8759
J. Phys. Chem. 1993,97,8759-8763
Thermal Decomposition of Energetic Materials. 62. Reconciliation of the Kinetics and Mechanisms of TNT on the Time Scale from Microseconds to Hours T. B. Brill' and K. J. James Department of Chemistry, University of Delaware, Newark, Delaware 19716 Received: February 16, 1993; In Final Form: May 27, I993
An analysis is described of the kinetics and mechanisms of thermal decomposition of 2,4,6-trinitrotoluene (TNT) in the bulk phase. The basis is the decomposition kinetics of mono-, di-, and trinitrobenzene and the corresponding toluene derivatives in the temperature dimension. Below about 770 O C (1100 ps reaction time), the initiation chemistry is dominated by oxidation reactions of the methyl group. Above about 770 OC C-NOz homolysis dominates the initiation process. Shock initiation is therefore dominated by C-NO2 homolysis while -CHs group oxidation and C-NO2 homolysis compete in controlling impact initiation. Neither of these processes controls the time-to-explosion at times longer than 0.1 s. Instead, time-to-explosion is controlled by catalysis from the decomposition products of TNT. This analysis reconciles conflicting observations and statements previously made about the chemical mechanisms that control the explosive behavior of TNT.
I. Introduction Much contradictory and fragmentary information exists concerning the chemistry that controls slow thermal decomposition, impact initiation, and shock initiation of 2,4,6-trinitrotoluene (TNT). It is not clear from these data how the kinetic constants relate to the reaction mechanism, whether or how the mechanism changes with temperature, and which kinetic constants should be used for models of explosive behavior. This article addresses these issues.
II. Background
No2
Considerable evidence exists that thermal decomposition processes of explosives are related to their sensitivity to impact and shock energy possibly to the detonation properties,&l2 and to the time required for an explosion to occur at the given temperature (time-to-explosion or TTX).13-19 Consequently, thermal decomposition chemistry along with numerous physical factors are fundamentally important in the explosives field." By focusing on chemistry alone in this article, we do not wish to minimize the major role in the explosive behavior played by the physical characteristics of the material. Specific reactions of initiation have been extensivelydiscussed for TNT but are contradictory as to whether -CH3 oxidation reactions or C-NO2 homolysis is the rate-determining step (RDS). TNTmeltsat 81 OC withoutdecompositionsothatdecomposition studiesare conducted in the melt and vapor phase. Ample evidence is available for the occurrence of -CH3 oxidation up to at least 400 0C3J1.19JI-30 and the fact that it is the RDS under these conditions.22 Extraction and analysis of the residue reveal that 2,4-dinitroanthranil and trinitrobenzene derivatives possessing substituents from oxidation of the -CH3 group (Scheme I) are formed.19JJl-'3 There is also analytical evidence for reduction of the -NO2 groups because nitroso, oxime, nitrile, nitrone, nitroxide, azo, and azoxy groups are present in the condensedphase resid~e.19-3"~ Bimolecular reactions are also known to play a major role in the condensed phase.38 Consistent with the Occurrence of -CH3 oxidation and N - O scission, the primary fragmentation product of TNT in a mass spectrometer involves loss of OH.2628,30,39." The importance of reactions of the C-H bond in the impact sensitivity has been noted.' In other work, C-NO2 homolysis is proposed to dominate the impact and shock sensitivity*14' and even be the RDS41 These assertions that reactions of the -CH3 group and the C-NO2 bond are rate controlling are contradictory at first sight. 0022-3654/93/2097-8759$04.00/0
*
COOH 02N*N02
O2NVNO2
(11
No,
No,
SCHEME II H @O
+
H&
9f
Some useful insights about the possible behavior of TNT at high temperature is available from shock tube4445 and laser p y r o l y ~ i sstudies ~ . ~ ~ of 2-nitrotoluene (2-NT) at 8fXL1000 OC. 2-NT condenses to anthranil, but C-NO2 homolysis is a competitive process (Scheme 11).u.45 The rate constant for isomerization of the CsHrN02 bond has also been estimated [ k = lot3exp(-55980/RT) s - * ] . ~ Keeping in mind the reactions in Scheme 11, the global Arrhenius constants for thermal decomposition of TNT in the liquid and gas phase are given in Table I. The induction phase is found to be a thermally neutral, first-order process in which the -CH3 group reactions in Scheme I begin to occur. In the acceleratory phase, catalysis by these decomposition products of TNT (autocatalysis) is a major factor. Becauseof the accelerating Q 1993 American Chemical Society
Brill and James
8760 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993
TABLE I: Arrhenius Parameters for Thermal Decomposition of TNT below 500 OC T,OC E,,kcal/mol A,rl methoda ref 203-283 298490 231.5-216.5 140-180 220-260 245-269 240-255 203-283 215-310 390-401 275-310 280-310 190-210 220-260
Induction Phase (-CH3 Oxidation) 40.9 f 1.6 ESR TTX 41.4 1013.2 1012.19*0.32 EG 43.4 0.8 46 f 3.1 EG 46.5 1012.9 EG IDSC 46.5 2.4 DTA 50.1 2.4
* **
Awleratory Phase (Catalyst Dominated) 30.2 f 0.6 ESR 32.0 f 2.5 TTX TTX 34 1011.4 EG 34.4 A 2.5 34.5 108.45 EG 109.3 EG 34.6 35.6 109 EG
300-350 400-450 791-901
42.5 49.5 f 1.3 51.52
10'0.22 1012,410.4 1013.08
1,3-(N02)2C6H4 1,4-(NOz)&jHd 1,3,5-(NOz),C& 2-NT 3-NT 4-NT
2.4-DNT
61.3 f 4.1 65.1 68.2 f 1.7 68 f 3 IO 2.1 68.6 f 2.5 67.3 f 0.8 61.41 61.0 f 2.2 65 68 61.5 65.9 64.29 68.2 f 2.0 61.4 f 1.1
*
-
10 42-
F%
. - 20--4
.. . *
-8
48 14 this work 14 23 24 52
-10 -12 -14 -16
11 1
0.0006
C-NO2 ISOMERIZATION C-NO2 HOMOLYSIS CYCLIZATION 0.0008
0.001
.'z
0.0012
'.-'I
aI
I
0.0014
1/T K Figure 1. Arrhenius plots of the rate constants for decomposition of 2-nitrotolueneu by the C-NOz homolysis, cyclization (anthranil formation), and nitro-nitrite isomerization channels. 1.3 1.2 1.1
ref 25 58 44
TABLE IIk Arrhenius Data for C-NO2 Homo1 sis in the Gas Phase from Nitrobenzene and Nitrotoluene (NT) Compounds "pd T,OC E,, kcal/mol A, s-l ref CsHsNOz 410-480 69.1 f 1.4 lO"."fo.' 59 455-530 191-907 821-971 420-480 821-911 420-410 380-410 191-907 821-911 300-350 400-410 300-350 400-490 191-907 821-911 821-911
12
86-
48 49 50 51 52 53 54
a EG = Manometric measurement of evolved gas, TTX = time-toexplosion (exotherm), IDSC = isothermal DSC.
TABLE Ik Arrhenius Data for Condensation of 2-Nitrotoluene to Anthranil (Scheme II) T,'C E,, kcal/mol A, s-l
14 -
60 44 41 59 41 59 59 1014.82 44 1016"M.6 41 1017.88 25 1016.9 59 10'6.'3 25 1016.7 59 10'4.89 44 41 1015.3M0.4 41
1015.3 1015.'M.5 1016.8M.5 1014.5M.5 1017.iM0.5 10i7.2fo.2
nature of the process, first-order autocatalytickinetics is followed. The activation energies from several studies2,55*56are not included in Table I, because they are clearly altered by the experimental conditions and do not reflect the true kinetics of TNT. The rate of decomposition of TNT is hardly affected by pressure in the range of atmospheric to 50 kbar.57 The Arrhenius data for conversion of 2-NT to anthranil in the gas phase are given in Table 11. Table IIIcompiles Arrhenius data for C-NO2 homolysis of nitrobenzene and derivatives in the gas phase.
III. Relatiomhips between Thennal Decomposition Mechanisms and Kinetics of TNT Numerous reactions m u r when TNT decomposesin the melt phase below 400 OC. In reality, few details are firmly known. However, the dominating initiating processes reduce to those mentioned in the foregoing discussions, i.e., -CH, oxidation, C-NO2 homolysis, and catalyst-dominated readions. Evidence for the formation of 2,4-dinitroanthranil and various oxidation products of the -CH3 group is 3-fold. First is the primarydeuterium isotope effect ( k ~ /=k1.66 ~ f 0.2) observedzz in the induction phase of TNT compared to TNT-a-d3. This
n
1
'0 0.9 4
2
c-
0.8
0.7 0.6 0.5
0.4
b 0.3 0.2 0.1 0
O6
time(1og scale), sec Figure 2. A plot enabling analysis of the time required to complete the reaction or prows noted at agivcntemperature. Thecatalysis-dominated acceleratory phase c a s e s to be important below about lF3s. Timeto-explosion data from four studies are shown.
result indicates that the C-H bond ruptures in the RDS. This process is undoubtedly not unimolecular C-H bond homolysis but rather is a concerted or bimolecular reaction. Second is the fact that compounds in which the methyl group is partly or totally stripped of H are isolated from the residue.19*21-3>34 Third is the similarity of the Arrhenius data in the induction phase (Table I) to thosefor condensation of 2-NT to anthranil (Table 11). Taken together these results are solid evidence that the induction stage data in Table I are dominated at the temperatures studied by 2,4-dinitroanthranil formation and other oxidation reactions of the -CH3 group. C-NO2 homolysis is the primary decomposition channel of nitrobenzene molecules lacking a reactive substituent ortho to -N02. The activation energies for these molecules resemble the C-NO2 bond dissociation energy of 7 1 f 2 kcal/mol:' as might be expected from the Evans-Polanyi formalism.61 Catalysisby decomposition products is the dominating influence on the rate of the acceleratory phase (Table I). E, values in the acceleratory stage are smaller than those in the induction stage because of catalysis and bimolecular processes.52 Evidence for catalysis is further supported by the fact that 2,4-dinitroanthranil," 2,4,6-trinitrobenzaldehyde,l9and virtually all other sources of radicals strongly accelerate thedecomposition rate of TNT.14J2 Nitro-nitrite isomerization(Scheme 11) has not been confimed for TNT or any other nitrobenzene derivative in the condensed phase.62 The only evidence pertains to the gas phase and it is somewhat circumstantial.44947 However, decomposition of the nitrite is a plausible source of the large amount of NO(g) that
Kinetics and Mechanisms of TNT
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8761
2.4
2.2
-E
2.1
w
E-.l
d 4
0
-
2.3
2 -
1.9
-
1.8
-
1.7
-
A
1.5 1.4 1.3 1.2 1.1 1.6
+ + A
1
I
1.5
1.48
1.52
is generated by many polynitrobenzene derivatives upon rapid thermal decomposition.62
IV. Temperature Dependence of the Decomposition Reaction Rates To evaluate the relative importanceof the initial decomposition reactions at each temperature, the Arrhenius data for the three reactions of 2-NT shown in Scheme I1 were plotted in Figure 1. C-NO2 homolysis and the condensation reaction to anthranil are isokinetic at about 950 "C. Above 950 OC, C-NO2 homolysis is the favored reaction. Nitrwnitrite isomerization is not favored at any temperature, but we stress that the rate of this reaction has only been estimatedSu An analysis of the relative rates of reactions for TNT could be constructed similarly to that done in Figure 1 for 2-NT. It is more instructive, however, to present the data in terms of the reaction time at a given temperature because these variables are especially relevant to understanding the initiation behavior of an explosive. A modified version of the Arrhenius equation, eq 3,
T, =
ETHn-2,4,6-TNB ISOPROPYL-2,4,6-TNB
P -a
R(ln A
+ In At)
(3)
was employed to compute the time required to essentiallycomplete a process (At) at a given temperature (Tm). The rate data for 1,3,5-trinitrobenzene in Table I11 were chosen for the C-NOI homolysis reaction. The carefully determined rate data of Cook and AbeggNin Table I werechosen as representativeof the average for the induction stage which is dominated by -CH3 oxidation. The rate data of RobertsonI4are representative of the catalystdominated acceleratorystage. Figure 2 shows the result. C-NO2 homolysis and -CH3 oxidation for TNT are isokinetic at about 770 OC. Above 770 OC C-NO2 homolysis is favored. The time required to complete the reactions at 770 'C is about 0.1 ms according to Figure 2. By using Figure 2, the major initiation conditions (time-to-explosion, impact, and shock) now can be analyzed in terms of classes of reactions. Time-to-Explosion. The length of time before an explosion occurs at a particular temperature is an important hazards index of a material.l*163 The balance between heat generated by reactions and heat lost to the surroundingsis the principal physical factor that controls the TTX.13J5.16 Figure 2 shows TTX data
1.54
1.56
1.58
for TNT from Wenograd's data2 at about 1 ms to the data of Dacons et al. at 16 h.'9 This is eight logarithmic decades of time. For heating times longer than about 0.1 s, the catalyst-controlled kinetics (acceleratory stage) and not the -CH3 group oxidation kinetics (induction stage) describes the TTX behavior. For heating times in the vicinity of 0.01 s, the TTX data2 depart from being catalysis controlled and become increasingly dominated by the -CH3 oxidation and C-NO2 homolysis kinetics. Some uncertainty exists in TTX measurements at these short times because it is experimentally difficult to achieve rapid enough heat transfer to the explosive to be confident of the true temperature of the explosive.64 However, the general pattern is unmistakable over eight logarithmic decades of time. Catalysis stage kinetics should be used for TTX models of TNT when the heating time is greater than about 0.1 s. Induction stage kinetics (C-NO2 and/or -CH3 group oxidation) should be used for heating times shorter than about 0.01 s. The fact that catalyst-controlled kinetics determines the TTX of nitroaromatic explosives is the probable reason why Ea values from TTX never correlate with the structure and electronic features of the parent molecule,62 such as the substituent linear free energy relationships of the substituents. Impact Initiation. During mechanical impact, such as in the drop-weight impact test, the duration of energy input is about 0.025 ms2J and the temperature achieved by TNT may be about 1000 OC. The results in Figures 1 and 2 indicate that the rates of C-NO2 homolysis and -CH3 group oxidation are competitive under these impact conditions. This findingexplains contradictory assertionsabout the RDS of nitroaromaticexplosives under impact conditions. Kamlet and Adolph) and Bliss et al.65 found that nitroaromatic explosives possessing an a-CH bond ortho to-NO2 were generally more impact sensitive for a given oxidant balance than those without thisstructure. This result implies that reactions involving the -CH3 group have a significant role in controlling the impact ~ensitivity.~ A directly contradictory assertion was made by Owens et al.41.42and Murray et aL43who reported that impact sensitivity could be correlated with the electrostatic potential at the midpoint of the C-NO2 bond, v&, for a series of nitroaromatic compounds. vhd is related to the strength of the bond. Although directly contradictory to one another, Figure 2 indicates that these two conclusions are each at least partly correct. At the temperature and duration of energy input during
Brill and James
8762 The Journal of Physical Chemistry, Vol. 97, No.34,1993
TABLE I V Comparison of the Activation Energy for Thermal Decomposition of Akyl-2,4,~trinitrobemene Derivatives Measured by Two Different Metbods E,, kcal/mol alkyl group -CH3 -CHzCH3
EG24
34.5 20.0
E,, kcal/mol
TTX
alkyl
(this work) 34 22
group -CH(CH3)z -C(CH3)3
TTX EGZ4 (this work) 31.1 39.7
29 35
impact, there are significant contributions from both the -CH3 oxidation and C-NO2 homolysis kinetics. The reported correlations witb both processes are, therefore, expected and not at all in conflict. One might expect to find a diluted H/D isotope effect in the dropweight impact sensitivity of TNT. Bliss et al.65 suggest that -CHp may simplybe acting as a diluent of the oxidant balance. Shock Initiation. The energy input during shock initiation occurs in 1 ps or less, and the temperature exceeds 1000 oC.66 According to Figures 1 and 2, C-NO2 homolysis of TNT is kinetically favored and should be used for shock initiation models that contain chemical kinetics. In accordance, C-NO2 homolysis has been proposed to be the RDS in shock initiation on the basis of correlations of structure and electronic properties of several polynitrobenzeneexplosives.41 C-NO2 homolysis in the gas phase appears to leave the aromatic ring with considerable internal energy to drive further reactions.6' Directly contradicting the apparent dominant role of C-NO2 homolysis in shock initiation is the suggestion by Bulusu and Autera4 that the mechanism of slow decomposition and shock initiation are the same. They arrived at this conclusion on the basis of an observed deuterium isotope effect when the shock initiatiqn threshold pressure (H/D = 1.09-1.14) and detonation velqity (H/D = 1.04-1.06) were measured for TNT and TNTa-d3. The isotope effect for slow decomposition of these compounds is kH/kD = 1.66 f 0.2 in the induction phase22 and is predominantlycontrolledby-CH3 group oxidation. The isotope effect under shock and detonating conditions is clearly diluted, probably because of the important role of the C-NO2 homolysis channel. In fact, Storm et alO5found that TNT is an outlier on a plot of shock sensitivity vs impact sensitivity for 21 explosives. TNT is about twice as shock sensitive as predicted by its impact sensitivity. This result suggests that the dominant controlling processes in the shock and impact sensitivity of TNT are not the same. V. "X Data for TNT in the 1-5-s Range
TTX data for this article were determined for TNT and other alkyl-substituted 2,4,6-trinitrobenzene compounds by using the T-jump/FTIR method.68 Values of E, were calculated14from a plot of In TTX vs 1/ T. A film of sample of about 200-pg mass was heated at 2000 "C/s to the chosen temperature by resistive heating on a pt ribbon filament. The time lapse before the platinum filament sensed violent heat release was measured. A pressure of 20 atm of Ar gas in the cell suppressed evaporation to a large extent. The TTX data are shown in Figure 3. The resulting E, values are compiled in Table IV along with those determined by Maksimov and Pavlik" from the rate of gas evolution. The E, values measured by these two different methods are similar, but they bear no obvious relationship to the Hammett functions for the four substituents. The reason for this, as discussed above, is the fact that catalysis chemistry plays the dominant role in the reaction rate. However, the relative rates of gas evdution" and times-to-exotherm at a given temperature contain the useful insight. Within the substituents that posses an a-CH bond, the isopropyl derivative would be expected to, and does, decompose at the fastest rate at a given temperature. The rate of gas evolution when tert-butyl is the substituent is 30-50 times slower than that of theother three substituents which
all possess an a-CH bond." Consequently, the simple presence or absence of an a-CH bond is the factor that moat dominates the difference in the reaction rates. Likewise in Figure 3, note that for a given time below a temperature of 375 "C, the rertbutyl derivative must be T-jumped to a higher temperature than the other derivatives to achieve the exotherm. These TTX data and the EG data reflect the same pattern of sensitivity for this series of compounds, but E. alone by either measurementcontains no insight. This is because catalysis plays the dominant role in the reaction rates at these times and temperatures as opposed to primary reactions of the parent molecule.
Acknowledgment. We are grateful to the Wright Laboratory, Armament Directorate, Eglin AFB, FL (Dr.R. L. McKenney, Jr,), for samples and financial support of this work. References and Notes (1) Robertson, A. J. B.; Yoffe, A. D. Nature 1948, 161, 806. (2) Wenograd, J. Trans. Faraday Soc. 1%1,57, 1612. (3) Kamlet, M. J.; Adolph, H. G. Propellants Explos. 1979, 4, 30. (4) Bulusu, S.; Autera, J. R. J. Energ. Mater. 1983, 1, 133. (5) Storm, C. B.; Stine, J. R.; Kramer, J. F. In Chemistry and Physics of Energetic Materials; Bulusu, S., Ed.;Kluwer Academic Publ.: Dordrecht, The Netherlands, 1990; p 605. (6) Cook, M. A.; Horsley, G. S.;Partridge, W. S.; Ursenbach, W. 0.J. Chem. Phys. 1956, 24,60. (7) Cook, M. A.; Mayfield, E.B.; Partridge, W. S. J. Chem. Phys. 1975, 59, 675. (8) Zeman, S. Thermochim.Acta 1979, 31, 269. (9) Zeman, S.Thermochim. Acta 1980,39, 117. (10) Zeman, S. Thermochim.Acta 1980, 41, 199. (1 1) Zeman, S. Thermochim.Acta 1981,19, 219. (12) Zeman, S.; Dimun, M.; Truchlik, S. Thermochim.Acta 1981, 71, 181.
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SSSR,Ser. Khim. 1978,615. (60) Maksimov, Y.Y.;Sorochkin, S.B.; Titov, S.V. Tr. Imt.-Mosk. Khim.-Tekhnol. Inst. im. D. I. Mendeleeua 1980, 112, 26. (61) Evans, M. G.; Polanyi, M. Trans. Furaday Soc. 1938, 34, 11. (62) Brill, T. B.; James, K. J. Chem. Rev., in press. (63) Henkin, H.; McGill, R. Ind. Eng. Chem. 1952,44, 1391. (64) Shepherd, J. E.; Brill, T. B. Tenth Symp. (Inf.)Detonation; Office of Naval Research; Arlington, VA, in press. (65) Bliss, D. E.; Christian, S. L.; Wilson, W. S . J. Energ. Mater. 1991, 9, 319. (66) Kanel, G. I. Fiz. Goreniya Vzryua 1978, 14, 113. (67) Meyerson, S.;Vander Haar, R. W.; Fields, E. K. J . Org. Chem. 1972, 37, 41 14. (68) Brill, T. B.; Brush, P. J.; James, K. J.; Shepherd, J. E.; Pfeiffer, K. J. Appl. Spectrosc. 1992, 46, 900.
