J. Phys. Chem. 1986, 90,6848-6853
6848
Figure 4, the experimental points were simply normalized to the computed curves to look for differences in shapes. However, the relative yields of y-bands are available from the experiments and show that small additions of N O enhance the y-band emission, as expected from reaction 6. The experiments also show that the enhancement (relative to no added NO) persists even at the highest ) . is consistent with N O addition (1.76 X lOI5 molecules ~ m - ~ This reaction 3a as the N2(A) source but not reaction 8. This arises because additional NO introduces a large additional loss of N(%), whereas N(2D) is already being removed by C2N2and therefore the effect of the additional removal by N O is mitigated. Thus, although none of the evidence is conclusive, it all points to reaction 3a as the important source of N2(A) and hence N O y-band chemiluminescence in the photodissociated cyanogenoxygen system. It is interesting to note that the N O y-bands are a very weak feature of the chemiluminescence produced in the photodissociated C2N& system. Clearly, CN(A and B) and the triatomic emitters NCO, NCN, and C C N are more efficiently produced. In fact, from Figure 1 and the relative monochromator-photomultiplier sensitivity,it appears that the most intense chemiluminescent feature, CN(B) emission, has a quantum efficiency of 20.1. It is interesting to speculate that, since a major loss of N(2D) is
-
assumed to be reaction with C2N2, if the reaction proceeds by N(2D) + C2N2
Nz
+ C2N
AH = -109 kcal/mol (1 1)
there is sufficient exothermicity to produce the observed C2N(A-+X) emission at 466 nm. Furthermore, C2N will be very reactive and its reaction with the 0 atoms present C2N 0 CO C N AH = -115 kcal/mol (12)
+
-+
+
is sufficiently exothermic to produce the CN(A) and CN(B) that are observed. It is this same reaction that was invoked39to explain the C N emissions observed when C2N2is added to partially titrated active nitrogen (with NO giving oxygen atoms). Clearly, measurements of the type described in this paper for the y-bands and additional modeling studies would help to determine the chemiluminescent mechanisms for the other emissions.
Acknowledgment. This work was supported by Contract No. F29601-84-C-0099 with the Air Force Weapons Laboratory. R@tV NO. C2N2,460-19-5; 0 2 , 7782-44-7; C N , 2074-87-5; NCO, 22400-26-6; 0, 17778-80-2; N2, 7727-37-9; N O , 10102-43-9; CO, 630-
08-0. (39) Safrany, D. R.; Jaster, W. J . Phys. Chem. 1968, 72, 3305.
Thermal Decompogition of Energetic Materials. 17. A Relatlonship of Molecular Compodtion to H M O Formation: Bicycio and Spiro Tetranitramlnes T. B. Brit]* and Y. Oyumi Department of Chemistry, University of Delaware, Newark, Delaware 19716 (Received: July 14, 1986)
The IR-active gas products from the rapid thermal decomposition of six cyclic nitramines, five of which contain bicyclic and spiro ring systems, were quantified. Four of the compounds are structural isomers CsHloNsOs. Combining these results with 11 nitramines from our previous studies led to the discovery of a relationship between the H / N 0 2 ratio of the parent molecule and the amount of HONO released in the initial 0.2 s of fast decomposition. Further analysis suggests that HONO forms largely from the adventitious encounter of H' and NO2' rather than by a specific, well-defined reaction, that CH2 units may be more effective H' donors than CH3units in the nitramines studied to date, that by sterically crowding together the NO2 and CH2 units, the production of HONO might be enhanced, and that HONO is an effective source of NO via its secondary reactions.
Introduction Compounds containing the nitramine functional group, >NNO, can be important ingredients in chemical propellants. Recent studies suggest that the formation and further reactions of H' radicals play a prominent role in the thermal decomposition of these energetic The decomposition of octahydro1,3,5,7-tetranitro-1,3,5,7-tetrazacene (HMX) in the solid phase exhibits a primary kinetic isotope effect consistent with the rate-controlling step being homolysis of the C-H bond.' HONO was proposed to be the product stemming from the abstraction of H' from -CH2- by NO;.' Theoretical calculations of the bond energies of simple gas-phase nitramines along the reaction coordinate suggest that the thermal decomposition can be driven by autocatalytic-generated H atoms attacking the nitramine fragmentG2These studies cast light on some of the elementary reactions that may take place during what is, undeniably, a very complicated physicochemical process. The formation of HONO from a thermally decomposing nitramine has been proposed and discussed at some length in recent (1) Shackelford, S. A.; Coolidge, M. B.; Goshgarian, B. B.; Loving, B. A,; Rogers, R. N.; Janney, J. L.; Ebinger, M. H . J. Phys. Chem. 1984,89, 31 18. (2) Melius, C. F.; Binkley, J. S . Symp. (Int.) Combust., Z l s r , 1986, in press.
years by others3-' Hindering a more full understanding has been the fact that the experimental evidence for H O N O consisted of its identification as a minor product from the very-low-pressure pyrolysis of dimethylnitramine (DMNA)8 and indirect evidence This void from mass spectral studies on larger nitra~nines.~-'~ has been filled by the development of fast pyrolysis methods (3) Shaw, R.; Walker, F. E. J . Phys. Chem. 1977, 81, 2572. (4) Schroeder, M. A. ARBRL-MR-03370, Ballistic Research Laboratory, Aberdeen, MD, 1984; CPIA Publ. 1979,11(308), 17; Proc. 18th JANNAF Combustion Mtg, CPIA Publ. 1981, II(347). 395. (5) Goshgarian, B. B. Report AFRPL-TR-78-76, Air Force Rocket Propulsion Laboratory, Edwards AFB, CA, 1978. (6) Brill, T. B.; Reese, C. 0. J . Phys. Chem. 1980, 84, 1376. (7) Fifer, R. A. Prog. Astronaut. Aeronaut. 1984, 90, 171. (8) McMillan, D. F.; Barker, J. R.; Lewis, K. E.; Trevor, P. L.; Golden, D. M. Final Report on SRI Project PYU 5787, Stanford Research International, June, 1979. (9)Bulusu, S.; Axenrod, T.; Milne, G. W. A. Org. Mass Spectrom. 1970, 3, 13. (10) Stals, J.; Buchanan, A. S.; Barraclough, C. G. Trans. Faraday SOC. 1971, 67, 1756. (1 1) Vouros, P.; Petersen, B. A.; Karger, B. L.; Harris, H. Anal. Chem. 1977,49, 1039. (12) Bradley, J. N.; Butler, A. K.; Capey, W. D.; Gilbert, J. R. J . Chem. Soc., Faraday Trans. I , 1977, 73, 1789. (13) Farber, M.; Srivastava, R. D. CPIA Publ. 1979, (308). 59. (14) Farber, M.; Srivastava, R. D. Chem. Phys. Leu. 1979, 64, 307.
0022-3654/86/2090-6848$01.50/00 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6849
H O N O Formation on Thermal Decomposition coupled with real-time, rapid-scan infrared spectroscopy,15which enabled in situ studies of thermally decomposing nitramines to be carried out under reasonably practical conditions. For the first time the relatively transient cis- and trans-HONO molecule can be routinely detected when nitramines are decomposed at atmospheric pressure and above. H O N O is a frequent, and occasionally the dominant, early gas product. It is also poignantly clear that its instability requires these rapid, real-time diagnostics. In this paper, the results from six nitramines are reported, four of which are structural isomers of CsHloN808.These compounds help define the variation in the amount of HONO that can occur within a given stoichiometry of the parent nitramine. On the basis of these results and those from our previous studies of nitra m i n e ~ , l a~ relationship -~~ emerges between the composition of the parent nitramine and the relative amount of HONO formed during thermolysis. The pattern suggests that adventitious contact between NO2' and H' to form HONO may dominate any specific solid-state reaction sources when heterogeneous gas-condensed phase decomposition is occurring. Experimental Section Colorless, polycrystalline samples of 1,3-dinitroimidazolidine (DNCP)?l 2,4,8,1O-tetranitro-2,4,8,lO-tetraazaspiro[5.5lundecane (TNSU),21 1,3,7,9-tetranitro-l,3,7,9-tetraazaspiro[4.5]decane (TNSD),22,23trans-decahydro- 1,4,5,8-tetranitropyrazino[2.3-b]pyrazine ( t r ~ n s - l , 4 , 5 , 8 - T N A D ) , cis-(f)-decahydro-l,3,5,7~~ tetranitropyrimido[ 5.4-4 pyrimidine (cis-1,3,5,7-TNAD),Z5 and trans-decahydro- 1,3,5,7-tetranitropyrimido[ 5.4-dl pyrimidine (trans- 1,3,5,7-TNAD)25were generously supplied by Dr. Rodney L. Willer of Morton Thiokol, Elkton, MD. These compounds were used without further treatment. NO2
702
I
I
I
DNCP
trans- 1.3,5,7-TNAD
NO2
NO2
TNSU
NO2
I
I
NO2
NO 2
I
0 NOp 0
N,O
I
b NO
HCN
TIME, SEC
Figure 1. Concentration vs. time profile of the gas products (excluding H20, IR-inactive species, and HNCO) from DNCP heated at 170 K s-I under 15 psi of Ar.
A CH20
HCN
702
I
NO2
lrens-1.4.5.8-TNAD NO2
cis-l.3,5,7-TNAD
50 1
TNSD
The variable-pressure thermolysis cell has been described e1~ewhere.I~ About 2 mg of sample was spread on the nichrome ribbon filament. The IR beam was focused several millimeters above the sample. An atmosphere of Ar was swept into the cell and adjusted to a static, desired pressure. With the spectrometer scanning a t 10 scans s-l and accumulating two spectra per file at 4-cm-l resolution, the filament was activated and a fast-acting shutter blocking the IR beam was simultaneously opened. Thus, the first interferogram corresponded to the onset of heating of the sample. The heating rate (dT/dt) of the filament was calibrated with a thermocouple connected to a 100-MHz storage oscilloscope with the pressure and dT/dt as variables. The sample (15) Oyumi, Y.; Brill, T. B. Combust. Flame 1985, 62, 213. (16) Oyumi, Y.; Brill, T. B.; Rheingold, A. L. J . Phys. Chem. 1986, 90, 2526. (17) Brill, T. B.; Oyumi, Y. J . Phys. Chem. 1986, 90, 2679. (18) Oyumi, Y.; Brill, T. B. Combust. Flame, in press. (19) Oyumi, Y.; Rheingold, A. L.; Brill, T. B. J . Phys. Chem. 1986, 90, 4686. (20) Oyumi, Y.; Brill, T. B. Combust. Flame 1985, 62, 225. (21) Willer, R. L.; Atkins, R. L. J . Org. Chem. 1984, 49, 5147. (22) Edwards, A,; Webb, G. A. J. Chem. Soc., Perkin Trans. I 1977,1989. (23) Willer, R. L. NWC. TM4703, Naval Weapons Center, China Lake, CA, January, 1982. (24) Willer, R. L. Propellanfs, Explos., Pyrotech. 1983, 8, 65. (25) Willer, R. L. J . Org. Chem. 1984, 49, 5150.
TIME, SEC
Figure 2. Concentration vs. time profile of the gas products from TNSU (excluding H20, IR-inactive species, and HNCO) for dT/dt = 170 K s-' under 15 psi of Ar.
was ramp heated for 3-5 s at the rate desired until the final filament temperature (Tf) was reached. At this time, Tf was held constant for 5-7 s so that the total duration of heating was 10 s. The time of each interferogram was recorded by the computer clock. The ratio of the absorbance of each IR-active gas product to its absolute intensity was measured in each spectrum to determine the relative amount of the species." The concentrations of cisand trans-HONO were summed. H 2 0was not quantified because of the complications presented by the rotation-vibration fine structure. Five heating rates ranging from 50 to 180 K s-l were tested for each compound. The decomposition products from DNCP and trans-1,4,5,8-TNAD were found to be more dependent on dT/dt than is the case for the other compounds. There is less H O N O and more N O 2 from DNCP as dT/dt is reduced. trans- 1,4,5,8-TNAD appears to deflagrate with dT/dt values above about 100 K s-I. Data for the highest dT/dt values are displayed in this paper because they are most closely related to the practical conditions in which these materials are used. When dT/dt = 170-180 K s-', Tf = 950-1000 K. For the concentration-pressure profiles, a static Ar pressure was placed in the cell and the current to the filament was adjusted to provide a uniform dT/dr of 110 K s-l at each pressure. The concentrations of each product in the first file containing thermolysis information (initial 0.2 s of thermolysis) were plotted at each pressure. Thermal Decomposition Results The bicyclic compounds studied in this work decompose at or below their liquifaction temperature. trans- 1,4,5,8-TNAD turns brown on heating to 490 K and then deflagrates in the 500-520
6850
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
Brill and Oyumi
10
8
TIME, SEC
TIME, SEC
Figure 3. Concentration vs. time profile of the gas products from TNSD
(excluding H20, HNCO, and IR-inactive species) for dT/dr = 170 K s-' under 15 psi of Ar. '
°
0
Figure 6. Concentration vs. time profile of the gas products from HNCO, and IR-inactive species) trans-1,3,5,7-TNAD (excluding HzO, for dT/dt = 180 K s-I under 15 psi of Ar.
' 0 NO2
80 -
40 -
0
0
N20
v HONO
NO HCN
. A
C02
co
E30v)
[
0 NGp
20-
A
CG2
z
0 NO
0 N2G
HCN 7
CG
B
C
2
4 6 TIME, SEC
8
0
,
10 PRESSURE, PSI
Figure 4. Concentration vs. time profile of the gas products from
rruns-1,4,5,8-TNAD(excluding H20, HNCO, and IR-inactive species) for dT/dt = 170 K s-l under 15 psi of Ar.
Figure 7. Initial gas concentrations vs. Ar pressure for DNCP using a heating rate of 110 K s-l.
0 NOp
A
CH20
v HONO
0
NO
CO2 v CG
62
ob
2
4
6
8
I
10
TIME, SEC
Figure 5. Concentration vs. time profile of the gas products from cis-
1,3,5,7-TNAD (excluding H20, HNCO, and IR-inactive species) for dT/dr = 180 K s-l under 15 psi of Ar.
K range depending on the sample size. There is no evidence of a melt of liquifaction event. In this way, it resembles 1,3,3,5,7,7-hexanitro- 1,5-diazacyclooctane (HNDZ).*O Liquifaction accompanied by decomposition occurs in cis- and truns1,3,5,7-TNAD (509 and 524 K, respectively), T N S D (473 K), and T N S U (523-6 K). They resemble 1,3,5-trinitro-s-triazine (RDX) in this respect.l5 Figures 1-6 show concentration vs. time plots for the IR-active gas products (excluding HzOand a small amount of HNCO, for
PRESSURE, PSI
Figure 8. Initial gas concentrations (first 0.2 s) vs. Ar gas pressure for TNSU using dT/dr = 110 K-I.
which the IR absolute intensities are unknown) produced by the thermolysis of DNCP, truns-l,4,5,8-TNAD, cis- and trans1,3,5,7-TNAD, TNSD, and T N S U at 170-180 K s-l. The products from fruns-1,4,5,8-TNAD differ from the others at this heating rate because deflagration occurs instead of decomposition. A considerable amount of HONO and more NOz are produced by this compound when the heating rate is lowered to 70 K sW1 (not shown). Unlike the others, TNSU produces no NO2 a t 15 psi of Ar. Figures 7-12 show the relative concentrations of the initial gas products from these compounds as a function of pressure from 1 to 1000 psi of Ar when each is heated at a constant rate of 110 K s-'. Since the IR-inactive species and HzO were not
HONO Formation on Thermal Decomposition
o N ~ O V
HONO
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6851
HCN A
GO2
5!
PRESSURE, PSI
PRESSURE, PSI
Figure 9. Initial gas concentrations (first 0.2 s) vs. Ar gas pressure for
franr-1,3,5,7-TNAD using dT/dt = 110 K s-l.
TNSD using dT/df = 110 K s-l.
70
rl o
NO^
a NO
0 N20
HCN
.
v HONO
Figure 12. Initial gas concentrations(first 0.2 s) vs. Ar gas pressure for
A C02
co
ti?
of Ar and dT/dt = 110 K s-'. The greater HONO/NOZ ratio at the higher pressures suggests that in all of these compounds the formation of HONO is not a primary decomposition process. Instead, it appears that NO,' must form first before H O N O does. When the pressure is low, most of the NOz' diffuses away from the reaction zone and does not engage in further reactions with the condensed phase. As the pressure increases, the rate of diffusion of NO2 diminishes so that the time in which it can engage in reactions with the condensed phase increases. This causes the HONO/NO,' ratio to increase with pressure. Thus,in accordance with Shackelford et al.' and Melius and Binkley? this experimental observation suggests that the simple four- and five-centered unimolecular H O N O elimination steps3 do not contribute significantly to the thermolysis of these nitramines.
PRESSURE, Psi Figure 10. Initial gas concentrations(first 0.2 s) vs. Ar gas pressure for
trans-1,4,5,8-TNAD using dT/dr = 110 K
s-l.
70 I
four - center o NO? 0 N20
sHONO
a NO HCN
. A
COP
co
5!
440 8 -I
9 k
1 20
11'1
,
10
(>
, , , , , , ,,,
100
d 1000
PRESSURE, PSI
Figure 11. Initial gas concentrations (first 0.2 s) vs. Ar gas pressure for cis-1,3,5,7-TNAD using dT/dr = 110 K s-'.
quantified, no mass balance was attempted. For the objectives of this work, our intention was to obtain the relative concentrations to cast more light on the decomposition process.
Discussion From Figure 7-12 and from several previous studies,'6*26it is evident that the amount of HONO relative to NOz increases with increasing pressure between 1 and 30 psi of Ar. In fact, only tram-1,3,5,7-TNAD produces more HONO than NO, at 1 psi (26) Oyumi, Y.;Brill, T. B. Combust. Flame 1985, 62, 233.
-
t i v e center
The fact that rrans-1,3,5,7-TNAD (Figure 12) generates more HONO than NOz' at 1 psi of Ar may originate from unusual crowding of the 1,5-nitro groups and the 4,8-CH2 groups in this molecule.z Perhaps this proximity enhances the chances for NO; to abstract H' at the outset following N-N bond homolysis rather than depending on the secondary migration of the NO,' radical to some other site. The net concentrations of H O N O and NOz' were observed to decrease at higher pressure (Figures 7-12) owing to their reactivity. This reactivity is aply demonstrated by the change in their concentration with time (Figure 1-3, 5, and 6). [NO,] and [HONO] decrease because of secondary reactions at the hot filament while the later stage decomposition and combustion products CO, COz, NO, and, probably, N z increase in concentration. This same behavior is witnessed in the concentration vs. pressure profiles of the first-observed products. As can be seen in Figures 7-12, there is a mid-pressure range in which H C N and N O dominate, while CO, COz, and NzO are less prevalent. This is because the diffusion of the fust-formed gas products away from the hot filament is retarded by the higher ambient preshure in the cell. Therefore, further reactions among the initial products and the condensed phase occur to a greater extent. Owing to the similar structures and compositions of the parent compounds, the identity and concentration of the gas products are very similar in this pressure range. Above 500 psi Ar the gas-condensed phase reactions at the hot filament increase to such an extent that products normally associated with combustion (CO, COz, H 2 0 , and, probably, N,) are all that appear. It is noteworthy that a high concentration of NO is present in the initial 0.2 s of decomposition under 15 psi of Ar. Three
6852 The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 60
DMHDNA a
50
40
"
60
40
trans-TNAD
2 p 30
Brill and Oyumi
trans-TNAD DNCP
DNCP
$O2e 30
# TNDBN
/
20
/
N IDZ
DMEDNA
- / pDMEDNA TNDBN
20
-
I HNDZ
10
10 -
RDX
T$AZ
0
TNTQ
8
4
H/N02 Figure 13. Relative percent of H O N O produced vs. the ratio of H to NO2 in the parent solid, secondary nitramine when the compound is heated at 145-180 K s-I under 15 psi of Ar. T N A Z = 1,3,3-trinitro1,2-ethanediamine, oazetidine, DMEDNA = N,N'-dimethyl-N,N'-dinitr MBNA DMHDNA = N,N'-dimethyl-N,N'-dinitro-1,6-hexanediamine, = N-methyl-N-nitro-1-butanamine, D N P = N,N'-dinitropiperidine, T N D B N = 1,3,5,7-tetranitro-3,7-diazabicyclo[3.3. llnonane, TNTO = 2,4,6,8-tetranitro-3,3,7,7-tetrabis(triflc~ clo[3.3.0]octane. The remaining compounds are identified in the text.
-
common means by which N O can form are (a) NOz reduction, (b) HONO decomposition (2HONO N O + NOz + H20),27 and (c) if C-NO' groups are present, isomerization of C-NO, to C - O N O followed by CO-NO bond h o m o l y ~ i s . ~ ~When -~~ [NO2],is relatively large compared to [HONOIi, [ N o l i is usually small.1s~18~19 However, when [HONOIi is large relative to [NOzji, [ N o l i is greater.15*16*19*20 Therefore, the decomposition of HONO by (b) appears to be an important source of N O in these compounds. It can be said safely that many nitramines liberate HONO upon rapid t h e r m o l y s i ~ . ~ ~ When ~ ~ Jthese ~ ~ ~studies ~ are taken together, a relationship emerges between the elemental composition of the parent nitramine and the prodigiousness of HONO production. To illustrate this, the ratio of the number of hydrogen atoms to the number of NOz groups in 16 solid, secondary nitramine molecules is graphed in Figure 13 as a function of the percent H O N O produced by each relative to the other products. The percent HONO is the amount appearing in the first 0.2 s after the onset of decomposition with a heating rate of dT/dt = 145-180 K s-l under 15 psi of Ar. As the H / N 0 2 ratio increases, the relative amount of HONO tends to increase. The five compounds for which H / N 0 2 = 3 are reported in this paper and provide a measure of the range of percent HONO that can be expected for a given H / N 0 2 ratio. Even if excursions at other H / N 0 2 ratios occur with the magnitude observed for H / N 0 2 = 3, a correlation would still exist. The greatest variance is with DMEDNA,3Zwhere [NOzli(60%) is much greater than that of [HONOIi (22%). The leveling off in the percent HONO is attributable to the dilution by the other products that are produced. (27) Fifer, R. A. J . Phys. Chem. 1976, 80, 2717. (28) Batt, L. In The Chemistry of Function01 Groups; Patai, S., Ed.; Wiley: New York 1982; Supplement F, Part 1, p 417. (29) Dewar, M. J. S.; Ritchie, J. P.; Alster, J. J . Urg. Chem. 1985, 50, 1031. (30) Wodtke, A. M.; Hintsa, E. J.; Lee, Y . T. J. Phys. Chem. 1986, 90, 3549. (31) Oyumi, Y.; Brill, T. B.; Rheingold, A. L. J . Phys. Chem. 1985, 89, 4824. (32) trans-1,4,5,8-TNAD was not included because, according to Figure 10, it has mostly deflagrated when dT/dt = 170 K s-l. Also, 1,1,1,3,6,8,8,8-octanitro-3,6-diazaoctane (ONDO) in ref 31, l,7-dimethyl1,3,5,7-tetranitrotrimethylenetetramine(OHMX), 1,7-bis(azidomethyl)2,4,6-trinitrotriazaheptane (DATH), and l-(azidomethyl)-3,5,7-trinitro1,3,5,7-tetraazacyclooctane (AZTC) were not included because they deflagrate or detonate at heating rates substantially below 145-180 K s-'.
HMX
CIS-TNAD
J. Phys. Chem. 1986, 90, 6853-6857 mol-I) relative to the cis isomer (AHf = 19.4 kcal mol-').Z5 In several ways the thermolysis products shown in Figures 1-6 were unexpected. Owing to the presence of the NCH,N(N02)unit in five of the molecules, C H 2 0was an anticipatedI6 product. However, it was observed only from TNSU. Thus, DNCP, cisand trans-1,3,5,7-TNAD, and TNSD join the mixed C-N02 and N-NO, containing molecule, 1,3,5,5-tetranitrohexahydropyrimidine (DNNC),O as exceptions to this pattern. Perhaps the extensive production of HONO from the bicyclonitramines reduces the requisite methylene groups from participating in CHzO formation. Also, on the basis of the available values of the N-N bond distances34 and asymmetric NO, stretching frequencies, a significant amount of N-N bond fission leading to NO,' is expected17935 and observed from cis- 1,3,5,7-TNAD and TNSD, but less is expected from DNCP. However, DNCP was found to be a strong NO,' generator. Exceptions to this pattern seem to arise when the molecular backbone is especially stable so that C-N bond fission reactions that degrade the backbone do not occur in preferences to N-N bond fission.I8 We have stressed17 that exceptions to any qualitative notions about the relationship between the parent molecular structure and the liberated gas products are bound to be discovered given the extraordinary complexity of these thermal decomposition processes. (34) Lowe-Ma, C., personal communication, 1986. (35) Oyumi, Y.;Rheingold, A. L.; Brill, T. B. Propellants, Explos., Pyrotech., submitted.
6853
Conclusions A contribution to the understanding of HONO production from energetic nitramine molecules has been made with these studies. First, there is experimental evidence that H O N O production is not a primary decomposition reaction of nitramines. It appears that NO,' forms first followed by H' abstraction in a followup step. Second, the formation of HONO is advanced by increasing the H / N 0 2 ratio in the parent molecule, especially if the H atoms are present as methylene units. This result indicates that HONO production from a rapidly heated solid nitramine compound may be largely due to adventitious encounters of NO,' with H' sources. Third, it may be possible to enhance the amount of H O N O produced by a nitramine by forcing an encounter of the NOz group with an H atom source by designing sterically crowded nitramines. Fourth, the decomposition of H O N O appears to be an important and ready source of NO. Acknowledgment. We are genuinely appreciative of the work of Dr. Rodney L. Willer of Morton-Thiokol, Elkton, MD, who synthesized all of the compounds used in this study. We are grateful to the Air Force Office of Scientific Research for support of this work through AFOSR-85-0356. We are also grateful to Drs. Carl Melius and Charlotte Lowe-Ma for permitting citation of their work in advance of publication. Registry No. DNCP, 4164-37-8;TNSU, 84606-37-1;rrans-1,4,5,8TNAD, 83673-31-8; rrans-1,3,5,7-TNAD, 92902-06-2; cis-1,3,5,7TNAD, 104911-60-6;TNSD, 65479-81-4.
Extended X-ray Absorption Flne Structure Investigation of Solid and Gel Forms of Calcium Poly(a-Pgalacturonate) Lucilla Alagna, Tommaso Prosperi, Anthony A. G. Tomlinson,* Istituto di Teoria e Struttura Elettronica dei Composti di Coordinazione, Area della Ricerca di Roma del C.N.R., C.P.10 Monterotondo Staz., 00016 Rome, Italy
and Roberto Rizzo Dipartimento di Chimica, Universitci di Napoli, Naples, Italy (Received: February 20, 1986; In Final Form: June 12, 1986)
The Ca K-edge transmission spectra of calcium poly(a-D-galacturonate)in solid and gel forms, of calcium oligo(a-L-guluronate) in solid form, and of calcium 2-keto-~-gluconatetrihydrate (of known crystal structure) have been measured and interpreted. Detailed analysis of the extended X-ray absorption fine structure of the model compound shows that Ca-0 bond distance distributionscan be reliably partitioned. With the parameters extracted from the model compound, it is found that the distribution of Ca-0 distances is different in the solid poly(ga1acturonate) and the solid oligo(gu1uronate). In the former, the Ca2+is ten coordinate, with a 2:6:1:1 (or 2:5:2:1) partitioning of the Ca-O distances and having Ca-0 = 2.40 (2), 2.55 (2), 2.81 (2), and 2.95 (2) A (or 2.39 (2), 2.54 (2), 2.78 (2), and 2.95 (2) A). By way of contrast, the solid oligo(gu1uronate) has a more compact first shell of Ca-0 distances, with further Ca-0 distances well separated from this, to give a 3:3:2 Ca-0 bond distance distribution with Ca-O = 2.33 (2), 2.45 (2), and 2.76 (2) A. The gel form of calcium poly(a-D-galacturonate) gives results very similar to those of the oligo(gu1uronate). These differences in Ca-O coordination are ascribed to differences in the availability of oxygen atoms in the coordinating cavity caused by differences in polysaccharide chain conformation. Geometrical calculations assuming 31 and 2' conformations for the saccharide chains have been performed and compared with the distribution of Ca-O and Ca-C distances found from the EXAFS analysis. It is concluded that both oligo(gu1uronate) and the gel form of poly(ga1acturonate) have a 2' chain conformation, whereas the solid form of poly(ga1acturonate) has a 31 chain conformation. This provides direct structural evidence that a polymorphic phase transition occurs in calcium poly(&-D-galacturonate) on drying.
Introduction calcium polysaccharides play an important role in plants as the major intercellular component and are also of considerable technological importance. However, the materials are noncrystalline in nature and most information on them has been obtained with indirect physical techniques,' and as yet very little is known 0022-3654/86/2090-6853$01.50/0
as regards the coordination about the calcium ion. Particular attention has been devoted recently to pectin, which consists of a linear 1,4-linked D-galaCtUrOnate chain containing rhamnose defects,2together with some branched chains composed primarily (1) Aspinall, G. 0. Polysaccharides; Pergamon: Oxford, U.K., 1970.
0 1986 American Chemical Society
