J. Phys. Chem. 1986, 90, 4121-4126
4121
K, = 0.96, KZg= 2 X IO4, and K2t = 2 X (see Table I). We could not apply the same analysis to the racemic 2,3-dimethylsuccinic acid, since the methine-methine coupling does not distinguish between conformer VIa, with the carboxyls gauche, and conformer VIb, with the carboxyls trans to each other. Finally, we used the stereoisomers of cyclohexane- 1,2-dicarboxylic acid as model compounds for the gauche and trans conformer of succinic acid. Titration of 0.094 M acids yielded apparent second ionization constants of 6.2 X 10-6 for the cis and for the trans isomers.16 This corresponds to K2,/K2, 3.2 X = 0.19. These model compounds have conformations analogous to VIc and VIb, and the deviation from the K28/K2,ratio estimated from N M R data for succinic and meso-2,3-dimethylsuccinicacid is, therefore, not surprising.
Cyclohexanedicarboxylic acid was recrystallized from water: mp 229-231 O C (lit.16 215 "C). The racemic 2,3-dimethylsuccinic acid as received contained about 10% of the meso isomer; since this is much leas water soluble, it was removed by filtration after the addition of a small volume of water. The product thus obtained exhibited only small 13CN M R peaks characteristic of the meso isomer; mp 122-123 O C (lit." 121-122 "C). cis-1,2-Cyclohexanedicarboxylic acid was obtained by refluxing its anhydride in water for 5 h: mp 199-201 OC (MI6 190 "C). Spectroscopy. The 'H N M R spectra of D 2 0 solutions of the 2,3-dimethylsuccinic acids were recorded by A. Malz a t the College of Staten Island, City University of New York, on a WP-200 IBM N M R spectrometer at 25 OC.
Experimental Section Materials. All chemicals were obtained from the Aldrich Chemical Co. meso-2,3-Dimethylsuccinicacid was recrystallized from water: mp 210-211 "C (lit." 197-201 "C). trans-1,2-
Acknowledgment. We are grateful to the National Science Foundation for their support of this work through Grant D M R 85-007 12, Polymers Program. Registry No. Succinic acid, 110-15-6;mes+2,3-dimethylsuccinic acid, 608-40-2;cis-cyclohexane-1,2-dicarboxylicacid, 6 10-09-3; rrans-cyclohexane- 1,I-dicarboxylic acid, 2305-32-0.
(16) Kuhn and Wassermann (Helu. Chim. Acra 1928,11,50) obtained K2 = 0.17 X lod for the cis and K2= 1.2 X 1od for the trans homer. We cannot account for the large discrepancy between these and our values.
(17) Eberson, L. Acfa Chim. Scad. 1959, 13, 40.
CHEMICAL KINETICS Deuterium Kinetic Isotope Effect in the Thermal Decomposition of 1,3,5-Trlnitro-l,3,5-triazacyciohexane and 1,3,5,7-Tetranltro-l,3,5,7-tetraazacyclooctane: Its Use as an Experimental Probe for Their Shock-Induced Chemistry7 Suryanarayana Bulusu,* David I. Weinstein, Joseph R. Autera, and Rudolph W. Velicky Large Caliber Weapon Systems Laboratory, US.Army Armament Research and Development Center, Dover, New Jersey 07801 -5001 (Received: October 15, 1985)
An isothermal thermogravimetric analysis (TGA) study of the decomposition of RDX, HMX, and the respective deuterated analogues, RDX-$ and HMX-d,, has been carried out. In RDX, a kinetic isotope effect (KIE) (Kh/&) of 1.5 was observed for the first time in the temperature range 199-216 O C . In the HMX decomposition,an isotope effect of -2.0 was consistently obtained in the temperature range 237-282 O C with no apparent temperature dependence. In both substances the KIE produced a small but definite decrease in shock sensitivity measured by the exploding metal foil method. These results indicate that the rate-determining steps in the processes of thermal decomposition and the chemical process of initiation are likely to be the same. Thus, the kinetic isotope effect served as a novel experimental probe to test the similarity, or otherwise, of the rate-limiting steps of the slow decomposition and the rapidly accelerating reactions of the initiation. Possible rate-limiting steps and reaction mechanisms are discussed.
Introduction 1,3,5-Trinitro-l,3,5-triazacyclohexane(RDX) and 1,3,5,7tetranitro-1,3,5,7-tetraazacyclooctane(HMX), both secondary cyclic nitramines, are well-known as high explosives and chemically interesting materials. They both undergo thermal decomposition in the solid state below their respective melting points and give rise mostly to low molecular weight gaseous products. In the solid state, HMX, which is the higher melting of the two nitramines, while RDX has four conformational isomers (j3,a,y, and
has been sh0wn~9~ to have one metastable conformer (8) in addition to the room-temperature stable one (a).The unique conformational properties and polymorphic transformations have been extensively studied recently by Brill and ceworkerss using FT-IR, laser Raman, 14NNQR, and 'H N M R (solid state) spectroscopies.
Presented at the 8th International Symposium on Detonation, Albuquerque, NM, July 15-19.1985, and the Symposium on the Role of Chemistry in Shock Wmomena, 190th National Mating of the American Chanical Society, Chicago, IL, Sept. 8-13, 1985.
(4) Karpowicz, R.J.; Serglo, S.T.; Brill, T. B. Ind. Eng. Chem. Prod.Res. Deu. 1983, 22, 363. ( 5 ) Landers, A. G.; Apple, T. M.; Dybowski, Cecil; Brill, T. B. Magn. Reson. Chem. 1985, 23, 158 and additional references therein.
~
~~
~
Smith, L. C. Report No. LAMS-2652; Los Alamos Scientific Laboratory: L a Alamos, NM, 1962. (2) Bedard, M.; Huber,H.;Myers, J. L., Wright, G. F. Can. J . Chem. (1) Cady, H.H.;
1962, 40, 2778. (3) McCrone, W . C. Anal. Chem. 1950,22, 954
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Other structural,68 electronic: and crystallographic1*I3 properties have also been reported in the past. The thermal decomposition mechanism and kinetics of RDX and HMX, which are dealt with in this paper, have been the subject of numerous investigations extending over the past 20 years and were reviewed in the literature by several authors.I"l6 Despite the large body of experimental data on the kinetics and decomposition of the nitramines under study, the rate-determining step in their controlled decomposition is not known with certainty. Previous work published" from this laboratory using 15N-labelingexperiments indicated that in the solid-phase decomposition of both RDX and HMX the N-N bond survives intact in the major nitrogenous products of decomposition, NzO and N,. In order to understand the molecular processes involved in the initiation of detonation in an explosive, it is helpful to identify the first bond-breaking step as well as the rate-controlling step in the degradation, both of which define the energy barrier to the overall process. This paper describes the application of the deuterium kinetic isotope effect (DKIE) to focus attention on the rate-limiting step in the thermal decomposition. Furthermore, the relevance of the observed primary isotope effect in the slow thermal decomposition to the shock-induced degradation has been examined by comparison of the shock sensitivities of the fully deuterated and normal samples of RDX and HMX. Similar work on 2,4,6-TNT and 2,4,6-TNT-a-d3 was recently reported by us.18 Shackelford et al.I9 recently reported a differential scanning calorimetry (DSC) study of DKIE in the decomposition of HMX. In the present work, experiments on both HMX and RDX, chiefly by isothermal TGA, are reported. The TGA method is justified because nearly all of the products are gaseousz0 under the experimental conditions used. The DKIE's observed in this work in the H M X decomposition show considerable differences in quantitative detail from those obtained by Shackelford et al.I9 However, the deuterium kinetic isotope effect in the thermal decomposition of RDX-h, and RDX-d, has not been reported in the literature.2' Experimental Section
Synthesis of RDX-d6. A mixture of N D 4 N 0 3(0.42 g), acetic anhydride (0.76 g), and acetic acid-OD (CH3COOD) (16.25 g) was added to a previously made mixture of anhydrous D N 0 3 (0.36 (6)Orloff, M.K.; Mullen, P. A.; Rauch, F. C. J. Phys. Chem. 1970, 74, 3189. (7)Filhol, A.; et al. J. Phys. Chem. 1971, 75, 2056. (8) Iqbal, Z.;Bulusu, S.; Autera, J. R. J. Chem. Phys. 1974, 60, 221. (9)Stals, J.; Buchanan, A. S.;Barraclough, C. G. Trans. Faraday SOC. 1971, 67, 1756 and preceding papers in the series. (10)Cady, H. H.; Larson, A. C.; Cromer, D. T. Acta Crystallogr. 1963, 16, 617. (1 1) Choi, C. S.;Boutin, H. P. Acta Crystallogr. Sect B Struct. Crystallogr. Cryst. Chem. 1970, 26, 1235. (12) Choi, C. S.;Prince, E. Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 1972, 28, 2857. (1 3) Cobbledick, R. E.; Small, R. W. H . Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 1974, 30, 1918. (14) (a) Schroeder, M. A. In Proceedings of the 16th JANNAF Combustion Meeting, Monterey, CA, Chemical Propulsion Information Agency; The Johns Hopkins University: Laurel, MD, 1979;CPIA Publication No. 308, Vol. 11, pp 17-34. (b) M. A. Schroeder, In Proceedings of the 2lst JANNAF Combustion Meeting, Lourel, MD, Chemical Propulsion Information Agency, The Johns Hopkins University: Laurel, MD, 1984;CPIA Publication No.412. (15)Shaw, R.; Walker, F. E. J. Phys. Chem. 1977,81, 2572. (16)Oyumi, Y.;Brill, T. B. Combust. Flame, in press. (17) (a) Bulusu, S.; Graybwh, R. J.; Autera, J. R. Chem. Ind. (London) 1967, 2177. (b) In Proceedings ojthe Army Science Conference (UC'RD), West Point, NY,Office, Chief of Research and Development, Department of the Army: Washington, DC,1968;Vol. 2, p 423. (18) Bulusu, S.;Autera, J. R. J. Energetic Mater. 1983, 1 , 177-205. (19)Shackelford, S.A,; Coolidge, M. B.; Goshgarian, B. B.; Loving, B. A.; Rogers, R. N.; Lanney, J. L.; Ebinger, M. H. J. Phys. Chem. 1985, 89, 3118.
(20)Bulusu, S.;Graybush, R. J. In Proceedings of the 36th International Congress on Industrial Chemistry, Brussels, Belgium, 1967; C. R . Ind. Chim. Belee. 1967. 32. 647650. (21) A referee drew our attention to a presentation by S. N. Rodgers at the 1985 AFOSR/AFRPL Rocket Research Meeting in which a temperature-dependent KIE of 1.7-2.1was reported in the decomposition of RDX-d6 studied by the DSC method.
Bulusu et al. TABLE I: Deuterium Kinetic Isotope Effect ( K , / K , ) in the Isothermal Decomposition of RDX by TCA" temp, 10zKh, 102Kd, "C 199 201 202 206 210 214 216
min-' 1.68 2.25 2.54 4.06 5.56 8.87 12.04
mi& 1.11 1.49 1.69 2.61 3.49
5.51 8.35
1.51 1.51 1.51 1.55 1.59 1.59 1.44 av 1.50
OKhand Kdrepresent the rate constants (at 25% decomposition) with an apparent order of reaction of zero.
mL) and ND4N0, (0.42 g). The temperature of the above mixture was slowly raised to 64 OC under stirring and held there while the following three solutions were added a little at a time sequentially over approximately 7 min: (a) 2 g of hexamine-d12 in 3.3 g of CH3COOD, (b) 4.25 g of DNO, plus 3.32 g of ND4N03,and (c) 11.47 g of acetic anhydride. At the end, the reaction mixture was stirred at 64 O C for 40 min more, cooled to near 0 OC by means of an ice-water bath, and diluted with 50 mL of chilled DzO. It was then heated under reflux for 30 min and cooled. The precipitate of RDX-d, was collected by filtration, washed with water, and dried (yield 3.5 g). The RDX obtained by this method contained about 12-13% HMX as by-product. The two were separated by extraction with ethylene dichloride (1400 mL) in which HMX was very much less soluble. RDX was then recrystallized from 40% aqueous acetone (100 mL for each gram of RDX) by heating at 60 OC until dissolved and letting it cool slowly. Mass spectral analysis showed 95% D enrichment. The synthesis of hexamine-dlz used in the above procedure was camed out from formaldehydedz (CDzO) and ammonia-d3 (ND,) by the same general method as described for 15N-labeledhexamine previously.z2 Synthesis of HMX-d8. As stated above, the procedure for RDX-d6 usually gives about 12% H M X as a by-product. However, when larger quantities of H M X are desired, the preferred method is the one previously described for 15N-labeled RDX/ H M X with appropriate substitution of (CD20),, hexamine-d12, ND4N03,DN03, CH3COOD, and DzO. This modified procedure yields approximately 85% HMX and 15% RDX which are readily separated by the extraction of RDX with ethylene dichloride. Control Samples of RDX and HMX. Control samples of RDX and HMX were purified just as the deuterated ones by an identical recrystallization of the "high purity" samples supplied by the Holston Army Ammunition Plant, Kingsport, TN. In each case, the normal and deuterated samples had identical particle size distributions (RDX: average particle size 33 hm, 92% of the crystals between 10 and 100 pm in size; HMX: average particle size 35 pm, 90% of the crystals between 15 and 110 pm in size). Thermogrwimetric Analysis. Isothermal decomposition curves for RDX, HMX, and the deuterated analogues were obtained on a Perkin-Elmer Model TGS-2 system operated under a 12-psig nitrogen purge. Convenient temperature ranges for RDX and H M X were found to be 199-216 and 230-282 OC, respectively. The isothermal temperature stability in this temperature range was stated to be 0.2 K in the short term. However, the temperature accuracy was estimated to be f l "C by using melting point standards on the DSC served by the same thermal analysis temperature controller (system 7/4). Because of this indirect method of calibration, the absolute temperatures of different runs could be even somewhat poorer than *l OC, making estimates of activation energies and frequency factors low in accuracy. However, at every temperature studied the normal and deuterium-labeled compounds were run immediately after each other so that rate constant could be obtained under as nearly identical conditions as possible. Samples of 0.5-1.5 mg were used for the (22)Bulusu, S.;Autera, J. R.; Axenrod, T. J. Lnbelled Compd. Radiopharm. 1970, 5 , 705.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4123
Thermal Decomposition of RDX and H M X TABLE II: Deuterium Kinetic Isotope Effect (Kh/Kd) in the Isothermal Decomposition of HMX by TGA"
temp, OC 235.0 237.0 239.3 242.5 249.6 265.9 273.2 276.2 279.8 282.3
phase
solid solid solid
solid solid
solid melt melt
melt melt
lozKb, min-l
102Kh,
1.04 1.22 1.51 2.03 2.67 11.74 87.48 233.28 329.11 385.56
0.54 0.59 0.79 0.94 1.40 5.60 43.74 103.68 153.31 184.45
min-l
Khl Kd 1.93 2.07 1.92 2.17 1.91 2.10 2.00 2.25 2.15 2.09 av 2.07
" Kh and Kd represent the rate constants (at 25% decomposition) with an apparent order of reaction of zero. runs in sealed aluminum pans containing pinholes in the covers of the escape of gaseous products. At run temperatures up to 240 OC, an equilibration time of 1 min was allowed before recording the weight loss. An examination of the induction periods in the TGA curves of H M X and RDX showed that sublimation, if any, makes less than 5% contribution to the weight loss. The processes of sublimation and melting do not affect the parameters related to the decomposition rate as in the case of DSC. Above 205 OC, RDX would be in a mixed-melt phase in the above experiments. The common stable form of H M X at room temperature is the @ polymorph which converts to the 6 form at 190 OC, and therefore, the latter was the one to undergo thermal decomposition in the temperature range studied except that at temperatures above 275 O C it would be in a mixed-melt phase. Shock Sensitivity Determinations. The shock sensitivities of normal and deuterated samples of RDX and HMX were compared by using the exploding foil driven flyer plate sensitivity Briefly, this test consists in applying incremental high-voltage (4-6 kV), high-current (-30-40 kA) pulses from a capacitor to 1-mil aluminum foil which instantly vaporizes. The expanding gas from this drives a 2-mil Mylar film to impact the explosive sample which is previously pressed into a steel washer to a known density, constant in a given series of tests. A steel witness disk is used to determine whether the sample has detonated or not. The voltage (or flyer velocity) required to produce 50% fires in 50 tests is used as a measure of shock sensitivity.
-
Results Thermal Decomposition of RDX-h6 and RDX-d6. The isothermal decomposition curves of RDX-h6 and RDX-d6 obtained by TGA are shown in Figure 1, a and b, respectively, and the decomposition rate constants and their ratios are summarized in Table I. In the temperature range of 199-216 O C RDX did not exhibit a pronounced induction period. Below 199 OC, the rate of decomposition was too slow to be measured conveniently, presumably because there was no melting which would have enhanced the rate. However, under isothermal conditions closer to the melting point, RDX appears to decompose at a rate initially constant most likely due to autocatalysis. The rates in Table I represent these constant rates measured arbitrarily at 25% decomposition. The values of the kinetic isotope effect shown in the last column are the ratios of these rates (Kh/Kd). Within experimental error, the isotope effect of 1.5 for RDX remained unchanged with temperature. Thermal Decomposition of HMX-h8 and HMX-d8. The isothermal decomposition curves obtained for HMX-h8 and HMX-d8 by TGA are shown in Figure 1 (c to f) in the two temperature (23) Voreck, W. E.;Velicky, R.W. In Proceedings of the 7th International Symposium on Detonation, US.Naual Academy, Anapolis, Md, June 16-29, NSWC MP-82-334, Naval Surface Weapons Center: White Oak, MD, 1981; NSWC MP-82-334,~924. (24) Weingar&, R.C.; Le,R.S.;Jackson, R. K.;Parker, N. L.In proceedings of the 6th Symposium on Detonation, Son Diego, CA, ONR, ACR-221, Office of Naval Research: Arlington, VA, 1976; p 201.
TABLE III: Deuterium Kinetic Isotope Effect (K,/Kd) during the Induction Period ( I ) in the Isothermal Decomposition of HMX by TGA I, min temp, KhlKd = OC HMX-ha HMX-dn IJIh 235.0 237.0 239.0 242.0 245.4 249.6 254.1
46.7 38.0 33.4 25.6 23.2 19.4 15.6
98.9 73.0 61.5 56.1 42.1 34.4 26.2
2.12 1.92 1.84 2.19 1.81 1.77 1.70 av 1.91
TABLE I V Kinetic Isotope Effect (Kh/Kd) Values Obtained from This Work and Shackelford et al.
This Work from rates at 25% decomposition (235-282 "C) 2.07 f 0.11 from induction periods (Table IV; 235-254 "C) 1.91 f 0.17 Shackelford et al.Ig induction period (278-280 "C) mixed-melt period (acceleratory; 278-280 "C) decay period (liquid) (278-280 "C)
2.21 f 0.36 0.85 f 0.30 1.15 f 0.15
regions 237-266 OC (c and d) and 273-282 OC (e and f). These curves are characterized by deviation from a typical sigmoid shape, but they do exhibit an induction period, acceleration, and decay regimes. The maximum rate was attained in each case at less than 25%decomposition, and it remained unchanged until about 80% of the material had decomposed in all the runs. Once again the rates measured at exactly 25% decomposition were used to calculate the DKIE. Table I1 gives the rate constants at 25% decomposition for HMX-h8 and HMX-d8 together with the DKIE in the last column. It can readily be seen that there was a significant deuterium isotope effect (2.07 average) as in the case of RDX and that it was also essentially invariant with temperature within experimental error. It appears also that the isotope effect (1.91) during the induction periods of HMX-h8 and HMX-d8 (Table 111) was not significantly different from that observed at 25% decomposition. For RDX, the induction period, if any, could not be measured with any precision. The induction periods in H M X were measured from zero time to the point of intersection of the initial straight-line portion of the weight loss curve and the extrapolation of the straight-line portion after the acceleratory period. It was assumed that the induction periods were inversely proportional to the reaction rates. Alternatively, induction periods can be estimated by measuring the time up to the point of inflection of the curve from the initial straight-line portion. However, the isotope effect calculated by this method was 1.81 and, therefore, was not significantly different from the value obtained by the other method (1.91). These results suggest that the principal reaction during the induction period was not different from the one occuring in the acceleratory period as supported also by the similarity of products determined by mass spectrometry (see below). A comparison of the isothermal TGA curves of H M X and the isotope effects in the present work with similar data obtained by isothermal DSC by Shackelford et al.19 reveals differences (Table IV) which illustrate the complexity of kinetics of solid-state decompositions. Attempts could not be made to compare and analyze the experimental curves of HMX-h8 and HMX-d, in their paper with those of this work because their experimental curves were not included. The reasons for the apparent differences summarized in Table IV could not therefore be readily explained. In the DSC exDeriments, the decomDosition rate constants are obtained from ihe heat evolution measurements as a function of time, whereas in the isothermal TGA method employed here, the decrease in the reactant (weight loss) is monitored as a function of time, which may be expected to more closely represent the process* That is, competing Of the ondary reactions are not ~ o n i t o r e dby TGA as 10% as the Products are gaseous. However, physical effects such as particle size, phase
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
4124
Bulusu et al.
ISOTHERMAL TGA RUNS OF HMX-h, (273 TO 2820 CI
e
I-
'
273O
280' 276'
28;
0
2
I TIME-MINUTES
8
8
TIMEMINUTES
I
ISOTHERMAL TGA RUNS OF HMX-hn 1237 TO 2660 C)
b
I
ISOTHERMAL TGA RUNS OF HMX-dn HMX-dl (237 TO 2880 CI
I
40
I
I
120 TIME-MINUTES
160
2W
Zi
ISOTHERMAL TGA RUW OF ROX: h 6
Mo
~.
220
TIME-MINUTES
I
\lW
0 TIMEMINUTES
I
I
I
I
I
15
30
15
M
75
90
I
1
105
120
TIME-MINUTES
Figure 1. Isothermal decomposition curves of RDX, HMX,and their deuterium-labeled isomers obtained by TGA.
transformations, sublimation, and nonuniform heat conduction can still complicate the initial period of decomposition, but efforts were made to minimize such effects. The complex nature of the kinetics can also be seen from the decomposition curves derived from the mass spectrometric measurement of the gaseous products of HMX in Figure 2, reproduced from an earlier paper,20 showing that the principal products of decomposition (HCHO, N20,N2,NO, C02, CO, and HCN) were not all produced at a constant ratio during the decomposition of HMX at 250 "C. NzO, N,, C 0 2 , and CO were formed at a constant ratio such that the curves of their amounts formed (as a fraction of the final amount) vs. time overlap to give curve 1 in Figure 2. Curve 2 shows that the rate of formation of formaldehyde was initially higher than the rates of the above products (curve l ) , but it became, in fact, negative in the later stages due,
presumably, to polymerization and other secondary reactions. In contrast, the nitric oxide (NO) formation (curve 3) lagged behind the products represented by curve 1. Curve 4 is a composite of curves 1-3 and is sigmoid in shape while curve 1 shows a constant rate (zero order) from approximately 15% to 93% of reaction and a very short decay period. Therefore, any rate constants derived from the overall reaction curve (curve 4) are likely to misrepresent the kinetics. As a result of various complications of the kinetics such as those mentioned above, it is not surprising that the magnitudes of the isotope effects observed in the isothermal DSC decomposition of HMX by Schackelford et aI.l9 shown in Table JV differ from those calculated in this work. However, the consistency of the kinetic isotope effect values during a substantial portion of the reaction over a wide temperature range (50 "C) obtained from the iso-
-
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4125
Thermal Decomposition of RDX and H M X , I
0
li
NeO,Nt.CO.COt
@ H C HI O
6
NO
@
TOTAL
’d
TABLE VI: Explpding Foil Shock Sensitivity Measurements of RDX, HMX, and Their Deuterated Analogues
L. ‘*.“4.
‘4
shock sensitivity sample
TlHE
-
NlNS
Figure 2. Mass spectrometric measurement of thermal decomposition productsZoof H M X at 250 OC as a function of time. PIP, represents fraction of a product formed (X 100) relative to its final amount.
TABLE V: Activation Energies and Frequency Factors for the Decomposition of RDX nnd HMX“ E., kcal/mol A, s-I entropy, eu RDX-h6 49.8 3.2 x 1019 30 RDX-d, HMX-hg HMX-dg
49.8 49.8 47.7
2.2 x 1019 2.7 x 1019 1.9 x 1018
27 19 18
”From rates at 25% decomposition
thermal TGA curves shown in Figure 1 by the methods employed in this work seems to support our conclusions regarding the existence of a positive and unchanging kinetic isotope effect in the entire reaction. The activation energies (E,) and the frequency factors derived from the T G A curves of RDX-h6, RDX-d6, HMX-h8, and HMX-d8 are summarized in Table V. These values are presented merely to indicate that they are of the same order of magnitude as previously reported in the literature and are obviously consistent with each other within the experimental errors unavoidable in studying the kinetics of solid decompositions. Effects of Deuterium Substitution on the Shock Sensitivities of RDX and HMX. The shock sensitivities determined for RDX, H M X , and their deuterated analogues by the exploding foil method are shown in Table VI together with the standard deviations obtained by the B r ~ c e t o nmethod. ~~ In each of the two series of tests with RDX-h6 and RDX-d6, the deuterated analogue was lower in sensitivity and the HMX-d8 showed the same effect compared to HMX-h8. The flyer velocity value shows a small but definite increase relative to the control sample, indicating lower shock sensitivity. For example, the order of magnitude of the difference in sensitivity is the same as that between Class A RDX (particle size
