Detection of HexamethyleneTriperoxide Diamine by Nitrogen-14 Magnetic Resonance

BACKGROUND OF INVENTION

[0001] Hexamethylenetriperoxide diamine (HMTD) (3,4,8,9,12,13-hexaoxa- 1 ,6-diazabicyclo[4.4.4]tetradecane) is a very unstable explosive compound that can be made from hexamethylenetetramine, hydrogen peroxide, and citric acid. It has the structure:

CH ₂ -O-O-CH ₂

/ \.

N-CH ₂ -O-O-CH ₂ -N ^X CH ₂ -O-O-CH ₂

HMTD is a high explosive organic chemical compound, first synthesised in 1885 by Legler. The explosive material languished in relative obscurity for a hundred years, until its ease of preparation and stability relative to other organic peroxides attracted the interest of terrorists, most notably the perpetrators of the London bombings of 2005. HMTD is an interesting molecule to the physical chemist: it is helically chiral in solution but crystallizes with disorder [Wierzbicki A, Cioffi EA. J. Phys. Chern. A 1999; 103: 8890-8894] in an achiral space group [Schaefer WP, Fourkas JT, Tiemann BG. J. Am. Chern. Soc. 1985; 107: 2461-2463]; and it possesses a comparatively planarized tertiary nitrogen, largely a result of the electron- withdrawing character of the peroxide groups ringing its molecular equator.

[0002] Its structure lent itself well to acting as an initiating or primary explosive. While still quite sensitive to shock and friction, it was a relatively stable compared to other initiating explosives of the time, such as mercury fulminate, and proved to be relatively inexpensive and easy to synthesize. As such, it was quickly taken up as a primary explosive in mining applications. However, it has since been superseded by even more stable compounds such as tetryl.

[0003] Despite no longer being used in any official application, it remains a fairly popular home-made explosive and has been used in a large number of suicide bombings throughout the world. The New York Times reported it as the planned explosive in the 2006 transatlantic aircraft plot. HMTD is easily synthesized from unrestricted ingredients including hydrogen peroxide, citric acid or dilute sulfuric acid as a catalyst and hexamine fuel tablets. Like other organic peroxides such as acetone peroxide, HMTD is an unstable compound that is sensitive to shock, friction, and heat. This makes the substance extremely dangerous to manufacture. It also reacts

with most common metals, which can lead to detonation. HMTD degrades too quickly for modern commercial and industrial applications, becoming useless.

[0004] With this explosive it is also possible to create blasting caps. Its primary shock wave will detonate the base charge in the caps. The base charge of the cap is normally RDX. or some other high explosive. The base charge needs to be powerful and stable, but still sensitive to the primary detonation wave. The 6700 M/sec. plus base charge detonation velocity of HMTD, will set off the main charge. As such, HMTD is one of the better initiating explosives but has some definite drawbacks. HMTD is not stable at even slightly elevated temperatures. Room temperature will even cause a decrease in performance with storage time. As one would imagine, due to the extreme excess of oxygen, the corrosion of metals in contact with the peroxide is a problem. The metals that will cause problems are aluminum, zinc, antimony, brass, copper, lead and iron. These metals in contact with the HMTD even when dry, will cause corrosion.

[0005] Analysis schemes to identify HMTD explosive have been reported (Reutter, D. J., Bender, E. C, and Rudolph, T. L. Analysis of an unusual explosive: Methods used and conclusions drawn from two cases. In: Proceedings of the International Symposium on the Forensic Aspects of Explosives Analysis. U.S. Government Printing Office, Washington, DC, 1983, pp. 149-158; Zitrin, S., Kraus, S., and Glattstein, B. Identification of two rare explosives. In: Proceedings of the International Symposium on the Forensic Aspects of Explosives Analysis. U.S. Government Printing Office, Washington, DC, 1983, pp.137-141), but in this case it was necessary to identify each precursor. Because peroxides are highly corrosive, care was taken to use a method that would not damage instruments during the chemical analysis. Infrared (IR) and Raman spectrometry techniques have also been used to detect HMTD. Other methods for the detection of HMTD are disclosed in U.S. patents Nos. 7,129,482; 6,773,674; 6,767,717; 6,406,918; 5,648,636; 5,175,230 and 4,591,645.

[0006] Nuclear quadrupole resonance (NQR) is an explosives detection method based on nitrogen quadrupole detection. A weak radio frequency (RF) signal NQR is detected from the quadrupole nuclei present in the explosive material. NQR is sometimes referred to simply as quadrupole resonance (QR.)

[0007] When quadrupole nuclei are exposed to a pulsed RF field, they move to a higher energy state. Upon removal of the RF field, the nuclei return to their

original lower energy state and the excess energy is released. The released energy is of a characteristic energy, which is dependent upon atom type and crystal structure. The advantages of NQR include the following: (1) no ionizing radiation source is used; (2) it is highly specific for the identification of explosive compounds; (3) there is little interference from other nitrogen-containing materials that may also be present; (4) there is a very low false alarm rate, and the probability of detection for a given explosive mass is shape independent.

[0008] Nuclear quadrupole resonance -NQR [ J. A. S. Smith, "Nuclear Quadrupole Resonance Spectroscopy, General Principles," Chemical Education, No. 48, 1971, pp. 3949; Y. K. Lee, "Spin-1 Nuclear Quadrupole Resonance Theory with Comparisons to Nuclear Magnetic Resonance," Concepts in Magnetic Resonance, No. 14,2002, pp. 155-171] is a magnetic resonance phenomenon related to NMR and its offspring, MRI. In NMR and MRI, a large static magnetic field (0.05-20.00 T, 0.5- 200.0 kG) orients the nuclei so that slightly more are in the low energy state (aligned parallel to the static field) than are in the higher state (opposed to the field). This population difference corresponds to a weak diamagnetism of the nuclear spins, with a classical magnetization vector aligned along the static magnetic field. The magnetic field corresponding to this nuclear diamagnetism can be observed by applying a resonant radio frequency (RF) pulse (at the Larmor frequency and at right angles to the static field), causing the magnetization to rotate away from the axis of the static magnetic field. The magnetization then precesses freely in the static field, at the Larmor frequency, and this time-dependent flux induces a weak voltage in an RF pickup coil perpendicular to the static field.This induced signal is the NMR signal.

[0009] NQR is similar to NMR but has some important distinctions. In NQR, the splitting of the nuclear spin states is determined by the electrostatic interaction of the nuclear charge density, p(r), with the external electric potential, V(r), of the surrounding electron cloud. A moment expansion of this electrostatic interaction shows that the important coupling is between the nuclear quadrupole moment, and the second derivative of the electric potential (equivalently, the gradient of the electric field). This is a key result. The quadrupole moment, nonzero only for nuclei with spin quantum number I greater than or equal to 1 , is a nuclear physics parameter describing the distribution of charge in the nucleus. The second term, the coupling to the electric field gradient of the valence electrons is largely based on chemistry, although the local crystal packing also plays a role.

[0010] Contrasting the chemical specificity of NQR with that of NMR, while NMR provides highly detailed information about chemical structure, the range of "chemical shifts" is generally small. Hydrogens in any arbitrary organic structure differ by a range of about 10 ppm away from their nominal NMR frequency, e.g., a 6- kHz range of frequencies in a 600-MHz NMR spectrometer. However, for 14N NQR, the NQR frequencies can range from zero to 6 MHz, depending on the symmetry of the molecule. Indeed, one of the difficulties of NQR is that it can be too sensitive to the chemistry of the compound of interest.

[0011] There are also some significant subtleties in NQR compared with NMR: For NQR the nuclear spin is greater than or equal to 1 , and the spins are quantized along the principal axis system of the electric field gradient, rather than for the NMR or MRI case that (commonly) involves spin-1/2 nuclei quantized along the static magnetic field.

[0012] It suffices to regard NQR as NMR without the magnet.

[0013] One significant advantage of NQR is the absence of a magnet: Even if the NMR approach were thought to give some advantage to detecting explosives, projecting a large static magnetic field is difficult. But the main advantage is that NQR provides a highly specific and arguably unique frequency signature for the material of interest.

[0014] Not all explosives exhibit an NQR signal. Both the NQR frequency and the relevant NQR relaxation times Tl and T2 are functions of temperature. The relaxation time Tl determines how rapidly the pulse sequence can be repeated, and T2 restricts the maximum length of the "spin echo" sequence used for detection.

[0015] Other approaches include combined NQR-NMR methods. For these a weak polarizing magnetic field is used, on the order of 10-100 G. Variants allow a cross relaxation between the nitrogen transitions and hydrogen NMR transitions, to either decrease the nitrogen Tl relaxation time or to detect the nitrogen NQR transition as a perturbation on the much stronger proton NMR signal. These techniques have been explored to some extent in the laboratory for the past 50 years [Nolte et al., "1H14N Cross-Relaxation in Trinitrotoluene: A Step Toward Improved Landmine Detection,", Phys. D: Appl. Phys., No. 35, 2002,pp.939-942].

[0016] NQR systems for bulk explosives detection are commercially available, e.g., from Quantum Magnetics Inc. (QR500).

SUMMARY OF THE INVENTION

[0017] It is an object of the invention to provide efficient and safe methods and systems for the detection of HMTD utilizing NMR and NQR detection techniques.

DETAILED DESCRIPTION OF THE INVENTION [0018] The present invention is predicated on the discovery, using high-field NMR of the magnitude of the nuclear quadrupole interaction in HMTD, and that NQR may be employed for the facile detection thereof.

[0019] The invention will be illustrated by the following non-limiting examples wherein: HMTD was prepared by standard methods; NMR experiments were conducted at 14T in a Bruker Avance spectrometer equipped with high power amplifiers for solids. The probe had a 1.1 cm diameter X 1 cm length coil, tunable over the range 35 - 90 MHz. The ¹⁴ N π/2 pulse, measured for ¹⁴ NH ₄ Cl at the Zeeman frequency, was 8 μs; because the NMR signal of HMTD was expected greatly to exceed the bandwidth of the instrument, and thus only single transitions excited, an effective single transition π/2 pulse of (8 μs/V2) ~ 5.7 μs was used in all experiments. Although there are no reports that high radiofrequncy fields can induce detonation of this material, to guard against this possibility, the NMR probe, with the screws on the top removed to allow it to blow out, was placed behind an explosion-proof screen, and the sample subjected to 30 μs pulses, once per second for 10 minutes, at a power level 2 dB above operating levels, before it was inserted into the NMR magnet. A Hahn echo sequence (π /2 - τ- π - τ - ) was employed, with τ delays of 80 ms and delays between acquistions of 8 s; typically 1000 transients were averaged to obtain a spin echo. Echoes were obtained every 200 kHz over two regions, each spanning 1 MHz, at 1.9 MHz above and below the Zeeman frequency, where computations suggested the perpendicular edges of the Pake doublet were likely to lie. Once the peak intensities of the Pake doublet were located, higher-quality spin echo spectra were obtained centered at or near the estimated frequency.

[0020] Fig. 1 shows the results of the echo sweep. Two pronounced maxima are apparent; the lower maximum, at 41.36 MHz, is less intense than the upper maximum at 45.36 MHz, partly because of the relative Boltzmann polarizations and detection efficiencies of the two transitions, but more importantly because of the better coincidence of the upper transition maximum with the Pake doublet

perpendicular edge (see below). Such frequency sweep measurements are necessarily coarse-grained and limited in accuracy. The spectra shown in Fig. 2 (a) are Fourier transforms of spin echo FIDs acquired at frequencies of 41.406241 MHz and 45.406241 MHz, respectively. These spectra allow careful examination of the Pake doublet perpendicular edge, and they were fit assuming an axially symmetric quadrupolar coupling tensor to second order in perturbation theory, using a finite pulse width correction for the spin-echo response using the measured 5.7 μs π/2 pulse length; fit parameters were C _Q = 5.3346 ± 0.001 MHz, T ₂ ^* = 177 μs , and σ* (offset of the HMTD perpendicular edge relative to NH ₄ Cl, presumably as a result of chemical shielding tensor of HMTD) = +0.002 ± 0.001 MHz The simulations are quite satisfactory, but the experimental spectra appear to contain an approximately 10 KHz additional frequency dispersion, manifest as a rather flat topped lineshape which is not accounted for in the simulation; the most probable explanation for this is a dispersion in quadrupole constants due to the known crystallographic disorder. Using a ¹⁴ N quadrupole moment eQ of 2.05 ± 0.02 fm ² [Cummins, P.L.; Bacskay, G.B.; Hush, N.S.; Ahlrichs, R. J. Chem. Phys. (1987) 86, 6908] and the conversion factor KQ = 2349647.8 Hartree-lBohr2fm-2s-l, the zz element of the EFG tensor in atomic units (Hartree Bohr-2) is given by Vzz = CQ/(eQKQ) = 1.107 ± 0.01 a.u.

[0021] HMTD calculations used the optimized D ₃ structure published previously: In order to examine the effects of larger basis sets and/or higher levels of correlation, EFGs for trimethylamine (TMA) optimized at the B3LYP/6- 311++G(2d,p) level were also examined; TMA optimized at the same level with the nitrogen constrained to 20 pm above the plane containing the carbon atoms (thus mimicking the angles around the nitrogen in HMTD; denoted by the abbreviation TMA20); and ammonia. Calculations used the augmented, correlation consistent polarized valence multiple zeta (aug-cc-pV/ϊZ) basis sets of Dunning and coworkers [Legler, L. Ber. Dtsch. Chern. Ges. 1885; 18: 3343-3351]; unlike the Pople basis sets, these allow a systematic comparison of EFGs calculated at different levels of theory, converging to the complete basis set (CBS) result. Electron correlation was introduced using the MP2 second order perturbation theory method, as well as (where possible) coupled cluster singles/doubles methods (CCSD), the latter accounts very accurately for the effect of correlation on EFGs. DFT calculations using the B3LYP functional are provided for purposes of comparison.

[0022] Unfortunately, coupled cluster calculations were not feasible for HMTD itself, but were run at the valence double zeta (aug-cc-pVDZ) level for TMA and TMA20, and at the aug-cc-pVDZ and valence triple zeta (aug-cc-pVTZ) levels for ammonia.

[0023] Increasing the basis set size increased the EFG at nitrogen for all molecules at all levels of correlation. Unfortunately, the three Hartree-Fock calculations of HMTD are insufficient to do a CBS extrapolation. We were however able to obtain CBS extrapolations for the Hartree-Fock results for TMA, TMA-20 and NH ₃ . The CBS result exceeds the aug-cc-pVTZ value by. It seems therefore reasonable to conclude the CBS result for HMTD exceeds the CBS result by XXX.

[0024] While this is not the case for smaller basis sets, results for TMA and ammonia indicated that basis sets at the aug-cc-pVTZ level, corrections for basis set incompleteness and electron correlation were largely additive. The difference between CCSD and MP2 EFGs fall in the range 0.0428 - 0.0469 with the lowest values being for ammonia.

The (CCSD -MP2) efg also appeared to be almost independent of basis set size. An average of the four computations for which both CCSD and Mp2 calculations were available were used to estimate VzzCCSD - VCzz,Mp2 = XXX.

[0025] Using these two corrections, a CCSD/CBS value was estimated for HMTD of 1.11 , and YYY for TMA. These in both cases exceed the experiemtnal value by about 4%; vibrational averaging, particularly by the umbrella mode along the molecular C3 axis, probably accounts for the difference. If the CCSD calculations are taken as the 'gold standard' for such calculations, MP2 EFG calculations lie below the CCSD value by 0.0428 - 0.0469 (average 0.0444). Starting from the best electron correlated calculations for HMTD (MP2/aug-cc-pVTZ; ), and applying estimates for basis set incompleteness of (0.0235 + 0.0012), and for CCSD of 0.0444, a best estimate for the CCSD/aug-cc-V5Z field gradient of HMTD is obtained of 1.157 ± 0.02 a.u.. This is in fair agreement with the experimental value; the 4% reduction in the experimental value may be largely a result of librational/vibrational averging in the solid state. For comparison, the same corrections applied to TMA (MP2/aug-cc- pVTZ) yield Vzz= 1.134 + (0.0235 + 0.0012) + 0.0444 = 1.203 a.u., compared to the gas phase value of , computed from the computed experimental CQ as described above. See tables below:

Tables

[0026] Nuclear Quadropole Resonance (NQR) is the examination of nuclei utilizing radio frequency electronics and a computer system. The peroxide explosive,

HMTD, used in improvised explosive devices, contains nitrogen atoms which have one of the largest, if not the largest nitrogen 14 (14N) nuclear quadrupole interaction (NQI) yet recorded. The NQI is an interaction of the atomic nucleus with its molecular environment. Because of this unusually high NQI, HMTD has a unique nuclear magnetic resonance frequency signature at 4 Megahertz, which can be detected by nuclear magnetic resonance spectrometers such as are being presently used to detect the TNT in land mines and the like, but at a much higher frequency and thus with greater sensitivity.

[0027] NQR spectrometers are presently commercially available or can be built cheaply from commonly available electronic components. The frequency signature of HMTD can be detected at a distance by its frequency signature, and thus inexpensive remote screening detectors can be built to detect IEDs containing this substance.

[0028] The main drawback of NMR/NQR detection is poor sensitivity, but the intrinsic sensitivity of this substance is quite high, as it has favorable NMR properties (high NQR frequency, short relaxation time). Until now, no one knew what the NQR frequency of HMTD was, or had detected its frequency signature in practice. This value was calculated using quantum mechanical methods to predict where it would lie, and then successfully measured.

[0029] The experimental quadrupolar coupling constant, 5.334 MHz, is in excellent agreement with quantum chemical calculations. The predicted single zero- field transition frequency should lie in an almost empty region of the 14N NQR spectrum; the spin relaxation rate is reasonably fast.