J . Phys. Chem. 1986, 90, 2526-2533
2526
Thermal Decomposition of Energetic Materials. 9. Polymorphism, Crystal Structures, and Thermal Decomposition of Polynitroazabicyclo[3.3.1 Inonanes Y. Oyumi, T. B. Brill,* and A. L. Rheingold Department of Chemistry, University of Delaware, Newark, Delaware 19716 (Received: September 19, 1985)
Five thermally interconvertible polymorphs of 1,3,5,7-tetranitr0-3,7-diazabicyclo[ 3.3. llnonane (TNDBN) were discovered between 163 and 490 K by using variable-temperature FTIR spectroscopy and differential thermal analysis. Some probable structural differences between several of the polymorphs can be ascertained from the IR spectra. The structure of one of the polymorphs was determined by single crystal X-ray diffraction: orthorhombic, Pbca, a = 14.639 (3) A, b = 15.485 (2) A, c = 21.576 ( 5 ) A, 2 = 16, RF = 0.047, RwF = 0.049. The asymmetric unit consists of two independent molecules; in one the nitramine groups are exo/exo, in the other exo/endo. In the exo/exo molecule one of the C-bound NO2 groups is rotationally disordered. Polymorphism, not previously known, was discovered in 3,7-dinitro-1,3,5,7-tetrazabicyclo[3.3. llnonane (DPT). The slow thermal decomposition of solid TNDBN and DPT was analyzed by IR spectroscopy. Concentration-time and concentration-pressure profiles for the gas products formed by high heating rate thermal decomposition were also constructed. The distribution of products from TNDBN differs significantly from DF’T but is understandable in terms of the compositional differences. The importance of the unit A is discussed.
Introduction Polymorphic phases influence the efficacy of energetic materials and are highlighted by the well-known examples of ammonium nitrate (five phases),’,2bis(@nitroxyethyl)nitramine, called DINA, (four phase^),^ and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazOCene, called HMX, (four phases)! However, HMX is especially interesting because the tetrazocene ring of the /3 polymorph has the chair conformations while the a,y and 6 phases have the chair-chair c ~ n f o r m a t i o n . ~ - ~ Additional examples of conformational polymorphism might be found among compounds that are structurally and compositionally similar to H M X (1). Two such compounds are 3,7/NO,
/NO,
1
NO,
for polymorphism, nor is its crystal structure known. DTA and variable-temperature IR spectroscopy studies of DPT and TNDBN were undertaken order to look for the thermally interconvertible solid-phase forms between 163 K and the decomposition point. A single crystal X-ray diffraction study of a room temperature stable phase of TNDBN was also conducted. The results show that not only is TNDBN richly polymorphic, but DPT also exhibits polymorphism. Within the asymmetric unit of the structurally characterized TNDBN polymorph, there are two molecular conformations. An additional goal of our recent studies on energetic materia l ~ has ~ been ~ - to~ relate ~ the structure and composition to the decomposition mechanism. This ambitious plan is undeniably made difficult by the fact that parallel decomposition reactions can be capriciously related to the parent molecule. Nevertheless, a study of the decomposition of TNDBN and DPT using low and high heating rates and variable pressure with FTIR diagnostics provides insight when the results are compared to other molecules having similar chemical and structural features. Such related molecules include H M X ( l ) , hexahydro- 1,3,5-trinitro-s-triazine (RDX) (4),19 l-(azidomethyl)-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane (AZTC) (5),14 l11,1,3,6,8,8,8-octanitro-3,6-diazaoctane (ONDO) (6),”1,3,5,5-tetranitrohexahydropyrimidine(DNNC) (7),20and 1,3,3,5,7,7-hexanitro- 1,5-diazacyclooctane (HNDZ) (8).20
3
dinitro- 1,3,5,7-tetrazabicyclo[3.3. llnonane, DPT (2),1° and 1,3,5,7-tetranitro-3,7-diazabicyclo[3.3.1]nonane, TNDBN (3).” An earlier DSC study of DPT reported no solid-solid phase transitions between room temperature and the decomposition point at 470 K.I2 The crystal structure of DPT at room temperature is known.13 To our knowledge TNDBN has not been scrutinized (1) Volfkovich, S . I.; Rubinchik, S . M.; Kozlin, V. M. Bull. Acad. Sci. USSR Diu.Cfiem.Sci. 1954, 209, 167. (2) Brown, R. N.; McLaren, A. C. Proc. R . SOC.London, A 1962, 266, 329. (3) McCrone, W . C. In Microchemical Techniques;Cheronis, N. D., Ed.; Wiley-Interscience: New York, 1962. (4) McCrone, W. C. Anal. Cfiem. 1950, 22, 1225. (5) Choi, C. S.; Coutin, H. P. Acta Crystallogr., Sect. B 1970, B26, 1235. (6) Cady, H. H.; Larson, A. C.; Cromer, D. T. Acta Crystallogr. 1963, 16, 617. (7) Cobbledick, R. E.; Small, R. W. H. Acta Crystallogr. 1974,830, 1918. (8) Goetz, F.; Brill, T. B. J . Pfiys. Cfiem. 1979, 83, 340. (9) Landers, A. G.;Apple, T. M.; Dybowski, C.; Brill, T. B. Magn. Reson. Cfiem. 1985, 23, 158. (IO) Bachmann, W. E.; Horton, W. J.; Jenner, E. L.; MacNaughton, N. W.; Cott, L. B. J . Am. Cfiem.SOC.1951, 73, 2769. ( I 1) Cichra, D. A.; Adolph, H . G.J . Org. Cfiem. 1982, 47, 2474. (12) Hall, P. G.Trans. Faraday SOC.1971, 67, 556.
Experimental Section A sample of TNDBN was obtained from H. G. Adolph” of the Naval Surface Weapons Center. DPT was prepared in this laboratory by S. F. Palopoli using a previously devised procedure.’O Infrared spectroscopic characterizations were conducted on a Nicolet 60SX FTIR spectrometer employing an MCT-B detector. Variable-temperature spectra were obtained on neat polycrystalline samples burnished in the case of DPT or evaporated from CH3CN or acetone in the case of TNDBN between two KBr plates by using (13) Choi, C. S.; Bulusu, S . Acta Crystallogr., Sect. B 1974, B30, 1576. (14) Brill, T. B.; Karpowicz, R. J.; Rheingold, A. L.; Haller, T. M. J . Pfiys. Chem. 1984,88, 4138. (15) Oyumi, Y.; Brill, T. B.; Rheingold, A. L.; Lowe-Ma, C. J . Phys. Cfiem. 1985, 89, 2309. (16) Oyumi, Y.; Brill, T. B. J . Pfiys. Cfiem. 1985,89, 4325. (17) Oyumi, Y.; Brill, T. B.; Rheingold, A. L. J . Pfiys. Cfiem. 1985, 89, 424. (18) Cronin, J. F.; Brill, T. B. J . Pfiys. Cfiem. 1986, 90, 178. (19) Oyumi, Y.; Brill, T. B. Combust. Flame 1985, 62, 213. (20) Oyumi, Y.; Brill, T. B. Combust. Flame 1985, 62, 225. (21) Oyumi, Y . ;Brill, T. B. Combust. Flame 1985, 62, 233.
0022-3654/86/2090-2526$01.50/00 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 1 1 , 1986 2521
Thermal Decomposition of Energetic Materials
TABLE I: Crystal Data, Data Collection, and Refinement of TNDBNOII)
formula formula wt crystal system space group a,
4
C7HION608
b, c, A
v,A3
z NO
J2
NO,
7
6
0N ,
0,N-N,
ON ,
NO,
A
IN-NO,
p(calcd), g temp, ‘C crystal size, mm diffractometer radiation scan limits, deg scan speed, deg m i d scan method std reflns reflns collected unique reflns unique reflns (Fo B 2.5a(F0)) RF,R w ~GOF , g [w-l = a2(Fo) + g(FO)*l highest peak, final diff map, e.&-’ slope normal probability plot
306.1 orthorhombic Pbca 14.639 (3), 15.485 (2), 21.576 (5) 4891 (1) 16 1.66 22 0.21 X 0.28 X 0.38 Nicolet R3 Mo K a (A = 0.71073 A) 4 < 26 6 48 var 4-12 W
3 std/197 reflns ( 6 1 % decay) 3782 3348 2257 (LS parameters = 400) 0.047, 0.049, 1.224 0.0008 0.27 1.033
No,
0 N ,
8
procedures described elsewhere.15-22 Thirty-two accumulated interferograms were averaged at 2-cm-’ resolution. A homebuilt, calibrated cell containing an electrically heated nichrome filament was used for the variable-pressure, high-rate pyrolysis studies. The details of this cell and the procedures employed have been fully described.Ig The essential features for this work include Ar atmosphere, 2-5 mg of polycrystalline sample, 10 scans s-l with two spectra per file, and 4-cm-l resolution. The heating rates (dT/dt) and final filament temperatures ( Tf) are given in the text. For a typical fast heating run, the filament reached T f at the rate described and then remained constant at Tf for the 10-s duration of the experiment. Each gas product was quantified by integrating the intensity of a characteristic band and taking the ratio with its known absolute intensity.Ig H 2 0 , which is definitely present, especially after deflagration, was not included in these plots because of the difficulty in calibrating its concentration. Of course, IR inactive molecules, such as N2, will not be detected. A colorless crystal of TNDBN(II1) obtained by crystallization from acetone was affixed to a fine glass fiber with urethane varnish for X-ray diffraction analysis. Preliminary photographic data revealed mmm Laue symmetry, and systematic absences uniquely defined the centrosymmetric orthorhombic space group, Pbcu. The unit-cell parameters given in Table I were obtained from a least-squares fit of the angular settings of 25 well-centered reflections, 25O I28 I30°, which contained three sets of Friedel-related reflections to check on optical and diffractometer alignment. An octant of all available data was collected. The asymmetric unit consists of two crystallographically independent TNDBN molecules (see text). Unique, observed reflections (2091 at F, I2.5a(F0)) were used in the direct methods solution and blocked-cascade refinement of the structure. Hydrogen atoms were fixed in idealized and updated locations (d(C-H) = 0.96 A); all non-hydrogen atoms were refined anisotropically without constraint. Rotational disorder in one of the eight nitro groups was resolvable by a twoposition model; occupancy of the oxygen atom sites was refined with a unit summed total. The 0(7),0(8a) pair is the major orientation at 61% occupancy. N o systematic dependencies in parity group, or Miller index were the data on sin 8, (F/Fmax)l/z, observed. The final difference Fourier synthesis was featureless. Other data collection, reduction, and refinement details are given in Table I. Fractional atomic coordinates are provided in Table 11. Selected bond distances and angles are summarized in Tables (22) Karpowicz, R.J.; Brill, T.B. Appl. Spectrosc. 1983, 37, 79.
TABLE 11: Atom Coordinates (X104) and Temperature Factors x 103)
atom
X
Y
Z
C(1) c(2j C(3) C(4) C(5) C(6) C(7) C(1’) C(2’) C(3’) C(4’) C(5’) C(6’) C(7’) N(l) N(2) N(3) N(4) N(5) N(6) N(1’) N(2’) N(39 N(4’) N(5’) N(6’) 0(1) O(2) O(3) O(4) O(5) O(6) O(7) O(7a) O(8) O(8a) O(1’) O(2’) O(3’) O(4’) O(5‘) O(6‘) O(7’) O(8’)
5306 (3) 4433 (3j 4657 (3) 5585 (2) 6003 (3) 5048 (3) 5911 (2) 3550 (3) 2557 (2) 2107 (2) 3145 (3) 1905 (2) 1910 (2) 2886 (2) 4439 (2) 4736 (2) 3708 (2) 4257 (2) 6860 (2) 5090 (3) 3472 (2) 2951 (2) 4090 (2) 3296 (2) 2970 (2) 975 (2) 3813 (2) 2996 (2) 4462 (2) 3636 (2) 7202 (2) 7236 (2) 5736 (4) 5561 (14) 4337 (7) 4688 (21) 4804 (2) 3882 (2) 3868 (2) 3020 (2) 3598 (3) 2490 (3) 659 (2) 600 (2)
3666 (21 2419 (2j 1591 (2) 2844 (2) 2184 (2) 1824 (2) 3020 (2) 537 (2) 67 (2) -1313 (2) -820 (2) 155 (2) -364 (2) 104 (2) 3261 (2) 2344 (2) 3792 (2) 2288 (2) 3425 (2) 969 (2) 124 (2) -1382 (2) -517 (2) -2204 (2) 635 (2) -296 (2) 4566 (2) 3456 (2) 2787 (2) 1765 (2) 3539 (2) 3611 (3) 503 (3) 770 (10) 537 (11) 899 (15) -543 (2) -994 (2) -2338 (2) -2736 (2) 462 (2) 1240 (3) 428 (2) -960 (2)
205 (21 648 (2j -362 (2) -797 (2) 222 (2) 275 (2) -140 (2) 2564 (2) 1704 (1) 2292 (2) 3185 (2) 2782 (2) 2173 (2) 3019 (2) 339 (1) -767 (1) 445 (1) -1309 (2) -178 (1) 631 (2) 1960 (1) 2653 (1) 1803 (1) 2699 (2) 3613 (1) 1867 (1) 353 (1) 618 (1) -1726 (1) -1335 (1) -680 (1) 295 (1) 516 (4) 958 (9) 603 (16) 1080 (15) 2091 (1) 1376 (1) 3106 (2) 2322 (2) 3955 (2) 3688 (2) 1801 (1) 1689 (1)
(A2
Ui,,. 46 (11
“Equivalent isotropic U defined as one-third of the trace of the orthogonalized U,, tensor.
111 and IV. Tables of anisotropic thermal parameters, and observed vs. calculated structure factors have been deposited as
2528
The Journal of Physical Chemistry, Vol. 90, No. 1 I , 1986
Oyumi et al.
04
03
Figure 1. Top and side views of the two crystallographically inequivalent molecules in TNDBN(II1)
supplementary data (see note at the end of the paper). Computer programs used are contained in the SHELXTL (4.1) library of programs (G. Sheldrick, distributed by the Nicolet Corp., Madison, WI). Neutral atom scattering factors were taken from standard sources.23 Crystal Structure of TNDBN(II1) The asymmetric unit of TNDBN(II1) contains two molecules shown in Figure 1 each adopting the chair-chair conformation which is the most stable conformation for bicyclo[3.3.l]nonane systems of this type.24,25 However, the two molecules structurally differ in that they are ( I ) nitrogen invertomers of one another and (2) contain a different degree of disorder in the C-nitro portion. The two conformations of TNDBN frozen together in the crystal lattice make it a variation on the concept of conformational polymorphism in an energetic material. The TNDBN molecule accommodates endo/exo and exo/exo positioning of the aza-bonded NO2 groups as a result of the location of the partially localized additional electron density on the aza nitrogen atom. Packing forces probably are responsible for the differing N O z positions because of the small inversion barrier at the amine nitrogen atom when an electron-withdrawing group, such as NO,, is attached.26 The other significant difference between the two TNDBN molecules in the asymmetric unit involves the C-N02 groups. (23) International Tables f o r X-Ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV. (24) Bhattacharjee, S . K.; Chacko, K. K. Tetrahedron 1979, 35, 1999. (25) Livant, P.; Roberts. K. A.; Eggers. M. D.; Worley, S. D. Tetrahedron 1981, 37, 1853. ( 2 6 ) Lehn, J. M.; Riddell, F. G.; Price, B. J.; Sutherland, I . 0. J . Chem. So?. B, 1967. 387.
TABLE 111: Bond Lengths (A) 1.530 ( 5 ) 1.519 ( 5 ) 1.531 (5) 1.521 (5) 1.510 (5) 1.532 (5) 1.536 (5) 1.539 (5) 1.520 (5) 1.523 (5) 1.541 (5) 1.523 (4) 1.370 (4) 1.224 (4) 1.223 (4) 1.206 (4) 1.215 (7) 1.292 (13) 1.373 (4) 1.217 (4) 1.232 ( 5 ) 1.210 (5) 1.222 (4)
C(1)-N(1) C(2)-N( 1) ~(3)-~(2) ~(4)-~(2) C(5)-C(7) C(7)-N(5) C(1’)-N( 1’) C(2’)-N( 1’) C(3’)-N(2’) C(4’)-N(2’) C(5’)-C(7’) C(7’)-N(5’) N(2)-N(4) ~(3)-ow N(4)-0(4) N(5)-0(6)
1.445 (5) 1.464 (5) 1.463 (4) 1.465 ( 5 ) 1.519 (5) 1.525 (4) 1.455 (4) 1.452 (4) 1.464 ( 5 ) 1.469 (4) 1.526 ( 5 ) 1.528 (5) 1.365 (4) 1.223 (4) 1.220 (5) 1.195 (4)
N ( l’)-N(3’) N(3’)-0(2’) N (4’)-0 (4’) N(5’)-O(6’) N(6’)-0(8’)
1.385 (4) 1.220 (4) 1.226 (5) 1.182 (5) 1.226 (4)
NO, disorder exists for one of the molecules but not for the other. The relative positions of these NO2 groups are not influenced strongly by the rest of the molecule because the NO, torsion angle, as well as the degree of disorder, varies widely among these and related molecule^.^'-^^ The barrier to rotation of NO2 about the ~~
( 2 7 ) Kaftory, M.; Dunitz, J. D. Acta Crystallogr.,Sect. B 1976, 8 3 2 , 1. (28) Reynolds, P.A . Acta Crsytallogr.,Sect. A 1978, A34, 242.
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2529
Thermal Decomposition of Energetic Materials TABLE IV: Bond Angles (deg)
C(7)-C(l)-N( 1) C(6)-C(3)-N(2) C(6)-C(5)-C(7) C(2)-C(6)-C(5) C(2)-C(6)-N(6) C( 5)-C (6)-N( 6) C( 1)-c(7)-c(5) C( l)-C(7)-N(5) C(S)-C(7)-N(5) C(6’)-C(3’)-N(2’) C(7’)-C( 1’)-N( 1’) C( 3’)-C (6’)-C( 2’) C( 2’)-C( 6’)-C( 5’) C( 2’)-C( 6’)-N( 6’) C(4’)-C(7’)-C( 1’) C( l’)-c(79-c( 5’) C( 1’)-C( 7’)-N( 5’) C( 1)-N( 1)-C(2) C(2)-N( 1)-N(3) C(3)-N(2)-N(4) N(l)-N(3)-0( 1) 0(1)-~(3)-0(2) N ( 2)-N (4)-0(4) C(7)-N(5)-0( 5 ) 0(5)-N(5)-0(6) C(6)-N(6)-0(7a) C(6)-N(6)-0(8) O(7a)-N (6)-O( 8) 0(7)-N(6)-0(8a) C(4’)-N(2’)-N(4’) C( 2’)-N( 1’)-C( 1’) C( 1’)-N( l’)-N(3’) N(2’)-N(4’)-0(4’) N( l’)-N(3’)-0( 1’) O( l’)-N(3’)-0(2’) C(7’)-N(5’)-0(6’) C(6’)-N(6’)-0(7’) 0(7’)-N(6’)-0(8’)
108.7 (3) 108.6 (3) 105.8 (3) 11 1.4 (3) 106.4 (3) 108.6 (3) 110.9 (3) 106.5 (3) 107.3 (3) 108.7 (3) 109.3 (3) 114.5 (3) 109.7 (3) 103.8 (3) 113.8 (3) 11 1.0 (3) 104.5 (3) 118.9 (3) 117.0 (3) 114.9 (3) 117.5 (3) 125.0 (3) 117.7 (3) 119.1 (3) 122.7 (3) 128.9 (10) 112.9 (1 1) 116.5 (16) 121.3 (14) 115.0 (3) 116.1 (3) 118.9 (3) 117.0 (3) 117.3 (3) 125.5 (3) 119.6 (3) 117.0 (3) 124.3 (3)
C(6)-C(2)-N(l) C(7)C(4)-N(2) C(2)-C(6)-C(3) C(3)-C(6)-C(S) C(3)-C(6)-N(6) C(l)-C(7)-C(4) C(4)-C(7)-C(5) C(4)C(7)-N(5) C(7’)4(4’)-N(2’) C(6’)-C(2’)-N(lt) C(6’)-C(5’)-C(7‘) C(3’)-C(6’)-C(5!) C(3’)-C(6’)-N(6’) C(S’)-C(6’)-N(6’) C(4’)-C(7’)-C(5’) C(4’)-C(7’)-N(5’) C(St)-C(7’)-N(5’) C(1)-N(1)-N(3) C(3)-N(2)-C(4) C(4)-N(2)-N(4) N(l)-N(3)-0(2) N(2)-N(4)-0(3) 0(3)-N(4)-0(4) C(7)-N(5)-0(6) C(6)-N(6)-0(7)
107.2 (3) 108.7 (3) 113.4 (3) 111.4 (3) 105.2 (3) 112.9 (3) 110.8 (3) 108.0 (3) 108.9 (3) 110.1 (3) 104.7 (3) 111.2 (3) 108.1 (3) 109.2 (3) 111.2 (3) 106.8 (3) 109.1 (3) 117.3 (3) 120.9 (3) 115.5 (3) 117.5 (3) 117.7 (3) 124.6 (3) 118.2 (3) 116.3 (4)
0(7)-N(6)-0(8) C(6)-N(6)-0(8a)
110.3 (8) 119.3 (13)
C( 4’)-N( 2’)-C( 3’) C(3’)-N(Zt)-N(4’) C(2’)-N(l’)-N(3’) N(2’)-N(4’)-0(3’) 0(3’)-N(4’)-0(4’) N(l’)-N(3’)-0(2’) C(7’)-N(5’)-0(5’) 0(5’)-N(5’)-0(6’) C(6’)-N(6’)-0(8’)
122.4 (3) 114.6 (3) 117.8 (3) 117.2 (3) 125.8 (3) 117.1 (3) 116.9 (3) 123.0 (4) 118.6 (3)
A
IOU
L * j Q 2 m
a7
1310 970 WAVENUMBER
1650
Variable-Temperature IR Spectra of the Solid Phase Tables V and VI give the frequencies and tentative assignments for the mid-IR absorptions of TNDBN and DPT. The assignments were made by comparing the frequencies for these two compounds and those of 1,34 6,’’ 7,j0 and gS0to the more firmly established assignments for 4.35 Extensive coupling exists among the motions of these molecules making them unassignable to localized modes.35 Only the NOz and CHz stretching modes are reasonably pure. A comparison is made in Figure 2 of the mid-IR spectra of five solid forms of TNDBN that were uncovered by heating and (29) Ammon, H. L.; Gilardi, R. D.; Bhattacharjee, S. K. Acta C~srallogr., 1680. (30) Oyumi, Y.;Brill, T. B.; Rheingold, A. L.; Haller, T. M. J . Phys. Chem. 1985, 89, 4317. (31) Tannenbaum, E.; Myers, R. J.; Gwinn, W. D. J . Chem. Phys. 1956, 25, 42. (32) Dakhis, M. I.; Levin, A. A.; Shlyapochnikov, V. A. J . Mol. Struct. 1972, 14, 321. (33) Brill, T. B.; Reese, C. 0. J . Phys. Chem. 1980, 84, 1376. (34) Iqbal, Z.; Bulusu, S.; Autera, J. R. J . Chern. Phys. 1974, 60, 221. (35) Trinquecoste, C.;Rey-Lafon, M.; Forel, M. T. J . Chim. Phys. 1975, 72, 689.
Sect. C 1983, C39,
630
Figure 2. Mid-IR absorption spectra of five polymorphs of TNDBN (neat polycrystalline samples) obtained by heating and cooling TNDBN(II1) and TNDBN(1V).
Ill
~
- IV
;;
388-393K ( I R )
v +decomposition
422-424K ( I R ) 425K (DTA)
490K
198K (IR)’
238K (DTA)
C-N bond axis is very which permits packing influences to control the position and degree of disorder of these NOz groups. The intermolecular heavy atom contacts C(N02), group probably influences the decomposition products to a large extent. The presence of the unit A in DPT, along with its absence in TNDBN, is probably responsible for the sharp differences between the thermolysis products of DPT and TNDBN. When the unit A is present, N 2 0 and C H 2 0 are favored for decomposition taking place in the heterogeneous condensed phase.19 N-N bond homolysis liberating NO2 and also, eventually, H C N becomes more prevalent for decomposition in the gas phase or perhaps when the initial gas products are able to diffuse away from the condensed phase. This pattern has been general, so far, for nitamines and can be qualitatively probed by varying the pressure during decomposition. As shown in Figures 9 and 10, lower pressure tilts the decomposition in favor of the gas-phase decomposition while higher pressure favors heterogeneous condensed-phase decomposition. These experiments were conducted so that the heating rate was about constant for each pressure. The concentrations of the products in the first 200 ms are plotted. Note that at subatmospheric pressures, the N 0 2 / H O N 0 concentration from TNDBN is high which is characteristic of more gas-phase
PRESSURE, PSI Figure 9. Concentration-pressure profiles of the first-observed thermolysis products (first 200 ms) from TNDBN (Ar atmosphere), using a constant heating rate of 110 K s-l.
1 ..\ . ,
-
_ . _ , , / , ,
t
K) Figure 10. Concentration-pressure profiles of the first-observed gas products (first 200 ms) from DPT (Ar atmosphere), using a constant heating rate of 110 K s-l.
decomposition (N-N bond homolysis) than condensed-phase decomposition (C-N bond homoly~is).'~ Likewise, the N 0 2 / H C N concentration from DPT is higher at lower pressure in accordance with the notion that N-N bond homolysis occurs more readily with conditions favoring gas-phase decomposition (lower pressure, higher heating rate, higher Tf) whereas C-N bond fission is more prevalent as a condensed-phase decomposition mechanism (favored by higher pressure, lower heating rate, and lower Tf). The concentration of carbon oxidation products increases with pressure while those of nitrogen decrease. We attribute this trend to the decrease in the mean free path of the gas products with increasing pressure forcing the products to remain in the vicinity of the hot filament for a longer period of time. As a result, more secondary reactions occur at higher pressure causing products resembling those of deflagration at higher pressure and decomposition at lower pressure. Acknowledgment. We thank Dr. H. G. Adolph (NSWC) and S . F. Palopoli of this laboratory for supplying samples of TNDBN and DPT, respectively. We are grateful to the Air Force Office of Scientific Research for support of this work through AFOSR-80-0258. The DTA studies were conducted in the laboratory of Dr. M. Jain. Registry No. 2, 949-56-4; 3, 8 1340-15-0.
Supplementary Material Available: Listings of the observed and calculated structure factors, the anisotropic temperature factors of non-hydrogen atoms, and the hydrogen atom coordinates (16 pages). Ordering information is given on any current masthead page.
