J. Phys. Chem. 1986,90, 2679-2682
-a
1
I
-20 T+
G
1
Figure 3. Orbital energies (in eV) of isolated TMA and TMA radical cation, as well as the radical cation of the TMA dimer at the optimized geometry (rN-N = 2.8 A).
value of 3.0 eV.I9 In order to rationalize the apparent selectivity in forming radical cation dimers, we performed calculations on an electron-deficient pair of T M A molecules. In the first case (I), both amines were constrained to be pyramidal in order to resemble the N bridgehead in ABCO.ZO In the second case (11), one of the TMA molecules was held planar in order to reflect the structural characteristics of the TEA radical cation, as determined from calculations. The results of the calculation indeed support the hypothesis that the formation of the radical cation dimer is energetically preferred in case I relative to case 11. A bound dimer with a separation of 2.8 is identified in case I, while in case 11, only a shallow minimum appears a t ca. 3.6 A. Presumably steric interference by the H atoms in the planar amine radical cation destabilizes the dimer relative to case I. If carried over (19) Alder, R. W.; Arrowsmith, R. J.; Casson, A.; Sessions, R. B.; Heilbronner, E.;Kovac, B.; Huber, H.; Taagepera, M. J. Am. Chem. Soc. 1981, 103,6142. (20) For the dimers, C,,symmetry was imposed on both the pyramidal (C-N-C bond angle 108.7O) and planar forms and calculations were performed for the N-N approach along the C3axis, with the two halves rotated relative to each other to provide a staggerred geometry in each dimer.
2679
to the ABCO, TEA, and ABCU systems, this analysis helps rationalize why dimerization is retarded in the latter two amines, which can more easily achieve planarity in the radical cation. It is significant to point out that, unlike ABCO, both TEA and ABCU fail to exhibit excimer emission even at appropriately high concentrations and low temperatures.21 Another significant result of the calculations of the TMA pairs concerns the orbital energies of the cation dimer relative to the monomer. This is shown in Figure 3 which indicates the relevant orbital energies of the isolated TMA neutral and radical, as well as the radical dimer for case I. The significant finding is that the transition predicted for the radical monomer (5.4eV) is sharply lowered in the dimer (2.2eV). While these calculations are only qualitative, they suggest that the existence of radical dimers may explain the low values of the transition energies observed in the unhindered cage amines ABCO and DABCO. Attempts to corroborate this suggestion by determining the transient absorption spectrum of ABCO at lower concentration were unsuccessful. At the lowest concentration studied (5.0 X M), the transient spectrum was similar to that shown in Figure 1. Possibly the equilibrium constant for the radical dimer formation is too large to allow for sufficient monomer concentrations a t experimentally attainable bulk concentrations. Another qualification should be placed on the interpretation of the DABCO system. First, DABCO excimer is not observed; in addition, while the DABCO radical cation spectrum reported here basically agrees with those in the literature, not all previous cases may have supported dimer formation; for example, Shida et al." produced the DABCO radical cation in a Freon glass at 77 K. The bulk DABCO concentration in this study was in excess of 0.1 M, and it is thus possible that short-range solute mobility could permit dimer formation. Acknowledgment. A.M.H. acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. (21) Halpern, A. M.; Nosowitz, M.; Ruggles, C. J., unpublished data. (22) Hunt, J. W.; Thomas, J. K. Radio?.Res. 1967, 32, 149. (23) Cornford, A. B.; Frost, D. C.; Herring, F. G.; McDowell, C. A. Can. J . Chem. 1971, 49, 1135. (24) D. Aue, private communication.
Thermal Decomposition of Energetlc Materials. 9. A Relationship of Molecular Structure and Vlbratlons to Decomposition: Polynltro-3,3,7,7-tetrakls( trlfluoromethyl)-2,4,6,8-tetraazabicyclo[ 3.3.0loctanes T. B. Brill* and Y. Oyumi Department of Chemistry, University of Delaware, Newark, Delaware 19716 (Received: December 16, 1985)
A qualitative correlation exists between the average N-N bond distance and the average frequency of the NOz asymmetric stretch in nitramines containing CzNNOzunits. Compounds containing long N-N bonds (high u,,(N02)) tend to liberate considerable NOz when they are rapidly heated. Other decomposition products replace or are in competition with NO2 for nitramines having shorter N-N bonds (lower u,(NOz)). These conclusions are bolstered by rapid-scan infrared spectroscopy studies of the initial gas decomposition products from 2,4,6,8-tetranitro-3,3,7,7-tetrakis(trifluoromethyl)-2,4,6,8-tetraazabicyclo[3.3.0]octane and its 2,6-dinitro analogue. The initial gas products evolved from the tetranitro compound are relatively independent of pressure (1-1000 psi) suggesting that N-N bond homolysis is the initial fast step that dominates thermal decomposition.
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The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
perimentally based connections can be found between the ground-state molecular parameters and the gaseous decomposition products. The present paper notes a correlation between the asymmetric stretching frequency of NO2 and the N-N bond distance for C 2 N N 0 2fragments. At one extreme of the plot are compounds that copiously liberate NO2 upon rapid thermolysis while at the other extreme lie compounds that do or do not evolve NOz but also produce large quantities of other products from competitive decomposition reactions. It is necessary to employ rapid, real-time, in situ analysis, such as is afforded by rapid-scan infrared spectroscopy,] to detect these differences because secondary reactions take place that alter the product distribution. The correlation is bolstered by thermolysis studies using fast heating rates and various pressures of two unusual nitramines, 2,4,6,8-tetranitro3,3,3,7-tetrakis(trifluoromethyl)-2,4,6,8-tetraazabicyclo[3.3.0]octane, 1, and 2,6-dinitro-3,3,7,7-tetrakis(trifluoromethyl)2,4,6,8-tetraazabicylo[3.3.0]octane 2. These compounds occupy yo2
TABLE I: Mid-IR Spectra (em-')of Solid 1 and 2 at 297 K 2
1
No2
2
an extreme position in the d ~ u a s ( N O zcorrelation ) and therefore are important in formulating the structure-decomposition dependency. In addition, the effect of pressure on the initial products evolved from rapid thermal decomposition of 1 suggests the predominance of the N-N bond fission step. Experimental Section Colorless polycrystalline samples of 1 and 2 were supplied by Drs. W. Koppes and H. Adolph of the Naval Surface Weapons Center. By differential thermal analysis (Mettler 2000B) neither of these compounds exhibits first-order phase transitions between room temperature and the decomposition or melting point. Infrared spectroscopy studies were conducted with a Nicolet 60SX FTIR spectrometer. The spectral methods, data handling procedures, and sample cells have been described elsewhere.' The IR spectrometer is capable of rapid scanning which is important for high-rate thermolysis studies. Quantitation of the gas-phase species is accomplished by scaling the absorption intensity to the absolute intensity and converting to a percent scale. Not included in the quantitation process are species whose I R intensities are unknown, IR silent molecules, and sublimed parent material. Therefore, a mass balance equation was not sought. For a typical experiment 1 mg of sample was placed on a nichrome filament. The atmosphere in the cell was Ar in all cases and the pressure was adjusted as desired. The temperature of the nichrome filament was calibrated with a 100-MHz storage oscilloscope and a thermocouple. Heating of the sample occurred at the rate noted in the text until the final filament temperature ( T f )was reached. The temperature was then held at T f so that the total duration of heating was 10 s. In effect, the sample was subjected to rapid ramp heating and then holding at Tf.The IR beam was focused a few millimeters above the filament surface so as to record the products as they are evolved. Slow heating experiments on the solid phase were conducted using a cell in which a thin film of sample was held in a semiconfined state between two NaCl plates? The sample was slowly heated (5 K m i d ) and, at prescribed intervals, 32 interferograms at 2-cm-' resolution were recorded. Vibrational Assignments Table I gives the frequencies and suggested assignments for the mid-IR spectra of solid 1 and 2 shown in Figure 1 at room ( I ) Oyumi, Y.; Brill, T. B Combusr. Flame 1985, 62, 213. (2) Karpowicz, R. J.; Brill, T. B. Appl. Spectrosc. 1983, 37, 79.
tentative assignments
3414 vs 3385 m, sh 3017 m, sh 3005 s 2922 w 1652 s 1638 s 1613 m 1602 s 1373 w 1316 vs 1283 s, sh 1259 vs 1253 vs
yo2
No2 No2
1
Brill and Oyumi
1115 s 1081 m 1053 m 981 m 973 w, sh 952 s 923 m 898 m 831 m 817 m 744 m 722 s
3021 vw 2913 w 1603 m 1581 vs 1458 m 1373 m 1318 vs 1287 m, sh 1268 vs 1236 vs 1198 vs 1163 s, sh 1152 s 1114 m 1077 m 958 s 940 m, sh 916 w 873 w 815 766 750 719
m m m s
2
-
-
yc
UL F
w -
9
p1760
1380 WAVENUMBER
620
Figure 1. Mid-IR spectra of neat solid 1 and 2 at room temperature.
temperature. An unusual feature of the spectrum of 1 is u,(N02) which, on average, is among the highest frequencies yet observed for a CzNN02-containingmolecule. The position of v,,(NOz) is probably influenced by crowding of the NO2 groups brought on by the combination of the four CF3 groups, the two fused five-membered rings, and tetranitration of the molecule. This steric hindrance was highlighted in a recent crystal structure determination of l.3 None of these molecular features alone appears to be solely responsible for the unusual NO2 asymmetric stretching frequency according to a comparison of related mol(3) K o p ~W.; , Gilardi, R.; George, C.; Flippen-Anderson, J.; Adolph, H., to be submltted for publication.
Thermal Decomposition of Energetic Materials
7
1001 80 -
0%
.No
"Hpo
'9
I
A '
6 60B 8 40-
s 201 20
I -
OO OO
4 6 8 10 4 6 8 10 TIME, SEC Figure 2. Concentration-time profiles of the gas products from pyrolysis of 1 (dT/dt = 145 K s-', Tf = 905 K). Fluorocarbon products are also present but not quantified for the lack of firm identification. 2 2
ecules containing these individual components. 1,3-Dinitroimidazolidine, 3, in which the NNOz groups are similarly posiNO2
I
NO2
3
4
NO2 NO2
I
1
No2
No2
4
tioned as part of a five-membered ring, has v,,(N02) averaging 1525 cm-'. truns-l,4,5,8-Tetranitro-l,4,5,8-tetraazadecalin, 4, in which the proximity of the four NO2 groups is similar to 1, has v,(N02) averaging 1559 cm-I. 2, having similarly disposed gem-CF3 groups, has vas(N02)averaging 1592 cm-'. On the basis of these comparisons the CF3 groups probably exert the greatest perturbation to v,,(NOz), but the combination of all of these ingredients is needed to raise the frequency to the extent that it is.
Thermal Decomposition Absorptions originating from decomposition products are detected beginning a t 358 K when a semiconfined thin film of solid 1 is slowly heated (5 K m i d ) . This temperature is below the liquefaction point (372-376 K) indicating that 1 lacks a true melting point. Once decomposition begins, the NO2 absorptions disappear at a faster rate than the backbone absorptions. In the end a prominent residual product appears to be NO), as evidenced by a strong absorption at 1342 cm-'. The IR spectra of 2 slowly heated under the same conditions indicates that melting occurs at 440-441 K about 10 K below the decomposition temperature. The lesser degree of nitration of 2 enhances its overall thermal stability compared to 1. Still, the intensity of the NO2 modes diminishes faster than the modes related to the backbone, which remain at 60-80 K higher than they do with 1. This difference in stability in understandable taking into account that liberation of NOz creates a radial site in the residue. Fewer of these radicals are created in 2 which alters the decomposition reaction and makes 2 more stable than 1. Figure 2 shows the concentration-time profiles of the three gas products whose IR intensities are known that are generated by high heating rate thermolysis (145 K s-l) of 1. NOz, which is derived from N-N bond homolysis, dominates. N,O and C 0 2 are in low concentration relative to NO2. All heating rates in the 50-170 K s-l range (Tf = 650-950 K) lead to similar product ratios. Secondary reactions at the hot filament are probably responsible for the conversion of NO2 to N O because the higher heating rates (higher Tf) result in N O appearing earlier and to a greater extent. In addition to these species, gaseous fluorocarbon fragments, whose exact identities and amounts are difficult to establish, are also liberated. The C-F stretching region qualitatively resembles a mixture of CF2=CFCF3 and CF3CHFCF3
I
"
01
"
I
u . "
1'0
,
100
i
,
,
o -
1000
PRESSURE, PSI Figure 4. Concentration-pressure profiles of the initial (first 200 ms) gas products from pyrolysis of 1 (dT/dt = 110K s-I).
which are reasonable fragments based on the composition of the backbone. These species are not shown in Figure 2. No H F was detected. A high-rate thermolysis study on 2 (Figure 3) was conducted for comparison with 1. 2 liberates the same products as 1 but the relative concentration of NO2,while still dominating, is less than from 1. Decomposition of NO2 to NO takes place earlier and to a larger extent which is probably the result of the higher decomposition temperature of 2. Also, the time to appearance of gas products from 2 is greater than 1 because of the difference in thermal stability. Similar fluorocarbon decomposition products as produced by 1 along with some sublimation occur with 2. The application of an external pressure of Ar in the thermolysis of 1 has a notably negligible effect on the relative concentrations of the products, NOz, N20, and C 0 2 (Figure 4). Also the fluorocarbon products remain equally prominent but there may be some redistribution among their relative concentrations. Consequently, the decomposition of 1 seems to be relatively independent of the balance of heterogeneous condensed-phase and gas-phase chemistry. Such behavior is characteristic of a compound whose decomposition mechanism is strongly influenced by one particular reaction, in this case probably N-N bond homolysis. A similar pressure-independent decomposition process was observed for several energetic azidooxetane compounds4 where the azide group dominates the initial reaction. Discussion Although many nitramines produce NOz (or HONO) upon high rate thermolysi~,'*~>~ the copious, pressure-independent for(4) Oyumi, Y.; Brill, T. B., Combust. Flume, in press. ( 5 ) Oyumi, Y.; Brill, T. B. Combust. Flame 1985, 62, 225. (6) Oyumi, Y.; Brill, T. B.; Rheingold, A. L. J . Phys. Chem. 1985, 89, 4824.
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The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
I
HMX .a/
t 14801 1.32
'
"
"
"
1.36
'
1.40
,d,
"
I
1.43
R
Figure 5. The correlation of the average N-N bond distance and the average value of v,(NOz) for C2NN02-containingmolecules in the solid state. NO2 is the most prominent product for compounds with high %(NO,).
mation of NO2 relative to the other products from 1 is unusual. We attribute this to the significant crowding of the NOz groups in the molec~le.~ The N-N bond distances are among the longest observed of a C2NN02nitramine. Likewise, the asymmetric stretching mode, v,(N02), has the highest average frequency that we are aware of for a C2NN02unit. In fact, a relationship shown in Figure 5 exists between the average N-N bond distance and the average value of v,(N02) for the C2NN02compounds studied to date. This relationship most likely arises from the fact that a compensatory increase in the 0-N multiple bond character takes place as the N-N bond distance and multiple bond character increase and decrease, respectively. The length of the N-N bond could play a role in the ease of NO2 evolution. However, such a connection could be complex because decomposition reactions of energetic materials often involve a concert of bond-breaking and bond-forming steps. Moreover, issues such as condensed- vs. gas-phase reactions, the decomposition temperature, and the influence of neighboring groups may be necessary to consider. Nevertheless, with a few exceptions there is a qualitative tendency for nitramines with long N-N bonds (higher v,(N02)) to be strong producers of NOz when heated rapidly (1,2, RDX), while compounds with shorter N-N bonds (lower v,(N02)) tend to liberate much more N 2 0 and CH20, among other species, during thermolysis. Two exceptions are 1-(azidomethyl)-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane, AZTC, and 3,7-dinitre 1,3,5,7-tetraazabicylo[ 3.3. llnonane, DIT.
Brill and Oyumi AZTC has energetic side chains involving both -CHIN, and "0,. No N02(g) at all could be detected during decomposition of AZTC perhaps because the decomposition of the -CHzN3group initiates the r e a ~ t i o n . ~Thermal decomposition of AZTC occurs at a rather low temperature apparently setting a path that is dominated by C-N bond fission rather than a mixture of N-N and C-N bond fission. The minimally energetic DPT molecule likewise does not produce NO2 but instead is dominated by C-N bond fission leading to N 2 0 and CH20.* Thus, connections between the parent molecular structure and the decomposition products can be found, but they are subject to the occasional exception because of the aforementioned complications. The pressure independence of the initial products evolved from 1, like that observed of several azido polymers? may be emblematic of molecules whose decomposition is dominated by one fast reaction step. For 1 this step is probably the N-N bond fission leading to NO2 while for the azidooxetane compounds it is probably reaction of the azide group to produce N2and the nitrene fragment which is also thermally labile.4 The gas products formed from 2 are more dependent on pressure and more typical of nitramines which suggests that the decomposition involves competition among multiple reaction schemes. In summary, lengthening the N-N bond in the C 2 N N 0 2unit as a result of spacial crowding or by the electronic effects of neighboring functional groups increases NO, production upon thermolysis. This tendency can be diagnosed by the frequency of the asymmetric stretch of the -NOz group. Within the caveat that thermal decomposition processes involve complex competitive steps which make conclusions qualitative, the weakening of a particular bond seems to enhance the tendency to have a predominent, identifiable path to decomposition. For instance with 1, the weakened N-N bond causes the N-N bond fission reaction to dominate over a wide pressure range. Acknowledgment. We are grateful to the Air Force Office of Scientific Research, Aerospace Sciences, for support of this work on AFOSR-80-0285 and AFOSR-85-0353. Drs. William Koppes and Horst Adolph generously supplied samples of 1 and 2. We thank Drs. C. George, R. Gilardi, and J. Flippen-Anderson of the Naval Research Laboratory for allowing citation of their crystal structures in advance of publication. Registry No. 1, 101347-48-2; 2, 101347-49-3; NO2, 10102-44-0. (7) Brill, T.B.; Karpowicz, R. J.; Rheingold, A. L.; Haller, T.M. J . Phys. Chem. 1984, 88, 4139. (8) Oyumi, Y.; Brill, T. B.; Rheingold, A. L. J . Phys. Chem., in press.
