Crystal Lattice Effects on the Orientation and Orbital Degeneracy of Nitric Oxide Trapped in Nitramine Single Crystals† Lev R. Ryzhkov*,‡ and John P. Toscano*,§
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 6 2066-2072
Departments of Chemistry, Towson University, Towson, Maryland 21252, and Johns Hopkins University, Baltimore, Maryland 21218 Received May 9, 2005
ABSTRACT: The orientation, structure, and location of nitric oxide (NO) generated by low-temperature photolysis in single crystals of two nitramines, RDX and HNIW, have been investigated by EPR spectroscopy. Following lowtemperature photolysis of single crystals of RDX and of the R- and β-polymorphs of HNIW, various NO species were observed after annealing up to 300 K. The orientation and orbital degeneracy of NO trapped in crystal lattices of these nitramines were inferred from its g tensor. These data illustrate the use of NO as a probe of reaction-generated stress in single crystals. In addition, analyses of R- and β-HNIW crystal packing and of the orientation and structural differences of NO trapped therein, were used to locate the NO within the R-HNIW crystal lattice, where it is shown to be trapped in intermolecular cavities and to survive storage for over 2 years at ambient temperature. Introduction Energetic materials such as RDX (1,3,5-trinitrohexahydro-s-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and HNIW (hexanitrohexaazaisowurtzitane or [2,4,6,8,10,12]-hexanitro-[2,4,6,8,10,12]hexaazatetracyclo[5.5.0.05,9.03,11]dodecane) have been extensively utilized as propellants and explosives in a wide variety of applications. Yet the detailed mecha-
nisms of their decomposition, particularly in the solid state (by far the most common state for their applications), are not well understood. Such mechanistic insights may prove useful for the rational design of better energetic substances and for understanding their degradative pathways in the environment.1 Obviously, the analysis of any mechanism requires a detailed understanding of the structure of intermediates involved. For processes that occur within a crystal lattice, this means not only identifying intermediates but also analyzing the influence of crystal packing and reaction-generated stress on their structure and reactivity. The orientation and spectral properties of intermediates and products in single crystals are highly dependent on these parameters, especially during the earliest stages of decomposition, while most of the lattice is still intact. This is especially true for photoinduced decomposition at very low temperatures. Nitric oxide (NO) and nitrogen dioxide (NO2) radicals are among the most common intermediates of both the thermal and photoinduced decomposition of nitramines, † Dedicated to Professor J. Michael McBride on the occasion of his 65th birthday. ‡ Towson University. § Johns Hopkins University.
in both the solution and the solid state. Here, we present a detailed analysis of the orientation and structure of NO radicals trapped in crystal lattices of two different nitramines, RDX and HNIW, as well as in two different polymorphs of the latter. As will be discussed below, NO is an ideal paramagnetic molecule for such a study. NO is detectable by EPR spectroscopy, but spectra are highly dependent on its orientation within the crystal lattice. Indeed, nonspinning, randomly oriented NO exhibits virtually no detectable EPR signal due to severe broadening, the consequence of very large g tensor anisotropy. In addition, the orbital degeneracy of NO is strongly influenced by its molecular environment and is shown to be sensitive to modulation of crystal lattice effects by reaction-generated stress. An additional advantage of studying NO in the crystal lattice of HNIW is that at least five polymorphs exist, two of which are conveniently prepared. Analysis of the NO EPR spectra, along with the similarities and differences between the packing and structure of HNIW in these two polymorphs, can assist in locating the NO intermediates within the crystal lattice, one of the more difficult aspects of mechanistic studies in the solid state. Experimental Section Materials. Caution! Although RDX and HNIW are not primary explosives, they are powerful ones and should be treated with extreme care. Crystals used in this work weighed less than 2 mg. RDX and HNIW were received from ARDECDover and NSWC-White Oak, respectively, and used for crystal growth without further purification. All solvents used in this study were obtained from commercial suppliers and used as received. K15NO3 (99% 15N) was obtained from Cambridge Isotope Laboratories. Concentrated sulfuric acid was obtained from Mallinckrodt. Fully labeled samples were prepared by exchange with 15NO2+ from two different sources: a H2SO4/ H15NO3 mixture in 65% (v/v) aqueous H2SO4 or a CF3SO3H/ H15NO3 mixture2 in CH3NO2. As described previously,3 the material obtained by the former method was used to prepare the hydrated R-polymorph, while the latter yielded HNIW suitable for growing the β-polymorph. RDX crystals were grown by slow evaporation from acetone. HNIW crystal growth
10.1021/cg050209u CCC: $30.25 © 2005 American Chemical Society Published on Web 09/30/2005
Nitric Oxide Trapped in Nitramine Single Crystals
Crystal Growth & Design, Vol. 5, No. 6, 2005 2067
Figure 1. Unpaired NO2 and NO radicals in RDX after photolysis at 20 K and annealing to temperatures between 165 and 300 K. All spectra were recorded at approximately 20 K with the crystallographic c axis oriented parallel to the applied field. Note that at this particular orientation the 240 K NO appears at higher field than the 260 K NO; however, the extreme g shift (observed at other crystal orientations) is larger for the 260 K NO, as reflected in its lower gzz value (Table 1). has been described in detail previously.3 To incorporate NO into unphotolyzed HNIW crystals, HNIW was dissolved in a deaerated methanol/acetone solution and the solution was then saturated with NO gas (Matheson). During crystal growth a slow stream of NO gas was maintained over the solution surface. EPR analysis of the resultant crystals showed no trace of NO2 signals. Instrumental Analysis. Low-resolution thermal desorption mass spectra were measured on a Hewlett-Packard 5898A mass spectrometer equipped with a direct ionization probe. All 1H and 13C NMR spectra were acquired on either a General Electric QE Plus (300 MHz) or Bruker AM500 spectrometer. 15 N NMR spectra were recorded at 50.7 MHz on a Bruker AM500 spectrometer equipped with a broad-band probe and a General Radio 1061 RF synthesizer. Natural-abundance 15NDMF in DMSO-d6 was used as the reference and locking solvent in a coaxial 10 mm NMR tube. EPR Spectroscopy. EPR spectra were acquired on a Varian Associates E-9 spectrometer at about 9.3 GHz. The spectrometer was interfaced to a Macintosh IIci personal computer equipped with a National Instruments LAB-NB I/O board. All spectra were digitized with 4K points, each of which was the average of 1300 measurements, requiring a total scan time of 7 min. Field sweep and spectral acquisition were controlled by LabView II software (National Instruments), and the spectra were processed with PRISM software.4 A commercially prepared sample of Cr3+ doped into MgO (g ) 1.9797)5 was used to calibrate g factors. Typically NO spectra
were acquired at 15-20 K with 0.5-1 G modulation amplitude, 5-10 mW microwave power, 1 s filter, and 200 G sweep width. NO2 spectra were acquired at 75-80 K with 2 G modulation amplitude, 5-15 mW microwave power, 1 s filter, and 200 G sweep width. Studies of signal intensities at different applied microwave power indicated that neither NO nor NO2 signals were saturated under these conditions. The crystals were cooled by a continuous flow of helium using Air Products Heli-Tran and Oxford ESR-900 cryostats. The crystal temperature was monitored by two Au/FeChromel thermocouples positioned 2-2.5 cm above and below the crystal. At low temperatures (
