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J . Phys. Chem. 1985,89,4317-4324
conformers. The effect of one low-frequency anharmonic vibration on another presents an interesting interplay of forces and when such interactions occur it is d o ~ u m e n t e dthat ~ ~ the treatment of either of the potential energy functions in a one-dimensional manner is at best only a first-order approximation of the energetics involved. We have recently reported spectral features observed for 3 - f l ~ o r o p r o p e n ethat ~ ~ must arise from similar interactions and we feel that in many cases, particularly when the energy difference between respective conformations is small, such interactions may in fact be the ones which govern the overall stability of these molecular systems.
Conclusions The infrared and Raman spectra of cyclobutylsilane provide clear evidence for the existence of two conformations in the fluid phases. These conformations have been identified as the axial (34) L. A. Carreira, I. M. Mills, and W. B. Person, J . Chem. Phys., 56, 1444 (1972). (351 J. R. Duria. -. Mennzhana - - Zhen, and T. S. Little, J . Chem. Phys., 81, 4259 (1984). (36) J. Laane, Appl. Spectrosc., 24, 73 (1970)
and equatorial forms and the latter has been shown to be the thermodynamically preferred form in the gaseous and liquid phases and is the only conformer present in the annealed solid. Although our analysis of the ring-puckering transitions, observed in the Raman spectrum of the vapor, is not a unique one, a potential has been suggested which reproduces the observed spectra, preferred conformation, ring-puckering angles, and energy difference. With the possible exception of c y c l ~ b u t a n o lto , ~ our ~ knowledge no evidence has been presented which suggests that any other monosubstituted four-membered rings exist with the substituent in the axial position. From an analysis of the sum and difference bands of the internal rotation of the SiH3 moiety with the Si-H stretching mode, barriers to internal rotation have been determined for both the high-energy axial and low-energy equatorial conformers. These barriers, as well as those results obtained for the ring-puckering fundamental, are compared to similar molecules. Acknowledgment. The authors gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-83-11279. Registry No. o-C,H7SiH3,288-06-2.
Crystal Structure and Molecular Dynamlcs of the Energetic Nitramine 1,3,3,5-TetranItrohexahydropyrimidine and a Comparison with 1,3,3,5,7,7-Hexanitro- 1,5-dlazacyclooctane and 1,3,3-Trinitroazetldine Y. Oyumi, T. B. Brill,* A. L. Rheingold, and T. M. Hailer Department of Chemistry, University of Delaware, Newark, Delaware 19716 (Received: April 12, 1985)
The structure and dynamics studies of four-, six-, and eight-membered cyclic energetic nitramines containing the >C(NO2?, fragment are reported. By X-ray diffraction 1,3,5,5-tetranitrohexahydropyrimidine(DNNC) crystallizes in the orthorhombic s ace group P212121with Z = 8 (two molecules per asymmetric unit), a = 11.130 (2), 6 = 11.227 (2), and c = 15.705 (3) Motion/disorder of the NO2 groups in the >C(NOJ2 fragment of DNNC is present in the two independent molecules. FTIR spectroscopy of polycrystalline DNNC reveals a first-order phase transition at about 195 K below which the positioning of the NO2 groups causes lower molecular symmetry. By differential thermal analysis, AH for the overall transformation is 0.8 kcal mol-'. The molecular structure of DNNC is similar in the solid phase at room temperature, in the melt phase, and as an aerosol. The variable low-temperature IR spectrum of DNNC in solution shows dynamic changes ( A H = 0.5-1.7 kcal mol-I) that produce a lower molecular symmetry as the temperature decreases. The IH NMR spectrum of DNNC exhibits no line broadening at low temperature. Spectral changes are also present in 1,3,3,5,7,7-hexanitro-1,5-diazacyclooctane with AH = 1.8 kcal mol-I but are less pronounced than in DNNC. The four-membered ring compound, 1,3,3-trinitroazetidine, appears to be more rigid in solution than the six- and eight-membered analogues.
1.
Introduction Cyclic nitramines have provided excellent examples of unusual chemical behavior including conformational polym~rphism,I-~ anomalously well-resolved dipole-dipole splittings in the 'H N M R ~ p e c t r u m structural ,~ changes with p h a ~ e ,and ~ . ~extended molecular clustering in the gas phase.8 In addition to being of fundamental interest, the thermal decomposition, combustion, and energy storage characteristics of nitramines have practical importance because two nitramines, hexahydro-l,3,5-trinitro-s-tri(1) Choi, C. S.; Boutin, H. P. Acta Crystallogr., Sect. E : Struct. Crystallogr. Cryst. Chem. 1970, 826, 1235. (2) Cady, H. H.; Larson, A. C.; Cromer, D. T. Acra Crystallogr. 1963, 16, 617. (3) Cobbledick, R. E.; Small, R. W. H. Acta Crystallogr.,Sect. B: Struct. Crystallogr. Cryst. Chem. 1974, B30, 1918. (4) Brill, T. B.; Reese, C. 0. J . Phys. Chem. 1980, 84, 1376. ( 5 ) Landers, A. G.; Apple, T. M.; Dybowski, C.; Brill, T. B. Mugn. Reson. Chem. 1985, 23, 158. (6) Karpowicz, R. J.; Brill, T. B. J . Phys. Chem. 1984, 88, 348. (7) Karpowicz, R. J.; Brill, T. B. Combust. Flame 1984, 56, 317. (8) Campana, J. E.; Doyle, R. J., Jr. J. Chem. Soc., Chem. Commun. 1985,
45.
azine, commonly called RDX, and octahydro- 1,3,5,7-tetranitro1,3,5,7-tetrazocine, known as H M X (see Figure I ) , are components of chemical propellants. A trademark of nitramine-containing propellants has been their notorious impassiveness to burn-rate accelerator^.^ Attempts to understand this inertness have sparked many studies of decomposition and combustion of cyclic nitramines, as well as efforts to synthesize new nitramines. One synthetic approach aimed at affecting the burn rate has focused on increasing the oxygen content of the nitramine by replacing an > N N 0 2 group with >C(N02),. A resultant compound, 1,3,5,5-tetranitrohexahydropyrimidine, is called DNNC'O.' (Figure 1) and is analogous to RDX, while H M X has an analogue in 1,3,3,5,7,7-hexanitro1,5-diazacyclooctane,called HNDZI2 (Figure I). There is no pure nitramine analogue of 1,3,3-trinitroazetidine,TNAZ (Figure l).I3
'
(9) McCarty, K. P. Report AFOSR TR 76-59; Air Force Rocket Propulsion Laboratory: Edwards AFB, CA, 1976. (10) Levins, D. A,; Bedford, C. D.; Staats, S . J. Propellants, Explos., Pyrotech. 1983, 8, 74. (11) Cichra, D. A,; Adolf, H. G. J . Org. Chem. 1982, 47, 2474. (12) Cichra, D. A.; Adolf, H. G. Synthesis 1983, 830.
0022-3654/85/2089-4317$0l.50/00 1985 American Chemical Society
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Oyumi et al. TABLE I: Crystal and Refinement Data
formula crystal system space group a,
R D X
H M X
A
b, A c,
‘A
v,A’
z
mol wt
p(calcd), g ~ m ‘ ~ temp, “C radiation 02N
HNDZ
0°C
N, 02N
-
i TNAZ
Figure 1. Skeletal structures (without hydrogen atoms) of the nitramines
discussed here. As a step toward characterizing the structure, dynamics, and molecular behavior of such cyclic nitroalkyl nitramines, the crystal structure of DNNC was determined here for comparison with that of HNDZ.14 The structure and dynamics of DNNC, H N D Z , and T N A Z were investigated by the use of variable-temperature FTIR spectroscopy on crystalline samples and dilute solutions between 155 K and an upper temperature limit exceeding the melting point of D N N C (>425 K) and T N A Z (>375 K) and the decomposition point of solid H N D Z (>550 K). Dynamic flexibility is found to be as characteristic of D N N C and H N D Z as it is of H M X and RDX6 while T N A Z is a more rigid molecule. Experimental Section A sample of D N N C was obtained from D. A. Levins, S R I International, Menlo Park, CA,’O and was recrystallized five tiines from ethanol. Samples of H N D Z and T N A Z were provided by H. G. Adolf, Naval Surface Weapons Center, Silver Spring, MD, and K. Baum, Fluorochem, Azusa, CA, respectively, and were used without further purification. X-ray Crystallography. A large, colorless, well-formed spheroid (0.38 X 0.40 X 0.49 mm) of DNNC, grown by evaporation of an ethanol solution, was affixed to a fine glass fiber with epoxy cement. Initial photographic work showed that the crystal belonged to the orthorhombic system and Laue group mmm. Systematic absences uniquely defined the space group as P212121. Tetragonal symmetry, suggested by a = b, was eliminated by the strong difference in intensity of the 080 and 800 reflections and the 060 and 600 reflections, among others. The unit cell parameters, derived from the least-squares fit of the angular settifigs of 25 reflections (22O < 26 < 30°), are provided in Table I along with the details of data collection, processing, and refinement. The same unit cell parameters are obtained for crystals grown from methanol. The structure was solved with some difficulty by using the direct-methods routine SOLV (SHELXTL, Nicolet Corp.); a productive selection of origin-fixing and multisolution reflections required several trial-and-error approaches. Phases generated from fragments of each of the two independent molecules ultimately revealed the locations of all non-hydrogen atoms through a series of difference Fourier syntheses. Blocked cascade refinement of a model with anisotropic parameters for all non-hydrogen atoms, and hydrogen atoms incorporated as fixed, idealized contributions, (13) Archibald, T. G.; Baurn, K. Report ONR-2-6 (Interim); Office of Naval Research: Washington, DC, 1984. (14) Ammon,H. L.; Gilardi, R. D.; Bhattacharjee, S. K. Acta Crystallogr., Sect. C: Crysf.Strucf. Commun. 1983, C39, 1680.
diffractometer abs coeff, cm-’ scan speed, deg/min 28 scan range, deg scan technique data collected scan width, deg unique data unique data with (Fobad 2.5dFObd) std reflcns Rn R,F, GOF
’
C4H6N608
orthorhombic p2 12121 11.130 (2) 11.227 (2) 15.705 (3) 1962.5 (7) 8 266.0 1.800 24 graphite-monochromated Mo Ka ( A = 0.71073 A) Nicolet R3 1.63 variable 3-10 4-48 w
+h, + k , +I 0.8 f A(a1 - a*)
2151 reflcns (2523 collected) 1685 3/97 (no decay obsd) 0.0564, 0.0581, 1.499
converged a t RF = 0.0822. However, several of the NOz group oxygen atoms, in particular 0(5), 0(6), 0(7), 0(8), O(S)’,0(6)’, O(7)’, and O(8)’, displayed extraordinarily elongated thermal ellipsoids. Although a final difference Fourier synthesis did not reveal peaks that could be used directly to construct a model for disorder, various models were investigated in which each of the affected oxygen atoms was bisected and separated along the major ellipsoid axis. While maintaining occupancy for each new atom, a minimum distance of separation was varied by 0.1-A increments until a distance was found that would remain approximately stable to further refinement without constraint to the 0-0 distances (the range of distances of separation was 0.48-1.36 A). All N - 0 distances in the affected nitro groups were refined as a single variable which, at convergence, was 1.18 A (cf. the unaffected N-O distances; average = 1.21 A). This disorder model converged at R , = 0.0564 and produced pairs of oxygen atom locations that were structurally reasonable. Atomic coordinates based on this model of disorder are provided in Table 11. Primed and unprimed refer to the two independent molecules and “a” and “b” to the bisected oxygen atoms. An inspection of the data revealed no trends with regard to parity group, Miller index, or sin 6/A. Determination of the correct enantiomorphic form was not possible. In the final difference map, the maximum residual was 0.21 e A-3 located 0.82 A from O(5a)’. Spectroscopy. The infrared spectra (4000-600 cm-’) were recorded on a Nicolet 60SX FTIR spectrometer with an MCT detector. Thirty-two interferograms were collected for each spectrum with an effective resolution of 2 cm-’ for the solid- and liquid-phase samples and 4-cm-’ resolution in the melt and aerosol-phase studies. The spectrum of DNNC in the aerosol was obtained by slowly heating D N N C in a glass pyrolysis cell at atmospheric pressure. The spectrum is that of vaporized DNNC, not DNNC condensed on the cell windows, because flushing the hot cell with N2 eliminates all of the absorption bands. Spectra of the solid samples were obtained by recrystallizing the nitramine onto an NaCl plate which fit into a sealed cell. This cell was then placed in an A1 block containing circulating coils through which blowoff from liquid N, was passed. The temperatures were continuously monitored with a type K thermocouple placed directly in the sample and connected to a Fluke digital thermometer. The spectrum of molten D N N C was obtained with a KBr microcavity cell of 0. I-mm path length and surrounded by nichrome wire.’ Spectra of the liquid samples were recorded by placing a solution of the sample between AgCl plates in a liquid cell with a 0.05-mm path length and cooling the sample as described above for the solids study. The only change was that the thermocouple was placed
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4319
Structure and Dynamics of Energetic Nitramines TABLE II: Atom Coordinates ( X lo‘)
atom
TABLE III: Bond Lengths
X
Y
z
2042 (4) 1927 (5) 2830 (4) 2990 (4) -90 (4) 693 (4) 2326 (6) 2255 (4) 2641 (12) 3093 (8) 2507 (18) 1628 (13) 278 (24) 478 (26) 639 (8) 247 (8) 1883 (3) 2878 (3) 742 (3) 2455 (4) 2349 (4) 844 (3) 8132 (4) 7190 (4) 5636 (4) 7746 (4) 6154 (4) 4926 (4) 5259 (3) 7122 (4) 9932 (15) 10106 (8) 9481 (10) 9372 (11) 8025 (20) 7756 (17) 9021 (6) 9497 (8) 6018 (3) 6517 (3) 5687 (3) 6289 (4) 9296 (4) 8473 (4)
959 (4) -27 (5) -1401 (4) 748 (4) -1306 (5) -2409 (4) -1670 (4) 222 (4) 2855 (11) 2786 (8) 2166 (15) 2284 (15) 497 (19) 667 (24) 2269 (5) 1828 (11) -1195 (3) -459 (3) -1671 (4) -652 (4) 2098 (4) 1264 (3) 392 (4) 685 (4) -150 (4) -599 (4) 2811 (3) 2066 (3) 73 (4) -303 (4) -714 (14) -103 (11) 648 (14) -352 (10) 1700 (24) 1919 (20) 2263 (6) 1588 (17) 847 (3) -421 (3) 1976 (3) -177 (3) 49 (5) 1513 (4)
-282 (3) -915 (3) 104 (3) 405 (3) -739 (3) 253 (4) 1735 (3) 1972 (2) -299 (8) -612 (10) -1486 (6) -1317 (8) 434 (10) 672 (10) 315 (7) -359 (7) -499 (2) 719 (2) -317 (3) 1528 (3) -758 (3) 129 (2) 7215 (3) 6536 (3) 7452 (3) 7816 (3) 6819 (2) 7720 (3) 9118 (2) 9429 (2) 7035 (13) 7275 (6) 6164 (8) 6106 (6) 8356 (8) 8162 (9) 7268 (5) 7204 (12) 6918 (2) 8091 (2) 7177 (2) 8931 (2) 6791 (3) 7698 (3)
in contact with the surface of the AgCl plate rather than inside the cell. Considerable care was taken to avoid evaporation of the solvent. The concentrations were controlled to a level that prevented crystallization of the compounds at the lower temperatures of the study. The saturation concentration of each compound was first determined at the freezing point of the solution, and then the solution was diluted by a factor of 10. In this way we ensured that the spectral changes at lower temperatures were not produced by crystallization of the sample from solution. The I R spectra of the pure solvent at each temperature were also recorded with identical spectrometer and cell conditions in order to subtract the solvent modes. For the solution spectrum shown here, this process eliminated base line slope in the NO2asymmetric stretching region that was caused by nearby absorption modes of the solvent. The spectra were otherwise unaffected. The absolute temperature accuracy is f4 K, and the relative temperature accuracy is f l K. N M R studies were conducted on a Bruker WM-250 M H z spectrometer with internal Me4% (6 0.00) as the standard. The temperature was measured by the methyl and hydroxyl signal separation of MeOH from 182 to 273 K. The differential thermal analysis was obtained by scanning 125-225 K with a Mettler 2000B analyzer. Results and Discussion Crystal Structure of DNNC. The structure of the two independent molecules in the unit cell of D N N C is shown in Figure 2. Tables 11, 111, and IV give the atomic coordinates, selected bond distances, and selected bond angles, respectively. The quality
C(I)-C(2) C(l)-N(5) C(2)-N(1) C(3)-N(2) 0(1)-N(3) 0(3)-N(4) 0(5a)-N(5) 0(6a)-N(5) 0(7a)-N(6) 0(8a)-N(6) N(l)-N(3) c(w(4) C(l)-N(6) C(3)-N(1) C(4)-N(2) 0(2)-N(3) 0(4)-N(4) 0(5b)-N(5) 0(6b)-N(5) 0(7b)-N(6) 0(8b)-N(6) N(2)-N(4)
(A)
1.494 (6) 1.520 (6) 1.464 (6) 1.433 (6) 1.210 (6) 1.196 (7) 1.161 (13) 1.160 (10) 1.169 (23) 1.188 (7) 1.408 (5) 1.528 (6) 1.519 (5) 1.437 (6) 1.447 (6) 1.221 (7) 1.224 (6) 1.155 (10) 1.207 (14) 1.159 (22) 1.196 (12) 1.372 (5)
C( 1)’-C( 2)’
C(l)’-N(5)’ C(2)’-N( 1)’ C(3)’-N(2)’ O( 1)’-N(3)’ 0(3)’-N(4)’ 0(5a)’-N(5)’ O(6a)’-N (5)’ 0(7a)’-N(6)’ O(8a)’-N (6) N( 1)’-N(3)’ C( 1)’-C(4)‘ C( 1)’-N(6)’ C(3)’-N( 1)’ C( 4)’-N( 2)’ 0(2)’-N(3)’ 0(4)’-N(4)’ 0(5b)’-N(5)’ 0(6b)’-N(5)’ 0(7b)’-N(6)’ 0(8b)’-N(6)’ N (2)’-N (4)’
1.531 (6) 1.507 (6) 1.447 (5) 1.436 (5) 1.209 (5) 1.216 (6) 1.174 (17) 1.211 (14) 1.167 (16) 1.239 (9) 1.382 (5) 1.520 (6) 1.517 (6) 1.462 (5) 1.449 (5) 1.207 (6) 1.222 (5) 1.191 (10) 1.169 (11) 1.172 (19) 1.188 (11) 1.371 (5)
TABLE IV: Bond Angles (deg)
C(2)-C( 1)-C(4) C( 4)-C( 1)-N( 5) C(4)-C( 1)-N(6) C( 1)-C( 2)-N( 1) C( 1)-C( 4)-N( 2) C( 2)-N( 1)-N( 3) C(3)-N(2)-C(4) C (4)-N (2)-N (4) O(I)-N(3)-N(l) O(3)-N( 4)-O( 4) O(4)-N( 4)-N( 2) C( l)-N(5)-0(5b) C( l)-N(5)-0(6b) O(5b)-N(5)-0(6b) C( 1)-N (6)-0(7 b) C( i)-N(6)-0(8b) 0(7b)-N(6)-0(8b) C(2)-C( 1)-N( 5) C(2)-C( 1)-N(6) N( 5)-C( 1)-N( 6) N( 1)-C(3)-N( 2) C(2)-N( 1)-C(3) C(3)-N( 1)-N(3) C(3)-N(2)-N(4) O( l)-N(3)-0(2) 0(2)-N(3)-N( 1) 0(3)-N(4)-N(2) C( 1)-N( 5)-O( 5a) C( l)-N(5)-0(6a) O(5a)-N( 5)-O(6a) C( l)-N(6)-0(7a) C( 1)-N( 6)-O( 8a) 0(7a)-N(6)-0(8a)
114.5 (4) 108.9 (3) 110.0 (3) 111.6 (3) 109.1 (3) 117.4 (4) 117.7 (3) 119.5 (3) 116.7 (4) 126.0 (5) 117.6 (4) 128.7 (8) 110.7 (8) 120.4 (1 1) 119.4 (14) 109.6 (6) 125.6 (15) 108.4 (3) 111.9 (3) 102.5 (3) 110.7 (4) 114.5 (4) 117.8 (4) 119.7 (3) 126.7 (4) 116.6 (4) 116.3 (4) 111.9 (7) 124.9 (9) 121.6 (11) 118.8 (12) 119.2 ( 5 ) 119.6 (12)
C(2)’-C(l)’-C(4)’ C(4)’-C(l)’-N(5)’ C(4)”C(l)’-N(6)’ C(l)’-C(2)’-N(l)’ C(l)’-C(4)’-N(2)’ C(2)’-N(l)’-N(3)’ C(3)’-N(2)’-C(4)’ C(4)’-N(2)’-N(4)’ O(l)’-N(3)’-N(l)’ 0(3)’-N(4)’-0(4)’ 0(4)’-N(4)’-N(2)’ C(l)’-N(5)’-0(5b)’ C(l)’-N(5)’-0(6b)’ 0(5b)’-N(6)’-0(6b)’ C(l)’-N(6)’-0(7b)’ C(l)’-N(6)’-0(8b)’ C(7b)’-N(6)’-0(8b)’ C(2)’-C(l)’-N(5)’ C(2)’-C(l)’-N(6)’ N(5)’-C(l)’-N(6)’ N(l)’-C(3)’-N(2)’ C(2)’-N(l)’-C(3)’ C(3)’-N(l)‘-N(3)’ C(3)’-N(2)’-N(4)’ 0(1)’-N(3)’-0(2)’ 0(2)’-N(3)’-N(l)’ 0(3)‘-N(4)’-N(2)’ C(l)’-N(5)’-0(5a)’ C(l)’-N(5)’-0(6a)’ 0(5a)’-N(6)’-0(6a)’ C(l)’-N(6)’-0(7a)’ C(l)’-N(6)’-0(8a)’ 0(7a)’-N(6)‘-0(8a)’
113.3 (3) 109.2 (4) 111.6 (3) 110.8 (3) 110.6 (3) 118.5 (3) 117.7 (3) 119.3 (3) 117.3 (4) 125.9 (4) 117.0 (4) 113.9 (6) 124.6 (7) 118.5 (8) 117.6 (11) 115.8 (10) 117.1 (13) 109.6 (3) 110.0 (4) 102.6 (3) 111.4 (3) 113.8 (3) 117.1 (3) 120.3 (3) 124.5 (4) 118.8 (4) 117.0 (4) 124.1 (10) 111.3 (7) 124.5 (12) 119.0 (13) 114.5 (5) 124.8 (14)
of the crystal structure is reduced somewhat by the motion/disorder of the NOz groups bonded to carbon. On the other hand, this feature is an important structural aspect of D N N C and will be discussed momentarily. The pyrimidine ring of D N N C adopts the chair conformation similar to that of the s-triazine ring of RDX,I5 causing the NO, groups of >NNO, to be axial and those of >C(NO,), to be axial and equatorial with respect to the ring. The atomic configuration about the amino nitrogen atom is essentially planar. The N-0 bond distances are normal for the >NNO, fragment.i-3J4-18 The (1 5) Choi, C. S.; Prince, E. Acta Crysrallogr.,Sect. B Struct. Crystallogr. Cryst. Chem. 1972, 828, 2857. (16) Cobbledick, R. D.; Small, R. W. H. Acta Crystallogr., Sect. 8: Struct. Crystallogr. Cryst. Chem. 1975, 831, 2805. (17) Haller, T. M.; Rheingold, A. L.; Brill, T. B. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1983, C39, 1559.
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Oyumi et al.
WAVENUM B E R
Figure 3. IR spectra of neat polycrystalline DNNC and HNDZ at room MOLECULE B
Figure 2. Projections of molecules A and B of DNNC showing the 30% probability thermal ellipsoids of each atom exclusive of H. Disorder of
NO2 was modeled as described in the text. The stippled set of oxygen atoms belong to one >C(NO,), conformation. N-N bond distances [ 1.37-1.40 ( l ) ] are shorter than the N-N single-bond distance of 1.45 A, suggesting that partial a-electron delocalization occurs over the entire > N N 0 2 unit. The other bond lengths in D N N C are normal for these molecule^.^-^^^^-^* The bond angles about carbon are close to tetrahedral except for C( 1) where the C(2)-C(l)-C(4) angle opens to 113-115’ while the N(5)-C( 1)-N(6) angle closes to 102’. Similar distortion occurs about the equivalent carbon atoms of HNDZ.14 The short N - O bond distances in the >C(N02), unit (Table 111) are an artifact of the motion/disorder. Models describing the NO2 groups as disordered and with exaggerated thermal motion were tested. Although the structure refines better with the disorder model, it is difficult to justify one model over another with the crystallographic data alone. Both motion and disorder can originate from a shallow or ill-defined potential energy surface. The greater rotational flexibility of the X - N O , bond compared to >N-NO, is tied to the extent of delocalization of the a-electron density which differs in these C-N and N-N bonds. The barrier to rotation of N O 2 in a 3 C - N 0 2 bond is known to be mall.'^ The existence of at least two rotomers in the >C(NO,), fragment of solid DNNC, each with exaggerated atomic oscillations, in contrast to only one rotomer with lesser thermal motion in HNDZ, is evidence that crystal packing forces and small differences in the intramolecular interactions probably control the twisting of the gem-NO2 groups with respect to each other. For instance, it is possible that the relatively short attractive intramolecular nonbonded distance between 0(7b)-N(4) and 0(7b)-.N(3) of 2.97 (1) and 3.06 (1) A, respectively, is a contributing factor in D N N C since this intramolecular interaction is less pronounced in H N D Z . A detailed description of the intermolecular contacts is difficult for D N N C because the majority of the cohesive interactions involve the oxygen atoms4 which have less reliable coordinates. In any case, there do not appear to be any unusual interactions. (18) Haller, T. M.; Brill, T. B.; Rheingold, A. L. Acta Crystallogr., Sect. C Cryst. Struct. Commun. 1984, C40, 517. (19) Tannenbaum, E.; Myers, R. J.; Gwinn, W. D. J . Chem. Pfiys. 1956, 25, 42. Dakhis, M. I.; Levin, A. A.; Shlyapochnikov, V. A. J . Mol. Strucr. 1972, 14, 321.
temperature. TABLE V: Symmetry Species for the NO2 Stretching Modes of the >C(NO,), Fragment in the Reasonable Rotomen
c. V,S
3A’
US
A“
(I,
c.
2B 2A
2A 2A
The pattern of O.-O and 0.-N contacts in the 3.0-3.2-A range that occurs in RDX15,20is also present in DNNC. The fact that the melting point of D N N C is about 50 K lower than RDX is further evidence of comparable, but somewhat weaker, intermolecular interactions in DNNC. Although the crystallographic data were not of sufficient quality to locate the hydrogen atoms, the infrared studies that follow suggest that C-He-0 hydrogen bonding is relatively unimportant in D N N C . Infrared Spectroscopy Studies. The crystal structure at room temperature suggests that DNNC might exhibit structural changes with temperature. A study of the infrared spectrum of polycrystalline D N N C from 148 to 425 K, the melt phase and the aerosol above 425 K, and various solutions of D N N C from 175 to 295 K was, therefore, undertaken. The vibrational mode symmetries were derived for the structures shown in Figure 2. One of the relative NO2 p.itions of >C(N02)2 produces effective C,point symmetry resulting in 66 normal modes spanning the irreducible representations 45A’ + 2 1A”. Because of coupling, few pure group frequencies will exist. However, the C-H and NO2 stretching modes are relatively independent of the other skeletal modes.21 Long-range coupling of vibrations, say between the NO2 stretching frequencies of the > N - N 0 2 groups, is expected to be small, and thus accidental degeneracy of many vibrations can be anticipated. Of the 15A’ 9A” stretching vibrations, 2(A’ A”) arise from N O 2 bound to N and 3A’ + A” arise from NO2 bound to C. The modes for C(3)-H transform as 2A’ while those of C(2,4)-H span 2(A’ + A”). There are 3(A’ A”) ring stretching modes and A’ A” N-N stretching modes. Two additional C-N stretches of A’ symmetry are associated with the >C(N02), fragment. The other N O 2 positioning in the > C ( N 0 2 ) 2 group has CIsymmetry. However, the vibrational frequencies arising from motions not involving the >C(NO,), fragment will be largely unaffected by the difference of symmetry within the >C(N02); group because the symmetry-breaking atomic features are localized in that fragment. Hence, the >NNO, and ring modes are probably similar for both structures. The
+
+
+
+
(20) Karpowicz, R. J.; Brill, T. B. J . Phys. Chem. 1983, 87, 2109.
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4321
Structure and Dynamics of Energetic Nitramines TABLE VI: Infrared Vibrationsa (4000675 cm-') DNNC(1) at 298 K
DNNC(I1) at 164K
3089 w 3066 w 3036 m 2972 w 2937 vw 2892 w 1590 m, sh 1577 s
3095 w 3066 w 3037 m 2979 w 2968 w 2939 w, sh 1539 s, sh 1584 m, sh 1579 s 1571 s, sh 1560 m, sh 1546 s, sh 1440 m 1427 m 1421 m, sh 1386 w, sh 1379 m 1355 m 1325 m, sh 1318 m 1299 s 1281 m 1256 m 1252 w, sh
1549 s, sh 1441 m 1427 m 1520 m, sh 1379 m 1355 m 1322 m, sh 1316 m 1297 s 1278 m 1252 m
995 m 946 m
1043 vs 1011 w, sh 996 m 946 m
896 s
897 s
868 w 858 w
869 w 859 w
842 w
842 w 807 w 769 w, sh 758 m 694 w, sh 687 w
DNNC melt at 425 K
HNDZ (solid) at 298 K
TNA (solid) at 298 K
3066 w 3033 m
3034 m 3015 m 2991 w
3037 m 3022 m
1587 s, sh 1577 s
2976 w 2889 w 1586 s, sh 1565 s
2976 w 2919 w , b 1590 s
1538 s 1442 m 1425 m
1444 m
1427 m 1410 w
1378 m 1350 m
1386 m 1337 m 1311
1382 w 1367 m 1340 s 1328 s
1271 s
1282 m
1208 w 1197 w, sh 1143 w
1219 m 1184 w 1175 w 1115 w 1088 w
1312 m 1288 s 1271 s, sh 1251 s
994 m 952 m, sh 945 m 895 s
I I 1
974 m v ring
908 w 911 m 867 m
758 m 689 w a
862 w 841 w 807 w
843 s
756 m 692 w
760 m
815 w 844m
1
6
N-NO, or C-NO,
6 N-NO, or C-NO,
and u ring
685 w
s = strong; m = medium; w = weak; vs = very strong; sh = shoulder; b = broad.
models of the >C(NO,), fragment must be. considered separately. The reasonable microsymmetries for >C(NO,), are C,,C,, and C,. Table V lists the distribution of N O 2 stretching modes for this fragment having each of these symmetries. Factor group splitting of the fundamentals of polycrystalline D N N C might occur, but H M X and RDX, where the intermolecular interactions are even larger than in DNNC$*20show few splittings caused by the site ~ y m m e t r y . ~ ' - ~The ~ molecular symmetry alone was, therefore, used to analyze the spectra. ( i ) Above 295 K . The IR spectrum of neat polycrystalline D N N C at room temperature is shown in Figure 3. The frequencies are tabulated with their tentative assignments in Table VI. These assignments were made by analogy to the a- and taking into account the expected difj3-polymorphs of RDX21*25 ferences caused by replacing a > N N O , group with >C(NO,),. Both RDX and D N N C possess the chair ring conformation. The asymmetric stretching modes, vu, of NO, in the >C(NOz), fragment are expected to produce intense absorptions in the 1570-1 660-cm-' while the corresponding modes of
>NNO, occur at 1530-1655 cm-1.28 The overlap of these regions complicates assignments because at least seven modes could be present in solid DNNC. The absolute intensities of the NO, modes vary markedly from compound to compound and are known not to be particularly useful for structural analysis.29 The spectral features of RDX and D N N C in the 1500-1 6 10-cm-' range are similar except that D N N C exhibits a broad absorption envelope whereas more sharply resolved bands occur with RDX. The broad, intense mode centered at 1577 cm-' in D N N C assuredly consists of multiple NO, asymmetric stretching frequencies of the >C(NO,), and > N N O z groups. The mode at 1549 cm-' is probably a mode of the > N N O , group because it also appears with approximately the same position and intensity in RDX. The symmetric stretching modes, vs, of N O 2 in a >C(NO,), fragment occur at 1300-1380 cm-',26927while those of > N N O , appear at 1250-1320 cm-1.28,29In D N N C this region contains four bands and a shoulder which are justifiably assigned to v,(NO,). The specific identity of each mode is ambiguous because none has a clear counterpart in RDX. Two NOz modes should occur from the > N N O , fragment. The absorption at 1278 and
(21) Rey-Lafon, M.; Cavagnat, R.; Trinquecoste, C.; Forel, M. T. J . Chim. Phys. Phys.-Chim. Biol. 1971, 68, 1573.
(22) Iqbal, Z.; Bulusu, S.; Autera, J. R. J . Chem. Phys. 1974, 60, 221. (23) Goetz, F.; Brill, T. B. J . Phys. Chem. 1979, 83, 340. (24) Rey-Lafon, M.; Trinquecoste, C.; Cavagnat, R.; Forel, M. T. J. Chim. Phys. Phys.-Chim. Biol. 1971, 68, 1533. (25) Karpowicz, R. J.; Sergio, S. T.; Brill, T. B. Ind. Eng. Chem. Prod. Res. Deu. 1983, 22, 363.
(26) Diallo, A. 0. Spectrochim. Acto, Part A 1974, 30A, 1505. (27) Loewenschuss, A,; Yellin, N.; Gabai, A. Spectrochim. Acta, Parr A 1974, 30A, 371. (28) Bellamy, L. J. "The Infrared Spectra of Complex Molecules"; Chapman and Hall: New York, 1980. (29) Rao, C. N. R. "Chemical Applications of Infrared Spectroscopy"; Academic Press: New York, 1963.
4322
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Oyumi et ai.
MELT 425 K
SOLI0 -415 K
A
1620
N
1660
i
1490
, -
,
1320 1150 980 WAVE NUMBE R
"
*1
1586 1569 1552 WAVENUMBER
1535
1518
Figure 5. v,(NO,) of DNNC as a function of temperature showing the first-order solid-state phase transition at about 195 K. The number of NO2 modes increases in the low-temperature phase
I
I
810
1603
640
Figure 4. Mid-IR spectrum of DNNC in various phases and temperatures. Striking similarity exists in the spectra.
HR = 2 KIMIN AH = 0 8 KCAUMOL
1253 cm-' were assigned to them. This leaves 1297, 1318, and 1325 cm-' for assignment to us(N02) in >C(N02), (five modes
are possible with a pair of D N N C molecules having effective C, and C2 symmetry). The remaining modes result from ring and C H 2 vibrations by analogy with the RDX spectrum. The C-N modes of a > C ( N 0 2 ) , unit appear at 837-860 cm-' 26 and were assigned in D N N C to the mode 858 cm-I. The NO2 asymmetric stretching modes of D N N C dissolved in various solvents occur 10-15 cm-l higher than in the crystal and appear to be comprised of fewer bands (vide infra). The C H 2 and u,(N02) modes experience random displacements of 0-5 cm-' upon dissolution. These spectral differences are consistent with averaging of the NO2 positions in the >C(NO,), fragment as well as a larger amount of conformational motion in solution compared to the solid phase. Unlike H M X and RDX, which rapidly decompose during melting, D N N C is thermally stable at its melting point of 425 K.l0 Rapid thermal decomposition does not take place until the temperature exceeds 440 K. A comparison of the structure of D N N C in the solid, melt, and aerosol phases is needed because the structure of H M X and R D X in the solid phase a t room temperature differs from the structures at high temperature and in the melt phase.'s3g7 Figure 4 shows a portion of the I R spectrum of solid D N N C at 295 and 415 K, of molten D N N C at 425 K, and as an aerosol at 4 15 K. The aerosol form of D N N C could result from sublimation followed by condensation into colloidal particles. The temperature of the cell is high enough to prevent condensation of D N N C on the cell windows. Spectra of the t p shown in Figure 4 suggest that no first-order solidsolid phase transitions occur between 295 K and the melting point and that crystals formed by sublimation are isostructural with the solvent-crystallized samples. As the temperature of the solid increases and the sample melts, the spectral resolution diminishes because of motional broadening. Small splittings occur in the N O 2 deformation mode at 602 cm-' and the C-N stretch at 858 cm-'. However, the most striking feature is the similarity among the spectra shown in Figure 4. The number of modes and their relative intensities hardly change. The frequencies of most of the modes exhibit only the expected slight shift to lower energy with increasing temperature. In contrast to H M X and RDX, these results suggest that D N N C has similar structural details in all phases of interest at room temperature and above. Some dif-
1 165
190 TEMPERATURE, K
195
Figure 6. Differential thermal analysis showing the solid-solid phase transformation of DNNC.
ferences in the >C(NO2)2 fragment could occur, but the loss of resolution in the u,,(N02) region makes the details impossible to specify. Further evidence of the weakness of C-H--0 hydrogen bonding in these nitramine molecules4 comes from comparing the C-H stretching modes of DNNC dissolved in CH2C12,in solid D N N C at room temperature, and in the melt phase. The frequencies in all cases are within 10 cm-' of one another. (ii)Below 295 K . The motion/disorder in the gem-dinitro group of D N N C prompted a study of the I R spectrum of DNNC in the crystal and in solution below room temperature. A polycrystalline sample of D N N C was slowly cooled from 295 to 160 K and each region of the spectrum examined. A slight increase in the frequency of most modes occurs probably because of the reduced amplitude of atomic oscillation. The intense broad mode centered at about 1575 cm-' and arising from v,(N02) gradually increases in breadth as Tdecreases to 195 K (Figure 5). It is evident from the band maximum that at least two modes diverge in frequency and become better resolved. A solid-solid phase transition then occurs sharply at 195 K, producing a new solid phase, DNNC(II), wherein six asymmetric N O 2 stretching modes are discernible. The resolution further improves as T decreases to 164 K which was the lower temperature limit of this study. The sharpness of this phase transformation is evident by the fact that DNNC(I), the high-temperature structure, is stable for at least 1 h at 193 K. The transformation of DNNC(1) to DNNC(I1) is completely reversible at about 195 K. The phase transformation was inde-
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4323
Structure and Dynamics of Energetic Nitramines
__--, 0 m
_-----2
.--
1620
1609
1598 1587 1576 WAVENUMBER
1565
\,
'92
\
188
1554
Figure 8. v,,(N02) of DNNC dissolved in CH2C12as a function of 1620
Figure 7. v,(NO,)
1603
1586 1569 1552 WAVENUMBER
1535
1518
of DNNC dissolved in acetone-d6as a function of
temperature. pendently confirmed by DTA (Figure 6) and can be seen to be somewhat more complicated than the resolution of the I R spectrum shows it to be. AH for I I1 is 0.8 f 0.1 kcal mol-'. The temperature of the phase transformation is the same by DTA and I R methods within the experimental accuracy. In addition to the dramatic splitting of the asymmetric stretching modes of NO,. the phase transformation causes several modes assigned to symmetric N O 2 stretching to broaden into unresolved doublets. Splitting occurs in the NOz the deformation modes at 758 and 687 cm-I. u, and u,, as well as deformation modes of CH2 split into partially resolved doublets in DNNC(I1). The ring modes seem to be largely unaltered by the phase transformation. The above comparisons suggest that the major differences between the molecular structure of DNNC(1) and DNNC(I1) are associated with the NO, groups and perhaps to a lesser extent involve the methylene groups. The ring conformation appears to be unchanged. The presence of at least six absorptions for uas(NO,) is possible only if the NO2 groups of the >NNO, units become inequivalent or the two crystallographically inequivalent molecules in the asymmetric unit possess >C(N02), fragments whose NO, groups lock into still lower symmetry (C, or C1,but not Cs). We prefer the latter explanation, although the >NNO, groups can become inequivalent without a change in the ring conformation if the lone pair on the amine nitrogen becomes stereochemically partially active and forces one of the NO, groups into an equatorial position while the other remains axial. The flexibility of the amine nitrogen atom is illustrated by the presence of axial, equatorial, and planar positions for the NO2 groups. in the sulfolane solvate of RDX.'* The I R spectra do not provide a conclusive answer to the structure of DNNC(II), but the behavior of the NO, groups differs markedly from DNNC(1). A study of the temperature dependence of the vibrational modes of D N N C in solution from room temperature to the freezing p i n t was undertaken. Vibrational absorption bands normally narrow with reduction in temperature. An increase in the number of bands suggests a shift in the equilibrium from a higher symmetry to a lower one. The energy of some of the vibrational modes can be expected to increase while others decrease. Fewer modes exist in solution compared to the solid because the crystallographic inequivalence is removed. Attention was focused on ua,(N0,) in this work because its line shape exhibited the greatest temperature sensitivity and the absorptions are intense. Dilute solutions of D N N C in acetone-d6 (fp 178 K), dichloromethane (fp 196 K), acetonitrile (fp 227 K), and methyl acetate (fp 175 K) were investigated. At room temperature, u,(NO,) has the same general
-
temperature. TABLE VII:
'H NMR Chemical Shifts (6) for DNNC in Acetone-d.
temp, K 213 206 184
C(3)-H 6.33 6.31 6.39
C(2,4)-H 5.48 5.59 5.60
appearance in all of these solvents with an intense band centered at 1582 cm-' and a weaker broad shoulder exhibiting greater solvent dependence at 1553-1563 cm-'. Both of these modes are higher in energy than those in the solid phase at room temperature. The spectral details of vas(N02)change as T decreases. Figures 7 and 8 show the behavior in acetone-d6 and methylene chloride, respectively. At lower temperature in acetone-d6 (Figure 7) the more intense NO, stretching mode broadens and splits into several components. The same behavior is witnessed in methyl acetate and acetonitrile solution. In methylene chloride solution (Figure 8), the more intense mode is split at room temperature but becomes an unresolved envelope with the appearance of a single band at lower temperature. The less intense lower energy mode becomes more resolved with reduction in temperature in all of these solutions. These spectral changes are entirely reversible. AH for the specific structural changes can be calculated by ratioing the intensity of related modes resulting from equilibrium structures as a function of temperature. We utilized this procedure to estimate AH for processes that could give rise to the observed spectral changes in Figures 7 and 8. Since the details of D N N C at low temperature have not been fully determined, the values of AH are not necessarily associated with a simple structural change. Moreover, subtle coupling among modes could contribute to the absorption intensity. For this reason, values of AH were computed from spectral changes at several different energies of the spectrum. Since the values are similar in magnitude, it seems unlikely that the effects of coupling are solely responsible for all of the changes. According to eq 1, the slope of the plot of In K vs. 1/ T gives AH. AH In K = -RT
aS +R
Deconvolution of modes is required of bands that overlap. Ratioing the intensities of 1575- and 1553-cm-I modes in acetone-d6solution yields AH = 1.7 kcal mol-' (cor = 0.98), while the 1589- and 1564-cm-I modes in methylene chloride solution give AH = 0.5 kcal mol-' (cor = 0.99). The magnitude of AH is of interest. These rather small values are consistent with a low barrier to rotation of the N O z groups of the >C(NO,), fragment, as well as the low barrier to inversion at the amine nitrogen atom when an electron-withdrawing group is attached,30although it is not (30) Lehn, J. M.; Riddell, F. G.; Price, B. J.; Sutherland, I. 0.J . Chem. SOC.B 1967, 381.
4324
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Oyumi et al.
TEMP. K Ih
298 263
253
1610
1601
1592 1583 1574 WAVE N UM BE R
1564
E;
1555
Figure 9. v,(N02) of HNDZ in acetone-d6as a function of temperature.
proven that these processes are responsible. Dynamic motion of the D N N C molecule with an energy barrier of less than 2 kcal mol-I is likely to be too rapid to observe on the N M R time scale at these temperatures. Indeed, the 250-MHz 'H N M R spectrum of D N N C in acetone-d6 and acetonitrile-d3 shows sharp singlets (Table VII) at all temperatures above the freezing point of the solvent. The spectrum of D N N C in solution as well as in the solid state is consistent with a shift from higher to lower molecular symmetry as T decreases. Given the motion/disorder present in the >C(NO,), fragment, it seems reasonable that this group plays a major role in these differences. Spectra and Dynamics of HNDZ and TNAZ The possibility of dynamic motion was investigated in H N D Z and TNAZ, which are molecules related to DNNC. H N D Z remains solid to at least 560 K where significant decomposition has already begun to occur. The crystal structure of HNDZ14 hints that the barrier to rotation of the NO2 groups might be larger than in D N N C because the oxygen atoms are refined with normal temperature coefficients and without disorder. The I R spectra of DNNC and H N D Z are compared in Figure 3. The frequencies for DNNC, HNDZ, and TWAZ are assembled in Table VI. The H N D Z molecule lies on an inversion center so that fewer IR-active modes should be observed than for DNNC. The number of modes is indeed fewer in H N D Z than DNNC, but the assignments in all three molecules are closely related. The IR spectrum of polycrystalline H N D Z at various temperatures reveals no first-order phase transitions in the solid phase between 155 K and the decomposition temperature of about 550 K. In acetone-d6 solution the NO, asymmetric stretching region of H N D Z (Figure 9) changes less dramatically than in DNNC. The shape of the more intense absorption a t 1590 cm-' remains constant while the shoulder at 1575 cm-l moves to 1569 cm-I and becomes better resolved as T decreases. The relative intensities of these two modes remain constant at each temperature. However, a dynamic, reversible change does occur in the lower frequency mode from which A H = 1.7 kcal mol-I was calculated by ratioing the intensity of the mode at 1569 cm-I to the intensity
243 233 223
213 208
203
;:: '98
,
, -
,
0 p..
1620
\JY' ,,i,
e;;;
1590 1560 WAVE NUMBER
1530
Figure 10. v , , ( N 0 2 ) of TNAZ dissolved in acetone-d6as a function of
temperature. at 1575 cm-I. This energy might be associated with conformational motion of the eight-membered ring because HMX is known ~ the details to be conformationally flexible in s ~ l u t i o n .Whatever of the molecular changes in H N D Z with temperature, their effect on the spectrum is much less marked than with DNNC. Solid T N A Z exhibited no phase transformations between 155 K and the melting point at 375 K according to the mid-IR spectrum. The v,,(N02) region of T N A Z dissolved in acetone-d6 shows the temperature dependence displayed in Figure 10. The modes become somewhat better resolved at lower temperature, but there is no significant change in their relative intensities. We interpret this to mean that T N A Z is relatively more rigid in solution than is D N N C and H N D Z , a fact consistent with the fewer degrees of conformational freedom of a four-membered ring compared to the six- and eight-membered ring compounds. The dynamics of these molecules relates to their thermal decomposition characteristics. In a separate study, we will report the thermolytic behavior of solid and molten TNAZ, DNNC, and H N D Z for comparison with H M X and RDX.31 Acknowledgment. We are grateful to the Air Force Office of Scientific Research, Aerospace Sciences (AFOSR 80-0258), for support of this research and to the National Science Foundation for partial support toward the purchase of the X-ray diffractometer. Dr. M. Jain provided the DTA data, and Dr. R. Crecely recorded the low-temperature N M R spectra. R-try NO. DNNC, 81360-42-1; HNDZ, 88371-89-5; TNAZ, 97645-24-4.
Supplementary Material Available: Listings of observed and calculated structure factors, the anisotropic temperature factors of non-hydrogen atoms, and the hydrogen atom coordinates (12 pages). Ordering information is given on any current masthead page. (31) Oyumi, Y.; Brill, T. B. Combust. Flame, in press.
