Spin trapping of .NO 2 radicals produced by uv photolysis of RDX, HMX, and nitroguanidine

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In a recent study, evidence was reported of aNO* free-radical formation in single crystals of RDX (I). The . NOz free radicals were produced in the RDX single crystals by uv photolysis at 77 K, but the EPR signal decayed when the single crystals were warmed to room temperature. Similar observations have been reported for . NO* free radicals which are trapped in a frozen matrix (2-4). Such evidence suggests that uv photolysis of the nitramines nitroguanidine [NQ; (H2N)&NN02], RDX [C3H6N606], and HMX [C4HsNsOs] can produce free radicals which rapidly decay before being detected by cw EPR spectroscopy. In this investigation we have employed the spin-trapping capacity of DMSO and DMSO-d, to indirectly detect -NO* free radicals which are generated from NQ, RDX, and HMX by uv radiation at room temperature (5). Nitramine solution samples were prepared by adding either DMSO or DMSO-d6 separately to milligram amounts of NQ, or RDX, or HMX. Typically 1 to 10 milligrams of RDX or HMX or 20 to 30 milligrams of nitroguanidine was dissolved into 1 gram of DMSO or DMSO-d,. (A greater quantity of nitroguanidine was necessary to produce a detectable EPR signal.) A l-ml portion of the nitramine solution was pipetted into a flat cell which was inserted into the EPR TM, r0 cavity. Broadband uv photolysis of the samples in the EPR cavity was accomplished by using a 600watt mercury-xenon lamp. EPR spectra and changes in the NMR spectra were observed only with samples containing DMSO or DMSO-ds solvent plus nitramine. The EPR spectra were recorded by using a Bruker ER 200 D spectrometer. A sample of Mn2+ was used as a g-value reference standard (6). In the first experiment, a sample of HMX in DMSO-ds (0.5% by weight) was uv irradiated in the EPR cavity and the spectrum shown in Fig. la was recorded. This EPR spectrum has 21 lines with unequal intensities. An 14N hyperfine coupling of 12.2 G, a smaller hyperhne coupling of 1.7 G, and a g value of 2.001 (4) are assigned to the spectrum. The intensity ratios for the seven lines of each triplet are 1:3:6:7:6:3: 1. This is consistent with the assignment of the smaller coupling to three equivalent deuterium atoms (I = 1). The EPR signal was detected within 2 minutes after beginning uv irradiation and persisted until the uv light was extinguished. The same experiment was repeated with an 0.5% solution of RDX in DMSO-ds and with an 0.25% solution of NQ in DMSO-d6. Both samples gave the same EPR pattern as shown in Fig. la. When the gain setting was increased by 10X, weak EPR signals