The synthesis and characterisation of halogen and nitro phenyl azide derivatives as highly energetic materials
2,4,6-tribromophenyl azide was synthesised as previously described and fully characterised using Infrared and Raman spectroscopy, elemental analysis, NMR spectroscopy (1H, 13C, 14N) and X-ray structural analysis. 2,4,6-tribromophenyl azide was recrystallised from ethanol to give pink needles which are monoclinic, with space group P21/n. The crystal packing diagram of 2,4,6-tribromophenyl azide shows that every terminal N(3) atom of the azide group has two intermolecular contacts with two bromine atoms of symetrically related molecules within the unit cell. The intermolecular distances are N(3) to Br(2)* (x – 0.5, -y-0.5, z-0.5) 3.605 Å and N(3) to Br(1)* (x + 1, y, z) 3.881 Å. These distances are both below the sum of the van der Waals radii of both atoms. The crystal packing diagram of 2,4,6-tribromophenyl azide is shown in the figure below. 2,6-diiodo-4-nitrophenyl azide was synthesised as previously described and fully characterised using Infrared and Raman spectroscopy, elemental analysis, NMR. spectroscopy (1H, 13C, 14N) and X-ray structural analysis. 2,6-diiodo-4-nitrophenyl azide was recrystallised from ethanol to give green needles which are monoclinic, with space group P21/c. There are two independent molecules in the asymmetric unit, one molecule has an ordered azide group and the other molecule has a disordered azide group. There is a most unusual intermolecular contact of 3.041 Å between O(11) and I(22) and between O(21) and I(12). This distance is well below the sum of the van der Waals radii of both atoms. The crystal packing is a chain with alternate ordered ands disordered molecules linked by this oxygen –iodine intermolecular contact (see figure below) The ab initio calculations of the vibrational frequencies for all of the halogen phenyl azides derivates were carried out at the self consistent HF level of theory using a 6-31G(d) basis set. Generally the agreement between calculated and experimentally observed (IR, Raman) frequencies is very good at HF/6-31G(d) level of theory for all derivatives prepared so that no scaling of the computed frequencies was necessary. The fact that the asymmetric azide vibration is calculated too high for 2,4,6-tribromophenyl azide, 2,4,6-trichlorophenyl azide 199 and 2,6-diiodo-4-nitrophenyl azide may or may not be explained by strong intermolecular interactions via the N3 group in these compounds as revealed by X-ray diffraction which due to the increased formal) charges on Nβ and Nγ weaken the terminal nitrogen-nitrogen triple bond. The thermal decomposition of three nitrophenyl azides, 1,3,5-(NO2)3-2,4,6-(N3)3-C6 (TNTA), 1,3-(NO2)2-2,4,6-(N3)3-C6H (DNTA) and 1,3,5-(NO2)3-2-(N3)-C6H2 (TNMA) was studied experimentally using gas-phase IR spectroscopy 1,3,5-(NO2)3-2,4,6-(N3)3-C6 →  6 CO + 6 N2 (1) 1,3-(NO2)2-2,4,6-(N3)3-C6H →  HCN + 4 CO + 5 N2 + C (2) 1,3,5-(NO2)3-2-(N3)-C6H2 →  2 HCN + 2 CO2 + NO2 + 3/2 N2 + 2 C (3) The combustion of 1,3.5-(NO2)3-2,4,6-(N3)3-C6 (TNTA) in an O2 atmosphere (two fold excess) yielded CO2, N2, very small amounts of NO2 and traces of N2O (eq. 4). 1,3,5-(NO2)3-2,4,6-(N3)3-C6 + 3 O2 →  6 CO2 + 6 N2 (4) In a further experiment exploring the potential of TATA as a solid fuel we mixed the material with the stoichiometric amount (cf. eq. (5)) of NH4NO3 1,3,5-(NO2)3-2,4,6-(N3)3-C6 + 6 NH4NO3 →  6 CO2 + 12 N2 12 H2O (5) The calculated enthalpy value of –908.9 kcal mol-1 makes the 1:6 molar mixture of 1,3-5- (NO2)3-2,4,6-(N3)3-C6 and NH4NO3 a very promising high energy density material (HEDM) the potential of which we are going to explore in more detail in future studies. In the drophammer testing of TNMA, DNTA and TNTA the order of the acoustic level is TNMA < DNTA < TNTA, but the values for DNTA and TNTA are very similar. Even the weakest of the investigated organic explosives (TNMA) is more powerful than AgN3 or Pb(N3)2, if the acoustic level is interpret as somewhat proportional to the detonation power. The calculated factor (F) and detonation velocities ( D) for TNMA, DNTA and TNTA were calculated using the Rothstein equation that calculates the detonation velocities of a variety of explosives on the basis of their molecular formulae. The calculated detonation velocities (D) are in agreement with the drophammer tests for these compounds i.e the values for detonation velocities (D) show TNMA < DNTA < TNTA, but the values for DNTA (9.185 mm µs-1) and TNTA (9.441 mm µs-1) are very similar. TNMA, DNTA and TNTA have higher calculated F factors and detonation velocities than several commercial explosives. In order to measure the 14N NMR spectrum of a pentazole, the pentazole was prepared in the NMR tube already in the probe. This was done by dissolving the diazonium salt in dichloromethane and placing this solution in the NMR tube and freezing it solid with liquid nitrogen and then the azide solution was added on top and also frozen solid, the NMR tube was then placed in the probe at –50°C. The two layers were allowed mix in the NMR tube as it obtained equilibrium at –50°C. The 14N NMR spectrum was recorded from the moment the tube was placed in the probe. In most of the spectra recorded using this method it was possible to see very small signals in the predicted region for the nitrogen atoms of the pentazole for about 30 minutes but again the strongest signals belong to the corresponding azide. The signals in the predicted pentazole region were not much more than the noise signal so we were not able to assign these with any confidence. However in the experiment using the above method for 2,4-dichlorophenylpentazole we were able to see three clearly distinctive signals in the expect range for the nitrogen atoms in a pentazole . This experiment was carried out several times and each time we observed the same spectra. As well as the three signals of the pentazole the are two signal of the 2,4-diclorophenyl azide present. The spectrum of this experiment can be seen in figure below. When the sample was allowed to warm up to room temperature the three signals for the pentazole disappear but the two signals of the azide remain. We can tentatively assign the signals in the spectrum of 2,4-dichlorophenylpentazole and 2,4 diclorophenyl azide as N1 ( δ = -176.0 ) ppm, N2 N5 ( δ = -72.0 ) ppm, N3 N4 ( δ = -24.0 ) ppm for the pentazole and Nβ ( δ = -135.0 ) ppm and Nγ ( δ = -141.0 ) ppm for the azide. Although the values we obtained for the 14N spectrum of the 2,4-dichloropenyl pentazole are shifted from the values previously reported, we are certain that the signals have been assigned correctly due to the fact that no other nitrogen containing substance could be present at –50°C and that the signals disappear as the temperature is raised which is in accordance with the pentazole decomposing to the azide. This is the first reported 14N NMR spectrum of a pentazole.
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