polymorphs.58 Polymorphism is an important factor in the understanding of the solid-state r 2011 American Chemical Society
structures and behaviors of explosives and associated compounds, as different polymorphs exhibit varying physical properties (e.g., explosive yield)9,10 and the existence of multiple polymorphs can complicate the interpretation of terahertz spectra. Even relatively simple ionic compounds can exhibit remarkably diverse polymorphic flexibility. In this article, the structure and low-frequency vibrations of the solid-state oxidizers ammonium nitrate (NH4NO3) and potassium nitrate (KNO3) are investigated using time-domain pulsed terahertz spectroscopy and analyzed using solid-state density functional theory (DFT) simulations. Potassium nitrate (KN) and ammonium nitrate (AN) are of interest because of their strong oxidizing properties and common usage as fertilizers. This makes these nitrates easily accessible and therefore frequently utilized in improvised explosives, such as black powder (a mixture of KN, charcoal, and sulfur) and ammonium nitrate/fuel oil mixtures. The investigation of these materials using terahertz spectroscopy was the subject of a previous work;4 however, analyses of these spectra have never Received: August 5, 2011 Revised: September 30, 2011 Published: October 18, 2011 12410
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The Journal of Physical Chemistry A been reported, and the origins of the spectral features remain unknown. Vibrational analysis and assignment to specific atomic motions is made more challenging by the rich polymorphic nature of both of these nitrates. A comparison of the terahertz spectra of KN and AN can be enlightening given the similarities in their crystalline structures but their intrinsic difference in terms of lattice dynamics because of the increased degrees of vibrational freedom introduced by the NH4+ ion versus the K+ ion. Crystalline KN has been reported to have seven known polymorphs existing over a wide range of temperatures and pressures. The polymorphs existing at normal atmospheric pressure are identified as KN-I, KN-II, and KN-III or as β-KN, α-KN, and γ-KN, respectively.1119 At room temperature and from normal atmospheric pressure to approximately 3 kbar, KN exists as orthorhombic polymorph II (KN-II), which is used in this work. Upon heating to ∼401 K, KN undergoes a phase transition to a trigonal KN-I structure. When KN-I is cooled, it passes through a KN-III metaphase structure existing between 373 and 397 K, before transforming back into KN-II. Both the I and II polymorphs of KN exist at pressures up to 8 kbar. Another orthorhombic polymorph, KN-IV, exists over the widest temperature range (up to 700 K) and can be observed when the sample is exposed to a pressure range of 340 kbar.18,19 Three additional high-temperature and high-pressure polymorphs (V, VI, and VII) have also been reported.18,19 Similarly, there are at least five (an additional disputed polymorph has been reported2022) known structural polymorphs of AN existing in the temperature range of 72442 K.11,2327 At temperatures below 255 K, AN exists as polymorph V (AN-V). Warming of the material induces a phase change, and at room temperature (255305 K),11,23 polymorph IV (AN-IV) has been established as the dominant form. Heating the sample further begins to induce some disorder into the crystal structure, but three distinct high-temperature polymorphs have been reported. These high-temperature polymorphs include polymorphs III (305357 K),26 II (357398 K),28 and I (398442 K).25,29 The focus of the current work is on the low-frequency (90%) translational in each of the reported modes. The nitrate motions are considerably more complicated, showing large amounts of mixed translational and rotational character that varies across all of the investigated modes. The absorption at 45.0 cm1 in the experimental data is characterized by large contributions of in-plane rotations of the nitrates (ac crystallographic plane for the N1 nitrate and bc plane for the N2 nitrate). The next spectral feature at 49.4 cm1 has a large translational contribution from the N1 nitrates and in-plane N2 nitrate rotations. The 68.9 cm1 feature consists primarily of translational motions of the nitrate groups. The final spectral feature at 87.8 cm1 is due to translational motions of the N1-nitrate ions and in-plane rotations of the N2-nitrate ions.
4. CONCLUSIONS Terahertz spectroscopy and solid-state DFT modeling have been utilized in the analysis of the crystalline polymorphs of potassium nitrate and ammonium nitrate at 294 and 78 K. At both temperatures, the terahertz spectra of KN-II reveal two absorption peaks (58.6 and 75.3 cm1 at 294 K and 62.1 and 84.0 cm1 at 78 K). The DFT simulations of the KN crystal structure and lattice dynamics yielded excellent reproduction of the experimental observations, including the temperature dependence of the peak positions. The lowest-frequency terahertz spectral absorption is assigned to in-phase nitrate rotations about the b axis, and the other higher-energy peak originates from inphase nitrate rotations about the c axis. Unlike KN, AN undergoes a phase change within this experimental temperature range from AN-IV (294 K) to AN-V (78 K), 12416
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The Journal of Physical Chemistry A and the influence of this structural modification of the AN crystal is evident in the terahertz spectra. The experimental cryogenic spectrum shows a very distinct set of narrow absorption features, in great contrast with the featureless room-temperature spectrum. The observed vibrations of the low-temperature AN-V polymorph were readily assigned using the solid-state DFT results to external rotations and translations of the NH4+ and NO3 ions. The solid-state DFT simulations of the room-temperature AN-IV polymorph revealed that the generally accepted Pmmn space-group symmetry of AN at 294 K is not a complete representation of the crystalline structure. The Pmmn structure represents a vibrationally averaged structure that is based on a lower-symmetry and lower-energy Pmn21 crystal structure. The rapid vibrational averaging caused by in-plane nitrate rotations of the Pmn21 structure gives rise to the experimentally observed Pmmn structure and also to the apparently featureless terahertz spectrum of AN at 294 K. The existence of this previously undiscovered structure of room temperature AN will aid in the understanding of the extensive polymorphic nature of AN. Whereas room-temperature terahertz detection of AN is clearly hampered by the symmetry-enforced selection rules of the Pmn21 space group of AN-IV, even moderate cooling (such as thermoelectric cooling) of AN results in well-defined spectral features originating from the AN-V polymorph that could greatly enhance AN detectability with terahertz spectroscopic methods.
’ ASSOCIATED CONTENT
bS
Supporting Information. Complete lists of experimental and calculated heavy-atom bond lengths and bond angles for KNII, AN-IV, and AN-V. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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