J. Phys. Chem. 1995,99, 15785-15789
15785
FT-IR Spectra of Photochemical Reaction Products of Crystalline RDX Myungjin Choi and Hackjin Kim* Department of Chemistry, College of Natural Sciences, Chungnam National University, Taejon, 305-764, Korea
Chinkap Chung Department of Chemistry, College of Natural Sciences, Keimyung University, Taegu, 704-701, Korea Received: June 9, 1995@
Photochemical reactions of hexahydro-1,3,5-trinitro1,3,5+triazine (RDX) are studied using FT-IR spectroscopy. Small single crystal and crystalline powder pelleted with KBr are irradiated with UV light at liquid nitrogen temperature, and the reaction progress is monitored by taking IR spectra. The same fragments, N20, NO, CO2, and N204, are produced irrespective of the crystal size. The reaction products suggest that the N-N bond fission and the heterogeneous reactions such as the C-N bond breaking are major primary pathways and that the concerted depolymerization into CHzNN02 is not effective. Physical states of the fragments produced in different sample conditions are discussed based on the IR band shapes.
Introduction
Hexahydro-l,3,5-trinitro-l,3,5+triazine(RDX) is one of the best-known energetic materials. A lot of experimental and theoretical studies have been carried out for the physicochemical properties of RDX.' Different reaction mechanisms have been suggested for the decomposition of RDX, depending on the reaction types and the physical conditions of RDX;2-8the N-N bond rupture, the concerted depolymerization into CH2NN02, and other heterogeneous mechanisms. In the KrF excimer laser photolysis of the gaseous RDX,2 emission of NO2, a product of the N-N bond rupture, is observed. Nitrogen dioxide is also detected in the EPR study of the photochemical reactions of single crystal RDX.3 The concerted mechanism is confirmed in the IR multiphoton dissociation of RDX in a molecular beam where HCN, HONO, N20, and CH20 are formed in the secondary reactions of CH2NN02.4 Heating rate and ambient pressure affect the distribution of products in the thermal decomposition of RDX, and accompanying secondary reactions make it difficult to identify the primary reaction^.^ Meanwhile, the energy barrier of the concerted mechanism is even lower than the weakest N-N bond f i ~ s i o nmany ; ~ other heterogeneous reaction pathways contribute in the reactions of condensed phase RDX.6-8 Chemical reactions of solid organic molecules are quite different from reactions in fluid media.'O Many features of molecular solids should be studied for understanding of the solid state chemical reactions. These might include intra- and intermolecular energy transfer, roles of local inhomogeneities due to stress and defects, and microscopic mechanisms for the spatial propagation of chemical reactions. Molecular mechanical energy transfer in the solid state occurs via many intra- and intermolecular processes." The stress plays an important role in the shock induced reactions of energetic molecules.12 Physical and chemical defects are considered to be crucial in hot spot formations in the detonation of explosive mate1ia1s.l~ The spatial propagation of chemical reactions results from the combination of many complex processes.I4 Various spectroscopic techniques have been employed in the studies of physical and chemical processes of energetic materia l ~ . 'Studies ~ with X-ray photoelectron spectroscopy show that @
Abstract published in Advance ACS Abstracts, September 15, 1995.
impact sensitivities of energetic materials are related to electronic structures.'6 Time-resolved optical absorption spectroscopy is used to examine electronic and chemical changes of shock compressed explo~ives.'~Laser interferometric techniques are utilized to investigate the propagation of shock and chemical reactions in energetic materials.I8 Ultrafast cinematographic methods are employed to study the detonation proces~es.'~ Mechanical energy transfer processes in explosives are investigated using picosecond Raman techniques.20 The IR spectroscopy is useful in the identification of small fragments, and several IR studies of chemical reactions of RDX have been r e p ~ r t e d . ~Excitations , ~ ~ - ~ ~ of molecular vibrations are key steps in thermal reactions, and the vibrational band shapes give information about the local environments and intermolecular interactions in the condensed phase. The band shapes are related to the orientation of crystalline molecule^.^^^^^ The stress produced by chemical reactions in molecular crystals is reflected on the vibrational spectrum of products.25 The inhomogeneous broadening of the vibrational bands is attributed to dipole-dipole interactions in the molecular solids.26 Experimental Section RDX, supplied from Hanwha Corp., is purified by recrystallization with spectrograde acetone several times. Single crystals of a few millimeters dimension are grown from the saturated acetone solution by slow evaporation at room temperature. X-ray diffraction data of the grown crystals show good agreement with the known lattice parameters of a-phase RDX crystal.27 A whole beam of high pressure Hg lamp (Oriel Hg arc lamp, Model 6285) is used for chemical reactions of the RDX sample in a liquid nitrogen Dewar. The IR from the Hg lamp is removed through a water filter, and the light is slightly focused to the sample. A schematic diagram of the liquid nitrogen Dewar used in the experiment is shown in Figure 1. A shroud with KRS-5 and quartz windows is rotatable under vacuum. After irradiation with the W light, the Dewar is moved to a Bruker IFS-48 FT-IR spectrometer, and spectra of 0.5 cm-' resolution are taken. The sample chamber of the spectrometer is purged with dry air to reduce atmospheric CO;?. Two different types of samples are reacted in the experiments. A single crystal of about 0.5 mm dimension attached to a KBr window and powdered crystals of a few micrometers order
0022-3654/95/2099-15785$09.00/0 0 1995 American Chemical Society
15786 J. Phys. Chem., Vol. 99, No. 43, 1995
n
Choi et al. TABLE 1: Relative Intensities of Reaction Product9
N20 C02 NO N204
HCN
single crystal sample irradiation time
pelleted powder sample irradiation time
10 min
30 min'
10 min
30 min'
pyrolysisb
1.00
1.00 [3.58]
1.00
1.00 [3.34]
0.22 0.42 1.49 0.00
0.30 [1.06] 0.20 [0.73] 0.82 [2.94] 0.00 [0.00]
0.24 0.24 0.35 0.00
0.28 [0.95] 0.18 [0.60] 0.30 [1.01] 0.00 [0.00]
1.o 0.4 4.5 40.8 2.5
a Relative intensities when the intensity of NzO peak is 1.00. Reference 21. Numbers in brackets are relative intensities when the intensity of N20 at 10 min is 1.00.
IR window
sample
Figure 1. Schematic diagram of low-temperature Dewar used in the experiments.
before reaction
N20
-
after reaction small crystal
-
after reaction powder
1
the single crystal sample decomposes faster than the powder sample, distribution of the reaction products is not affected by the sample size. Relative band intensities of the reaction products are given in Table 1. The reaction products grow linearly with the irradiation time for both samples until more than 50% of RDX decomposes. The band shapes of the reaction products have not changed during the reaction progress and reproduced well for both samples. However, the spectra of Figure 2 are different from the IR spectra observed in the C02 laser pyrolysis of crystalline RDX.,] The band intensities of the pyrolysis products are compared in Table 1. While N204 and NO grow faster than N2O in the pyrolysis, the N20 band is the largest one in the photolysis. Conspicuous formation of N2O4 in the pyrolysis of RDX supports the N-N bond fission as the primary reaction.,' Hydrogen cyanide, which is one of major products in other reactions of RDX, is not detected in the photolysis. The C-H bands of RDX broaden with reaction progress, but no new band growth is detected in the region between 2800 and 3500 cm-I for both crystal and powder samples. Methylenenitramine (CH2NN02), which is identified only in the C02 laser pyrolysis of RDX in the molecular beam: is regarded as an important intermediate not only in the concerted mechanism but also in other reaction pathway^.^^^^ Two reaction pathways are available for the secondary reactions of energetic CH2NN02. CH,NNO,
-
HCN
-
CH2NN02 1700
1900
2100
2300
wavenumber [cm-'1
Figure 2. IR spectra of main products. No differences due to the sample size are found.
pelleted with KBr are prepared. While the single crystal sample is exposed to vacuum, microcrystals of the pelleted sample are embedded in the KBr matrix. Results and Discussion FT-IR Spectra of Reaction Products. Figure 2 shows the FT-IR spectra of two different RDX samples before and after irradiation of the UV light. The samples are reacted at 90 K. Before irradiation, both small crystal and pelleted powder sample give the identical spectra which show the characteristic vibrational bands of the a-phase RDX crystal.28 The spectra of Figure 2 are taken after the samples are irradiated for 35 min at the intensity of about 200 Wlcm2. Although the expanded spectra of the products reveal some differences due to the sample conditions, the spectra of the same overall features are obtained for both samples after irradiation. The sample size is critical in the shock induced chemistry of energetic materials.29 While
+ HONO
(1)
+N20
(2)
CH,O
As in other IR studies of RDX reaction^,^,*^%^* any bands which could be assigned to CH2NN02 are not observed, and only N20 among the products of (1) and (2) is detected. The branching ratio of (1) to (2) is measured as about 5 in the molecular beam study.4 The branching ratio of above reactions would not change much in the reactions of condensed phase RDX because the volumes of activation for (1) and (2) seem similar. While HONO is very reactive, HCN is stable in the UV photoly~is.~~ Therefore, HCN should be found if CHzNNO, is produced as a reaction intermediate. Absence of HCN suggests that N20 is formed through reaction pathways other than (2). The following reaction pathways are proposed based on the kinetic behaviors of the product growth in the W photolysis of the matrix isolated RDX.22 RDX
-
4N0
RDX
-
+ 2CH20 + N, + CH,
N,O
+ N,O,
-
+
+
4 N 0 2CO N, CH, H, (3)
+ + + CO, + C,H2 + N, + 2H,
(4)
The products of (3) are considered to be formed by secondary reactions following the N-N bond fission, and the products of
J. Phys. Chem., Vol. 99, No. 43, 1995 15787
Photochemical Reaction Products of Crystalline RDX (4) result from heterogeneous reactions involving the C-N bond rupture. However, the above reaction pathways do not explain the spectra of Figure 2. The absolute intensity of C0,32which has a vibrational frequency at 2170 cm-I, is about twice as large as the intensity of NO33so that the CO band of similar intensity as the NO band should appear if reaction 3 is effective. Absence of CO implies that other secondary reactions of CH20 and CH2 exist or that reaction 3 is not effective in the photolysis of crystalline RDX. Formation of N20 and C02 cannot be explained with only reaction 4 because the number density ratio of N2O/CO2 estimated from the relative band intensities of Table 1 is larger than 5 . The absolute intensity of the v3 mode of C0234 is about twice as large as the intensity of v3 mode of N20.35 The relative intensities of the products imply that the heterogeneous reaction pathways in the photochemical reaction of the crystalline RDX are not stoichiometric as in the reaction of the matrix isolated RDX. Studies of thermal decomposition of condensed phase RDX798 suggest several heterogeneous reaction pathways for N2O formation such as
RDX
-
+ H,CN + 2N,O + 2CH,O ONDNTA - N 2 0 + C H 2 0 + others -N 2 0 + C H 2 0 + NO2 + NH2CH0
-
NO2
(5)
(6) (7)
The intermediate of (6), l-nitroso-3,5-dinitrohexahydro-striazine (ONDNTA), is an addition product of NO to the radical formed by the N-N bond cleavage of RDX. Reaction 7 is a catalytic reaction by products of other reactions. Heterogeneous reactions such as (5)-(7) are found important in the thermal reaction of gaseous RDX at high pressure where lifetimes of unstable products decrease so that products of heterogeneous reactions like the C-N bond breaking in~rease.~ A consequence of heterogeneous reactions is enhanced in the photochemical reactions of crystalline RDX by similar mechanisms. As the NO2 bands are obscured with the RDX bands, it is difficult to c o n f i i the formation of NO2 through the IR spectra. Although NO2 is not identified for both crystal and powder samples, the N-N bond fission is still considered important in the photolysis of crystalline RDX. Nitrogen dioxide easily decomposes into NO and 0 atom by the UV light36 and can form a dimer, N204 in the crystalline RDX lattice.2' As the absolute intensity of the NO band32 is 1/10 of the intensity of the v3 mode of N20,35the concentration of NO estimated from the relative intensity of Table 1 is comparable or larger than that of N20. The reactive oxygen atom from the dissociation of NO2 is presumed to participate in many catalytic reactions. Formaldehyde involved in many reaction pathways photochemically decomposes into hydrogen and C0.37 The absence of CO might be explained by the reaction of CO with the atomic oxygen to form C02. The distribution of the reaction products discussed above suggests that the major pathways of the UV photochemical reaction of the crystalline RDX are the N-N bond fission and the heterogeneous catalytic reactions accompanying the C-N bond breaking to produce N20. The concerted depolymerization into three molecules of CH2NNO2 is excluded as in other reactions of condensed RDX. Photochemically activated fragments from the N-N bond fission are considered to accelerate the heterogeneous reactions so that contribution of the heterogeneous reactions increases in the UV photolysis of the crystalline RDX than in the C02 laser pyrolysis. Temperature Dependence of FT-IR Spectra. Figure 3 shows the expanded spectra of N20 and C02 in the small crystal
G
C
CG
2300
2350
2400
wavenumber [cm-'1
Figure 3. (a) Spectra of N20 and (b) COz at 90 K (solid line) and 210 K (dashed line). The products are initially formed in the small crystal sample at 90 K. The vibrational frequencies of the gaseous species and the transverse mode frequencies of the crystalline species are marked with G and C.
sample at 90 and 210 K. The fragments are initially produced at 90 K, and the reacted sample is heated slowly. The RDX bands show inhomogeneous broadening by chemical reactions at 90 K but have not changed during heat-up. The melting point of RDX is -200 OC, and the vapor pressure of RDX is below lo-' Torr at 210 K.38The reaction products evaporate with temperature rise so that the band intensities decrease at 210 K. Sometimes the crystalline RDX molecules evaporate and decompose in the gas phase when irradiated with IR or W light.21,39The spectra of the single crystal sample exposed to vacuum show only products of the solid state reactions. No significant changes in the band shapes of N20 and C02 during heat-up support that the products are originally formed inside the crystalline RDX at 90 K. Both bands of N20 and C02 reveal the asymmetric decrease during heat-up. The molecules of the high frequency side which evaporate faster are supposed to be near the crystal surface and have weaker interactions with host RDX molecules. As the vapor pressure of N2O is larger than C02,@ more N20 evaporates than C02 at high temperature. Figure 4 shows the expanded spectra of N20 and C02 at different temperatures, which are initially produced in the pelleted powder sample at 90 K. The reacted sample is heated to 240 K and then cooled to 90 K. Invariance of the band area after cooling indicates that the products have not escaped during heat-up. While the band shape of COShas not changed during the heating and cooling cycle, the band shape of N20 changes greatly. The initial band shape of N20 at 90 K is slightly different in each sample, but the identical band maximum and the tailing in the blue side of the spectra are observed for all pelleted samples. The N20 band shows red shift and converts into a single Lorentzian profile at 240 K. The Lorentzian band shape indicates that the N20 of the pelleted sample is in a single phase at high temperature. The IR spectra of N20 at temperatures from 90 to 200 K produced by thermal reaction of hydroxylamine hydrochloride in alkali halide matrices were rep~rted.~'An identical spectrum with the band of the cooled N20 of Figure 4a was observed at 90 K in ref 41 where the reversible transition of the band shape
15788 J. Phys. Chem., Vol. 99, No. 43, 1995
Choi et al.
G
C
I
1
I
2200 0.15
2250
2300
CG I
1
0 2300
2350
2400
wavenumber [cm-'1 Figure 4. (a) Spectra of N20 and (b) CO? at different temperatures. The molecules are initially produced in the pelleted powdered sample at 90 K (solid line) and heated to 240 K (dashed line) and recooled to 90 K (dotted line). The vibrational frequencies of the gaseous and crystalline species are marked as in Figure 3.
by temperature change was explained by partition change in different trapping sites of the matrix. The N2O of the pelleted sample is initially produced inside the RDX lattice and turns into gas confined in the KBr matrix with temperature rise. When the sample is recooled, the N20 gas forms clusters in different trapping sites of the KBr matrix. The N20 band shape at 240 K of Figure 4a is similar to the Raman spectra of dense N20 gas.42 The spectra of N20 gas at pressures ranging from 8 bar to 2 kbar and temperatures from room temperature to 100 "C were reported, and the fwhm of the v3 mode of N20 gas was measured as 8.52 cm-I at 298 K and 2 kbar. The resonance vibrational energy transfer model explains well the larger fwhm of N20 gas at lower temperature and higher pressure.42 The resonance energy transfer model states that the bandwidth insensitive to temperature is proportional to the number density. According to the model, the bandwidth at 240 K is estimated to be about 10%larger than at 298 K. Although the pressure of the N20 gas may not be quantitatively determined from the measured fwhm, 19.6 cm-', it is clear that the pressure of the N2O gas is greater than 2 kbar. The mode Griineisen parameter gives more quantitative idea about the gas pressure. Referring to the Griineisen parameter of the v3 mode of solid NzO?~the shift of 3 cm-' observed in the spectra of Figure 4a corresponds to about 8 kbar. This estimation seems reasonable because the v3 mode of N20 gas shifts by 1 cm-l at 2 kbar.42 Such high pressures are observed for the fragments of photochemical reaction of organic molecular crystal.25 High internal pressure is considered to play an important role in the dynamics of molecular solid^.^^,^^ The C02 of the pelleted sample transforms in a different manner. Despite the broadening of the wing side at high temperature, the band shape of C02 is not fit to a single Lorentzian profile. If C02 is a high-pressure gas like N20 at 240 K, dense C02 gas would show higher vibrational frequency than the normal gas because the Griineisen parameter of the v3 mode of C02 is positive.44 The band maximum of C02 at 240 K suggests that a major part of COz is in the condensed phase at 240 K. The phase diagram for C O Z shows ~ ~ that C02 is
2200
2250
2300
wavenumber [cm"] Figure 5. Normalized N20 band in the crystal (solid line) and powder (dashed line) sample at 90 K. The spectra are identical with the spectra of Figures 3a and 4a but normalized to give the same band area. The vibrational frequencies of the gaseous and crystalline species are marked as in Figure 3. solid under the pressure of a few kbar. The wing side broadening at 240 K and the band shape of the recooled sample indicate that some part of C02 is gas at 240 K. The intensity increase of the blue side in the recooled sample is due to the condensed C02 in the KBr matrix. The normalized N20 bands of the crystal and pelleted sample at 90 K are compared in Figure 5. The N20 band of the pelleted sample shifts to the blue side by 5 cm-I from the band of the crystal sample. A smaller blue shift of 2 cm-' is observed for C02. Although the photochemical reactions of RDX do not depend on the sample size, the band shift results from the effect of the crystal size. The photofragments at grain boundaries suffer stress more than the molecules inside crystal lattice, and the bands of N20 molecules under greater stress appear at higher vibrational frequencies. Since the powder sample has larger surface area than the single crystal sample, the band shift of the powder sample is considered to be due to the stressed molecules. The blue shift by the molecules near the crystal surface is consistent with the asymmetric decrease of the band intensity shown in Figure 3 where molecules of higher frequencies evaporate faster by heat-up. The absorbance difference of the bands of Figure 5 is related to the fraction of molecules located at the grain boundaries. About 22% of the normalized area shifts from the red side of the crystal sample to the blue side of the pelleted sample. When the crystal shapes of the crystal and powder samples are presumed to be spherical, the fraction of molecules under the surface stress corresponds to the ratio of the surface volume to the whole volume of the particle, [(4nR2d)/(4nR3/3)]. Here R is the radius of the particle, and d is the thickness of the stressed surface. Then, the ratio of the shifted band area is approximately given by
[ ( 4 ~ ~ R ~ d ) l ( 4 ~ R , 3-1 3[(4~R,~d)/(4nR,~/3)] )] = (3d/R,) (3dIRJ x 3dlR, where R, and R, are the crystal radius of the crystal and powder sample and R, >> d. Therefore, the molecules at the surface within about 7% of the powder crystal radius experience the surface stress. This estimation, which is rough but reasonable, confirms that the band shift is caused by the sample size. Acknowledgment. This work was supported by grants from the Korean Science and Engineering Foundation (Grant 930300-009-2) and the Basic Science Research Institute, Ministry of Education (BSRI-95-3432). References and Notes (1) Chemistry and Physics of Energetic Materials; Bulusu, S. N., Ed.; NATO AS1 Ser. C, Vol. 309; Kluwer: Dordrecht, 1990.
Photochemical Reaction Products of Crystalline RDX
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