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Spin trapping of .NO2 radicals produced by uv photolysis of RDX, HMX, and nitroguanidine
JOURNAL
OF MAGNETIC
RESONANCE
52, 143-146 (1983)
Spin Trapping of - NO2 Radicals Produced by uv Photolysis of RDX, HMX, and Nitroguanidine M. D. PACE AND B. S. HOLMES Naval Research Laboratory. Code 6120, Washington, D.C. 20375 Received August 27, 1982; revised October 26, 1982
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 143
0022-2364183 $3.00 Copyright All
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0
1983 by
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Academic in any
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Inc. reserved.
144
NOTES
FIG. 1. (a) This first-derivative EPR spectrum was recorded during uv photolysis of the solution samples composed of nitramine (RDX, or HMX, or NQ) dissolved in DMSO-d,. (b) This first-derivative EPR spectrum was recorded during uv photolysis of a solution composed of RDX dissolved in DMSO solvent. (c) The spectrum in Fig. (1 b) was computer simulated. The simulation required four equivalent I = l/2 couplings and three I = 1 couplings (two of which were equivalent). Details are described in the text.
(not shown) from other free radicals were observed in addition to the spectrum in Fig. la. The EPR spectrum in Fig. 1a is attributed to a nitroxide free-radical designated I in Table 1. This free radical has been observed when gaseous - NO2 was bubbled through DMSO-d6 and simultaneously irradiated with uv light (5). The mechanism for the formation of I has been outlined in Ref. (5). It begins with a reaction of - NO2 and DMSO-d, . The reaction is suggested to proceed by the formation and subsequent reaction of methyl radicals. The decay of this nitroxide radical when the uv light is extinguished probably occurs by its reaction with another free radical (perhaps a methyl radical to form II in Table 1). When the same procedure was repeated by using DMSO as the solvent, a different result was observed. Samples of HMX and NQ did not give an EPR signal during the uv irradiation, but the RDX sample gave the EPR spectrum shown in Fig. 1b. This 3 l-line spectrum is complicated, but an assignment to the spectrum was made by matching the computer-simulated spectrum in Fig. lc with the experimental spectrum. The spectrum is assigned to three inequivalent hyperfme couplings. The largest coupling is assigned to an 14N hyper6ne coupling (I = 1) of 16.0 G and contributes a triplet splitting with each line of equal intensity. The second largest coupling (I = l/2) of 6.6 G is from four equivalent protons which contribute a quintet splitting. The intensity ratio of this quintet is 1:4:6:4:1. The smallest coupling is assigned to an 14N hyperhne coupling of 2.0 G from two equivalent nitrogens which contribute a quintet splitting with an intensity ratio of 1:2:3:2: 1. The free-radical spectrum in Fig. 1b is not attributed to an adduct radical. Rather, the structure is assigned to a fragment of the RDX molecule which
145
NOTES TABLE 1 FREE-RADICALANDMOLECULARSTRUCRJREASSIGNMENTS UPON EPR AND NMR b.WLTs
Designation
BASED
Structure
I
0 II
D,C -S ” -N -CD, II I 0 0 -CD,
“yNy III
.
“/I
H I’”
O,NONL~ON\~~e H’ ‘H
remains after loss of . NO2 from the ring. The possible structure of this radical is indicated as III in Table 1. A free radical with similar hyperlme couplings has been reported for uv-irradiated RDX in dioxane (7). In a second experiment, l-ml portions of the RDX or HMX solutions were uv irradiated and intermittently monitored by using 60-MHz proton NMR spectroscopy recorded with a JEOL FX60 Q NMR spectrometer. At intervals of 10 minutes each sample was removed from the uv light and a proton NMR spectrum of the sample was recorded. Figs. 2a to d show proton NMR spectra which were recorded for an 0.5% RDX solution (in DMSO-&, solvent). The NMR spectrum in Fig. 2a was recorded before uv irradiation. The peak at 2.50 ppm is assigned to the DMSO protons’ resonance (0.5% undeuterated DMSO which is present in the DMSO-d6). The peak at 3.31 ppm is assigned to water protons. The intense peak at 6.10 ppm is assigned to the ring protons on RDX. The short peak at 6.02 ppm is assigned to HMX ring protons. (The final product of the RDX synthesis contains several percent of HMX as an impurity.) After 110 min of uv irradiation, the RDX proton peak decreased to approximately the same intensity as the HMX proton peak (Fig. 2~). After 170 min of uv irradiation (Fig. 2d), almost no RDX proton signal was detected, but many new peaks were observed. This experiment was repeated by using an 0.5% HMX solution (in DMSO-d6 solvent). The results of this experiment were similar to the RDX results. Figs. 2a to d indicate that prolonged uv irradiation of the nitramine samples results in destruction of the ring structures and the formation of new products.
146
NOTES
FIG. 2. The proton NMR spectrum of a sample of RDX in DMSO-ds is shown: (a) before uv irradiation, (b) after 10 min of uv, (c) after 110 min of uv, (d) after 170 min of uv. The numbers indicated are ppm relative to DMSO at 2.50 ppm.
In Fig. 2d the disappearance of an RDX NMR signal before the HMX NMR signal provides evidence that RDX is more susceptible to damage by uv radiation than is HMX when these nitramines are dissolved in DMSO or DMSO-d, solvent. ACKNOWLEDGMENTS The authors thank Dr. A. D. Britt, Dr. A. Watterson, and Dr. B. Kalyanaraman cussions and Mr. D. R. Fat-tar for computer simulations of spectra.
for enlightening dis-
REFERENCES 1. 2. 3. 4. 5.
M. G. T. F. C.
D. PACE AND W. B. MONIZ, J. Magn. Reson. 47, 510 (1982). H. MYERS, W. C. EAXEY, AND B. A. ZILLES, J. Chem. Phys. 53, J. SCHAAFSMA AND J. KOMMANDEUR, Mol. Phys. 14, 525 (1968). J. ADRIAN, .I. Chern. Phys. 36, 1692 (1962). LAGERCRANTZ, Acta Chem. Stand. 23,3259 (1969).
6. W. Low, Phys. Rev. 105, 793 (1957). 7. C. DGREY, J. S. WILKES, L. P. DAVIS,
H. L. PIJGH,
W. R. CARPER,
118 1 (1970).
A. G. TURNER,
AND K.
E.
“Erogress Report on Research into Chemical Structure/Bonding Decomposition Relationships for Energetic Materials,‘* Frank J. !%ile.r Research Lab, Report FJSRL-TR-80-0026, 1980. SIEGENTHALER,
OF MAGNETIC
RESONANCE
52, 143-146 (1983)
Spin Trapping of - NO2 Radicals Produced by uv Photolysis of RDX, HMX, and Nitroguanidine M. D. PACE AND B. S. HOLMES Naval Research Laboratory. Code 6120, Washington, D.C. 20375 Received August 27, 1982; revised October 26, 1982
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 143
0022-2364183 $3.00 Copyright All
rights
0
1983 by
of reproduction
Academic in any
Press, form
Inc. reserved.
144
NOTES
FIG. 1. (a) This first-derivative EPR spectrum was recorded during uv photolysis of the solution samples composed of nitramine (RDX, or HMX, or NQ) dissolved in DMSO-d,. (b) This first-derivative EPR spectrum was recorded during uv photolysis of a solution composed of RDX dissolved in DMSO solvent. (c) The spectrum in Fig. (1 b) was computer simulated. The simulation required four equivalent I = l/2 couplings and three I = 1 couplings (two of which were equivalent). Details are described in the text.
(not shown) from other free radicals were observed in addition to the spectrum in Fig. la. The EPR spectrum in Fig. 1a is attributed to a nitroxide free-radical designated I in Table 1. This free radical has been observed when gaseous - NO2 was bubbled through DMSO-d6 and simultaneously irradiated with uv light (5). The mechanism for the formation of I has been outlined in Ref. (5). It begins with a reaction of - NO2 and DMSO-d, . The reaction is suggested to proceed by the formation and subsequent reaction of methyl radicals. The decay of this nitroxide radical when the uv light is extinguished probably occurs by its reaction with another free radical (perhaps a methyl radical to form II in Table 1). When the same procedure was repeated by using DMSO as the solvent, a different result was observed. Samples of HMX and NQ did not give an EPR signal during the uv irradiation, but the RDX sample gave the EPR spectrum shown in Fig. 1b. This 3 l-line spectrum is complicated, but an assignment to the spectrum was made by matching the computer-simulated spectrum in Fig. lc with the experimental spectrum. The spectrum is assigned to three inequivalent hyperfme couplings. The largest coupling is assigned to an 14N hyper6ne coupling (I = 1) of 16.0 G and contributes a triplet splitting with each line of equal intensity. The second largest coupling (I = l/2) of 6.6 G is from four equivalent protons which contribute a quintet splitting. The intensity ratio of this quintet is 1:4:6:4:1. The smallest coupling is assigned to an 14N hyperhne coupling of 2.0 G from two equivalent nitrogens which contribute a quintet splitting with an intensity ratio of 1:2:3:2: 1. The free-radical spectrum in Fig. 1b is not attributed to an adduct radical. Rather, the structure is assigned to a fragment of the RDX molecule which
145
NOTES TABLE 1 FREE-RADICALANDMOLECULARSTRUCRJREASSIGNMENTS UPON EPR AND NMR b.WLTs
Designation
BASED
Structure
I
0 II
D,C -S ” -N -CD, II I 0 0 -CD,
“yNy III
.
“/I
H I’”
O,NONL~ON\~~e H’ ‘H
remains after loss of . NO2 from the ring. The possible structure of this radical is indicated as III in Table 1. A free radical with similar hyperlme couplings has been reported for uv-irradiated RDX in dioxane (7). In a second experiment, l-ml portions of the RDX or HMX solutions were uv irradiated and intermittently monitored by using 60-MHz proton NMR spectroscopy recorded with a JEOL FX60 Q NMR spectrometer. At intervals of 10 minutes each sample was removed from the uv light and a proton NMR spectrum of the sample was recorded. Figs. 2a to d show proton NMR spectra which were recorded for an 0.5% RDX solution (in DMSO-&, solvent). The NMR spectrum in Fig. 2a was recorded before uv irradiation. The peak at 2.50 ppm is assigned to the DMSO protons’ resonance (0.5% undeuterated DMSO which is present in the DMSO-d6). The peak at 3.31 ppm is assigned to water protons. The intense peak at 6.10 ppm is assigned to the ring protons on RDX. The short peak at 6.02 ppm is assigned to HMX ring protons. (The final product of the RDX synthesis contains several percent of HMX as an impurity.) After 110 min of uv irradiation, the RDX proton peak decreased to approximately the same intensity as the HMX proton peak (Fig. 2~). After 170 min of uv irradiation (Fig. 2d), almost no RDX proton signal was detected, but many new peaks were observed. This experiment was repeated by using an 0.5% HMX solution (in DMSO-d6 solvent). The results of this experiment were similar to the RDX results. Figs. 2a to d indicate that prolonged uv irradiation of the nitramine samples results in destruction of the ring structures and the formation of new products.
146
NOTES
FIG. 2. The proton NMR spectrum of a sample of RDX in DMSO-ds is shown: (a) before uv irradiation, (b) after 10 min of uv, (c) after 110 min of uv, (d) after 170 min of uv. The numbers indicated are ppm relative to DMSO at 2.50 ppm.
In Fig. 2d the disappearance of an RDX NMR signal before the HMX NMR signal provides evidence that RDX is more susceptible to damage by uv radiation than is HMX when these nitramines are dissolved in DMSO or DMSO-d, solvent. ACKNOWLEDGMENTS The authors thank Dr. A. D. Britt, Dr. A. Watterson, and Dr. B. Kalyanaraman cussions and Mr. D. R. Fat-tar for computer simulations of spectra.
for enlightening dis-
REFERENCES 1. 2. 3. 4. 5.
M. G. T. F. C.
D. PACE AND W. B. MONIZ, J. Magn. Reson. 47, 510 (1982). H. MYERS, W. C. EAXEY, AND B. A. ZILLES, J. Chem. Phys. 53, J. SCHAAFSMA AND J. KOMMANDEUR, Mol. Phys. 14, 525 (1968). J. ADRIAN, .I. Chern. Phys. 36, 1692 (1962). LAGERCRANTZ, Acta Chem. Stand. 23,3259 (1969).
6. W. Low, Phys. Rev. 105, 793 (1957). 7. C. DGREY, J. S. WILKES, L. P. DAVIS,
H. L. PIJGH,
W. R. CARPER,
118 1 (1970).
A. G. TURNER,
AND K.
E.
“Erogress Report on Research into Chemical Structure/Bonding Decomposition Relationships for Energetic Materials,‘* Frank J. !%ile.r Research Lab, Report FJSRL-TR-80-0026, 1980. SIEGENTHALER,