5509
J. Phys. Chem. 1991,95, 5509-5517 surrounding medium, has time constants of the order of 10 ps for aliphatic solvents." During the interval between internal conversion and vibrational cooling, any primary merocyanine form may further isomerize because of its high temperature. Therefore it may not be possible to isolate the primary isomer, but instead a broad, nearly even distribution of isomers will be created. The temperature of a merocyanine isomer may be estimated from its vibrational spectrum and the internal energy. From a simulation of the vibrational spectrum, we estimate that UV excitation leads to an initial temperature Tinitial 940 K for a primary isomer; corrections for mode anharmonicities imply a somewhat lower value. Now assume a B1 B2 isomerization rate kbo = A exp(-Hh*/RTiihl) 5 X 10" s-l at Tinitw= 900 K. The frequency factor A should be higher than that for the ringclosure reaction, which entails a large negative activation entropy.' With A = 3 X IO" s-l, one obtains Hbo' = 30 kJ mo1-l. This compares well with barriers to &/trans isomerization in cyanines." The "roulette model" of the spiropyran-merocyanim conversion described so far is also compatible with the relaxation studies at
- -
(40)Sukowski, U.; Seilmcicr, A.; Elsaacseer, T.; Fischer, S.F. J . Chem. Phys. 1990,93,4094. 141) Schbffel. K.: Dietz.. F.:. Kroasner. Th. Chem. Phvs. Lcrr. 1990.172. 187 and referen& therein.
low temperatures, if control of isomer distribution is ascribed to the rigid medium: it blocks mt reaction paths with the exception of that leading to Bl. However neither our model nor the relaxation studies are compatible with resonance Raman studies," which concluded that only one isomer is present at room temperature in aliphatic solvents. At present we cannot give an explanation of this discrepancy. 5. Conclusion
Our results indicate that the photochemical ring-opening reaction of unsubstituted spiropyrans contains three major elements: (i) geometric complexity for the (likely) barrierless reaction in the S1state; (ii) internal conversion and generation of merocyanine products within 2 p;(iii) subsequent isomerization prior to vibrational cooling. Further experiments would greatly benefit from semiempirical calculations probing the potential energy and dipole moment of the SI state, around the spiropyran geometry. Acknowledgmenr. We thank Dr. B. Dick for stimulating discussions and for parts of Figure 2 and Prof. Alexander M[lller for the loan of equipment. We are grateful to Prof. F. P. SchHfer and the Deutsche Forschungsgemeinschaft for support through the Leibniz Prize Program.
Photochemical and Thermochemical Decomposition of 3-Nitre1,2,4-triazoCS-one and Perdeuterio-3-nltro-1,2,4-triazol-[i-one in Neat and Mixed Systems Joseph A. Menapace,* John E. Marlin, Frank J. Seiler Research Laboratory (AFSC), USAF Academy, Colorado 80840-6528
Dan R. Bruss,**+and Regina V. Dascher Albany College of Pharmacy, Albany, New York 12208 (Received: November 29, 1990) Electron paramagnetic resonance spectroscopy (EPR) and high performance liquid chromatography (HPLC) are used to (NTO), NTO-d2, NTO (NTO-d2) in acetone (acetone-d& and examine the decomposition of 3-nitro-l,2,4-triazol-S-one NTO (NTO-d&in 2,4,6-trinitrotoluene (TNT) (TNT-d5) under photochemical and thermochemical decomposition conditions. Room-temperature photochemical decomposition of NTO (NTO-d2)/acetone(acetonads) solutions monitored by EPR shows that the NTO nitro group abstracts hydrogen atoms from other NTO species and/or acetone as evidenced by the observation of EPR spectra associated with NTO hydroxy nitroxide radicals. The aryl hydroxy nitroxide and aryl benzyl nitroxide radical spectra observed in the NTO (NTO-d2)/TNT (TNT-d5) thermochemical decompositions at 370 K also show that nitro group/hydrogen abstraction reactions are operative in these systems. The hydrogen abstraction reactivity order present in these systems is found to be N 0 2 m / H m > N0-/CH3,m > NOm/NHm N02m/CH3,m z 0. These experiments also indicate that the mixtures undergo accelerated decomposition relative to the neat materials due to contributions from cross reactions occurring between NTO and TNT. The HPLC global kinetics studies conducted between 498 and 518 K show that the loss of NTO (NTO-d2) proceeds by a solid-stateglobal autocatalytic reaction scheme for which initiation and propagation reactions contribute to the observed kinetic behavior. The decompositions are found to possess an average global activation energy of 88.0kcal/mol. A primary kinetic deuterium isotope effect of 1.67 (2.44at 298 K)is also observed which indicates that nitrogen-hydrogen bond cleavage plays a significant role in the loss of NTO (NTO-d2).
I. Introduction
0
Problems d a t e d with catastrophic mishaps and sympathetic detonations have made common explosives such as 2,4,6-trinitrotoluene (TNT), hexahydro-l,3,5-trinitro-l,3,5-triazine (RDX), and octahydro-l,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) less attractive for use in certain applications and have s p u d efforts to synthesize new insensitive high explosives (IHE) which meet more stringent chemical and physical requirements.' 3-Nitro-l,2,4-triazol-5-one (NTO)Zis currently receiving considerable attention from both defense and civilian sectors as a 'Current address: Department of Chemistry, Central College, Pella, IA 50219.
H,NXNdH k=( No2
NTO
potential new generation IHE which is energetically comparable to RDX and which possesses sensitivity and stability characteristics (1) Coburn, M.D.; Harris, E. W.; La,K.-Y.; Stinecipher, M. M.;Hayden, H. H. I d . Eng. Chem. Prod. Res. hv.1986, 25, 68. (2) (a) Lee,K.-Y.; Chapman, L. E.;Coburn, M.D. J. Enrg. Marer. 1987, 5, 27. (b) La,K.-Y.; Coburn, M. D. 3-nitr~l,2,4-triazol-S-one,A Less Sensitive Explosive; LA-10302-MS, Los Alamos National Laboratory: h Alamos, NM, 1985. (c) Ritchie, J. P. J. Org. Chem. 1989, 51, 3553.
This article not subject to US. Copyright. Published 1991 by the American Chemical Society
5510 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 comparable to 1,3,5-triamino-2,4,6-trinitrobenzene(TATB).2.3 The stability characteristics of this class of compounds are of vital importance, as the materials can be subjected to extreme conditions while being transported, stored, or prepared for use. This paper reports our observations on the global kinetics and reaction intermediates present in thermochemical and photochemical decompositions of NTO and its perdeuterio analogue, NTO-d2. The global kinetics for the thermochemical decomposition of neat NTO (NTO-d2) occurring between 498 and 518 K are analyzed by using high performance liquid chromatography (HPLC). Electron paramagnetic resonance (EPR) spectroscopy is employed to identify paramagnetic decomposition intermediates appearing in the room-temperature photochemical decomposition of NTO (NTO-d2) in acetone (acetone-d6)and in the thermochemical decomposition of NTO (NTO-d2)/TNT (TNT-d5) mixtures at 370 K. Acetone (acetone-d6)is used in the photochemical decompositions since it is one of the few solvents in which NTO is slightly soluble.3b The acetone methyl substituents are also capable of donating hydrogen to NTO during the decomposition which makes it possible to study solvolysis reactions involving the NTO functionalities. Acetone-d6 causes the decompositions to slow down these solvolysis reactions to the point where the solvent acts as a spectator during the early stages of the decomposition due to kinetic deuterium isotope effects' (KDIE); vide infra. Thus, the KDIE makes it possible, through EPR radical intermediate identification, to characterize reactions occurring between NTO species without masking from NTO/solvent radical adducts. The NTO (NTO-dJ/TNT (TNT-d5) mixtures are being studied in the low-temperature thermochemical decompositions to elucidate the chemistry involved in reactions of NTO in the presence of other materials containing similar functionalities, as well as to assess the compatibility of these two energetic materials for use in melt-cast explosive mixture^.^ In particular, the mixtures were prepared to determine whether reactions between the NO2 and NH groups on NTO and the CH3 and NO, groups on TNT are likely to occur in the vicinity of the typical processing temperature for TNT melt-cast systems. These reactions, if present, can be crucial to the safety and reliability of this type of explosive mixture. While the thermochemical and photochemical decompositions of nitroaromatic and nitramine type explosives have been studied in detail for some time?' these experiments are the first involving (3) (a) Dobratz, B. M. LLNL Explosives Handbook Properties of Chemical Explosives and Explosive Simulants; UCRL-52997; Lawrence Livennore National Laboratory: Livennore, CA, 16 Mar 1981. (b) Minutes of the Technical Meeting for the Working Party for Explosives. Joint Ordnance Commanders' Group for Munitions Development, Working Party for Explosives, U.S. Air Force Academy, CO, 28-30 Jul 1987. (4) (a) Moore,J. W.; Pearson, R. G.Kinetics adMeehanism, 3rd ed.; Wiley-Interscience: New York, 1981. (b) Carpenter, B. K. &termination ofckgcmlc Reaction Mechanlsmr; John Wiley B Sons: New York, 1984. (c) Lowry, T.H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 2nd ed.; Harper & Row: New York, 1981. (5) The USAF Annamcnt Laboratory, Eglin AFB, FL, and Frank J. Seila Rwarch Laboratory, USAF Academy, CO, are currently performing and evaluating preliminary bomb loading and compatibility experiments on NTO/TNT mixtures. (6) (a) Dawns, J. C.; Adolph, H. G.; Kamlet, M.J. J . Phys. Chem. 1970, 74,3035. (b) Shackelford,S. A.; Coolidge, M.B.; Goshgarian. B. B.; Loving, B.A.; Rogm, R. N.; Janney, J. L.; Ebinger, M. H. J . Phys. Chem. 1985,89, 31 18. (c) Rogm, R. N . Anal. Chem. 1970.39,730. (d) Kimura, J.; Kubota, N . Propellanrs, Explos. Pyroteeh. 1980.5, 1 . (e) Gonzalez, A. C.; Larson. C. W.; McMillen, D. F.; Golden, D. M. J . Phys. Chem. 1985.89,4809. (f) Shackelford. S. A.; Goshgarian, B. B.; Chapman, R. D.; Askins, R. E.; Flanigan, D. A.; Rogers, R. N. Propellants, Explos. Pyrotech. 1989,14,93. (g) Rogers, R. N.; Janney, J. L.; Ebinger, M.H. Thermochim. Acta 1982, 59,287. (h) Rogers, R. N . AMI. Chem. 1972,II. 1336. (i) Beckmann, J. W.; Wilkes, J. S.; McGuire, R. R. Thermoehim. Acta 1977, 19, 1 1 1 . u) Bulusu, S.; Weinstein, D. I.; Autera, J. R.; Velicky, R. W. J. Phys. Chem. 1986, 90,4121. (k) Shackelford, S. A.; Beckmann, J. W.; Wilkes, J. S. J . Org. Chem. 1977, 12,4201. (1) Bulusu, S.;Autera, J. R. J . Energ. Mater. 1983, I , 133. Shackelford, S. A. J. Phys. (Paris)1987, 48, C4-193. (7) (a) Menapace, J. A.; Marlin, J. E. J . Phys. Chem. 1990,94,1906. (b) McKinney, T.M.;Warren, L. F.; Goldberg, 1. R.; Swanson, J. T. J . Phys. Chem. 1986, 90. 1008. (c) Guidry, R. M.; Davis, L. P. Thermochim. Aero 1979,32, 1. (d) Swanson, J. T.;Davis, L. P.; Dorey, R. C.; Carper, W. R. Magn. Reson. Chem. 1986,24,762.
Menapace et al. the kinetics and intermediate identification for the decompoQition of the nitro heterocyclic explosive, NTO, in both neat and mixed systems. The impetus for these studies centers upon elucidating the detailed nature of the decomposition process which may provide insight into the structural features that are important in the decomposition of NTO under a variety of stimuli. Elucidating the role of these structural features could spark potential synthetic avenues to more efficient energetic materials as well as a better understanding of critical parameters that affect the performance of this class of explosives. The present experiments are designed to a d d m four questions conceming the detailed nature of the decomposition of NTO (1) What is the temporal profile for the loss of NTO during the decomposition event? (2) What are the corresponding global decomposition kinetics parameters for the overall decomposition? (3) Is N-H bond cleavage central to a mechanistic rate-limiting reaction governing the loss of NTO? (4)What NTO reactions and functionalities are important in the decomposition under both photochemical and thermochemical conditions? 11. Experimental Procedures The NTO,ZbTNT,* and TNT-d5*samples used in the experiments were prepared by personnel from the Frank J. Seiler Research Laboratory Energetic Materials Research Division using established synthetic procedures. NTO was purified before use by recrystallization from distilled/deionized water (Nanopure) and by vacuum-drying for 8 h at room temperature. The recrystallized product possessed a melting point between 541 and 543 K (lit. 538-541 K).3 A 'H NMR spectrum of NTO in perdeuteriodimethylsulfoxide (DMSO-d6) yielded 12.76 and 10.92 ppm chemical shifts for the NH protons (lit. 12.7 ppm).) W a d 2 was prepared by dissolving previously recrystallized NTO in refluxing D20 (MSD Isotopes) for 1 h, after which time the solution was cooled to 273 K and the NTO-d2 collected by vacuum filtration. The recrystallized product was then vacuum-dried for 8 h at room temperature. Mass spectral analysis (direct insertion mass spectrometry) indicated 95% conversion to the perdeuteriated species with 5% monodeuteriated impurity. No evidence of the presence of protonated species was observed. The NTO-d2 was used without further purification. The TNT and TNT-d5samples were purified by recrystallization in an ethanol (HPLC Grade, Aldrich Chemical)/water (Nanopure) mixture and vacuum-dried for 8 h at room temperature before use. The acetone (HPLC Grade, 99.9+%, Aldrich Chemical) and acetone-d6 (99.996, Cambridge Isotope Laboratories) solvents were used without further purification. The 'H NMR analyses used to characterize the samples were conducted on a JEOL FX90Q FT-NMR 90-MHz spectrometer. Mass spectral characterizations were accomplished using a Hewlett-Packard 5985B GC-MS spectrometer equipped with a direct insertion probe accessory. HPLC Thermochemical Decomposition Kinetics of Neat NTO (NTO-d2). Global decomposition kinetics experiments on neat NTO were conducted isothermally at six temperatures: 498,503, 508,510.5, 513, and 518 K. The decompositions were carried out using reaction tubes prepared from standard Pyrex stock measuring either 200 X 4 mm or 200 X 8 mm. The two tube sizes were selected for use in the experiments to determine whether contact with the atmosphere or the sample tubes influenced the global decomposition kinetics of the neat samples. Each tube was flame-sealed on one end and oven-annealed. The reaction tubes were cleaned in a potassium hydroxide/methanol bath, washed with water, and dried overnight at 373 K prior to use. NTO samples (100f 0.4mg) were placed into each of 13 reaction t u b used in a particular temperature run. In some runs, the NTO samples were ground by an agate mortar and pestle prior to weighing and placement into the tubes to determine whether the global decomposition kinetics were influenced by sample surface (8) Dorey, R. C.; Carper, W. R. J . Chem. Eng. Data 1984, 29, 93. TNT-15 was synthesized by using toluene-d8 starting material.
Decomposition of NTO and NTO-d2 area or particle size. In other experiments, small NTO crystals were used. Twelve of the filled tubes were placed in a Tecam SBL-1 fluidized bath preheated to the specified temperature for the decomposition. The bath temperature was maintained within f0.5 K of the specified decomposition temperature by means of a Tecam TC4D fluidized bath controller. Temperature stability and accuracy were verified in situ by a Luxtron Model 750 fluoroptic thermometry system and a MIW type probe. All sample tubes placed in the fluidized bath were loosely fitted with corks to prevent sand from entering and contaminating the material while allowing for adequate ventilation of gas-phase decomposition products. The remaining reaction tube was used as a control sample to establish the HPLC calibration and time zero measurement for each decomposition. Individual reaction tubes were subsequently removed from the bath at specific times and immediately placed into ice. After cooling, the samples in the tubes were dissolved in DMSO and transferred to individual 5-mL volumetric flasks. To assure quantitative transfer of material from the tubes, each tube was washed with several aliquots of DMSO, and the washings were added to the volumetric flasks. Each volumetric flask was filled with additional DMSO to provide a total of 5 mL of solution. The samples were prepared for HPLC analysis by transferring 100 pL of each of the solutions to individual 4-mL vials. Each sample was subsequently diluted with 2.500 g of water. Decomposition reactions were similarly carried out with 50 f 0.4 mg of NTO at 503,508,510.5,513, and 518 K and for 100 f 0.4 and 50 f 0.4 mg NTO-d2 samples at 498, 508, 513, and 518 K. The loss of NTO (NTO-d2) during the decompositions was measured by use of a Hewlett-Packard 1090 or Beckmann 110B/ 166 HPLC chromatograph equipped with a UV/vis detection system operating at 254 nm. Material amounts were measured as elution peak areas as recorded by a Hewlett-Packard 3390A integrator or by a microcomputer interfaced with the HPLC using Beckmann System Gold software. The measured peak areas for each sample were converted to milligrams of NTO (NTO-d2) for use in subsequent analyses. All resulting weights were normalized to 100 or 50 mg, depending on sample amount used, to correct for small variations in weighing. In addition, a minimum of three repetitive HPLC assays were carried out on each sample to ensure precision in NTO (NTO-d2) loss determination. Elutions were carried out with an isocratic water/ methanol solvent system (9010 at 5 mL/min flow) using a 25-cm C-18 reverse phase column (Whatmann ODS-5, Partisil5), HPLC grade methanol (Aldrich), distilled/deionized water, and a 20-pL injection loop. The solvent system was acidified with acetic acid (1 vol %) for ion suppression of NTO during the elution^.^^,^^ In all HPLC runs, 100-pL injections were made into the injection loop to ensure proper/reproducible loading of the injector. EPR Identification of Photochemical and Thermochemical DecompositionIntermediates. EPR spectroscopic techniques were employed to identify radical decomposition intermediatespresent in the photochemical decompositions of liquid mixtures containing NTO (NTO-d2) with acetone (acetone-d6)and thermochemical decompositions of NTO (NTO-d2) with TNT (TNT-ds). EPR spectra of the paramagnetic decomposition intermediatespresent in both sets of experiments were collected by use of a Bruker ESP300 EPR spectrometer operating in the first-derivative mode. All spectra were obtained with the spectrometer modulation frequency set at 100 kHz. Depending upon the sample being analyzed, the spectrometer modulation amplitude was set between 0.10 and 0.25 G peak to peak, and the incident microwave power was set between 10 and 15 mW. The isotropic radical g values were calculated from their respective spectrum center magnetic field shifts relative to that determined for a solid 1,l-diphenyl2-picrylhydrazyl (DPPH) standard reference. The known DPPH g value9of 2.0036 f 0.0003 was utilized in the computations along with each experimental microwave frequency measured by a (9) Alger, R. S. Electron Paramagnetic Resonance: Techniques and A p plications; Wiley: New York, 1968; p 414.
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5511 Hewlett-Packard 5350B microwave frequency counter. The EPR spectra collected were analyzed by computer, and spectral simulations were conducted for the radicals observed using an EPR spectrum simulation and analysis package developed at this l a b oratory.1° The parameters used in the simulations were determined from the experimental EPR spectra and included measured hyperfine splittings for the various spin species present as well as the average measured first-derivative peak-to-peak line width. The applied algorithms assume that the EPR spectra possess features with Lorentzian line shapes. The photochemical decompositionswere accomplished by using 0.50mL of 0.005M NTO or NTO-d2 solution (acetone or acetone-d6 used as solvent) loaded into Suprasil quartz EPR sample tuba (Wilmad Model 707-SQ). After loading, individual samples were placed into a Bruker ER-4102ST TElo2rectangular EPR cavity equipped with a horizontal grid which allowed in situ ultraviolet irradiation of samples. Sample irradiation was conducted with an Oriel 1000-W Hg/Xe (Hanovia Model L5173) high-pressure arc source located 30 cm from the EPR cavity irradiation grid. A lower wavelength limit of 190 nm was established for the incident ultraviolet light used in the experiments by placing a water bath between the arc source and the EPR sample cavity. EPR spectra were collected during irradiation after sufficient radical concentration was obtained. In this set of experiments, sufficient radical concentration was typically obtained between 15 min and 1 h after the start of the irradiation. Low-temperature thermochemical decomposition experiments on NTO/TNT, NTO/TNT-d5, NTO-d2/TNT, and NTO-d2/ TNT-dS mixtures were carried out using 50-mg samples (5050 by weight) placed into Suprasil quartz EPR sample tubes. After loading, individual samples were placed into a Bruker ER4106ZR-AC cylindrical acentric EPR cavity heated to 370 K. In these experiments, sample/cavity temperature was estab lished and maintained by means of a Bruker Model ER-4111 VT temperature control accessory and a combination of bottled nitrogen, heating Dewar interconnections, and transfer Dewars to direct nitrogen gas heated to a particular temperature over the samples. Sample temperatures were verified by a Luxtron Model 750 fluoroptic thermometry system equipped with a MIW-type probe by placing the probe tip along side the EPR tubes in the cavity. Sample temperatures were found to be within f0.2 K of the temperature set for the experiments. After the samples equilibrated to 370 K and the decompositions yielded adequate radical EPR signal (typically between 40 min and 1 h), EPR spectra were collected while the decomposition was in progress. Similar control experiments were also conducted on neat NTO (NTO-d2) and TNT (TNT-dS)to determine whether significant decomposition was present in the neat samples at 370 K. 111. Results
HPLC Thermochemical Decomposition Kinetics of Neat NTO (NTO-d2).The global kinetics experiments conducted on the neat NTO (NTO-d2) samples yield temporal information typical of that represented by the 513 K results shown in Figures 1-3. The decomposition rates for the temperatures studied are found to be dependent upon the initial amounts of NTO (NTO-d2) present: in the 50- and 100-mg decompositions, the observed rates of NTO (NTO-d2) loss are inversely proportional to the initial amount of starting material used with the 50-mg runs decomposing at a rate 4 times faster than that for the 100-mg runs (cf. Figures 1 and 2). On the basis of this observation, the kinetic data for both the NTO and NTO-d2 decompositions are interpreted by using an empirical autocatalytic rate expression of the form4..l1 d(NT0) -- k(NTO)(B) (1) dt ((NTO) + (B))2 ~~
~
~~
~~
~~~
(IO) Menapace, J. A. EPRFIT An Electron Paramagnetic Resonance
Modeling and Analysis Package, Version 1.10; Frank J. Sciler Research Laboratory, USAF Academy, CO, 1990. Copiw available upon request. (1 1) Capellos, C.; Bielski. 9. H. J. Kinetic Systems; Wiley-Interscience: New York, 1972.
5512 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
temp, f0.5 K 498.0 503.0 508.0
510.5 513.0 518.0
1@k, mol/s 1.5 f 0.4
100 mg lOb(NTO),,, mol 759.2 f 0.5
C
IO’B,,, mol 373.0 f 1.0
1@k, mol/s
754.8 0.5 772.8 f 0.1 754.8 f 0.2 760.8 f 0.2
50 mg I06(NTO), mol
C
408.0 f 1.0 704.4f 0.8 338.4 0.7 122.4 f 0.4
I@& mol C
C
3.7 f 0.2 7.7 f 0.1 12.5 f 0.3 19.9 f 0.5 b
C
C
8.8 f 0.4 11.7 f 0.3 19.6 f 0.1 47.2 f 0.7
Menapace et al.
12.5 f 0.3 14.4 f 0.2 17.4f 0.4 3.6 f 0.1 b
382.3 f 0.3 381.0 f 0.3 372.0 0.5 379.2 f 0.4 b
*
NTO-d2‘ 100 mg I06(NTO-d2)& mol 747.7 f 0.5 733.2 f 0.5 746.4 f 0.4 765.0 f 0.7
lek,mol/s
temp, k0.5K 498.0 508.0 513.0 518.0
0.9 f 0.4 4.8 f 0.3 12.2 f 0.2 27.0 f 0.8
lo1&,,mol 173.8 f 0.8 185.4 f 0.7 244.2 f 0.5 66.0 f 0.1
50 mg 106(NTO-d2)0, mol
I@k, mol/s C
C
5.1 f 0.1 11.9 f 0.2 26.5 f 0.7
I@& mol C
381.6 f 0.1 391.8 f 0.4 375.0 f 0.4
46.2 f 0.3 23.4 0.3 18.6 f 0.4
*
OParamctcrs are the average of two experimental runs at each temperature. *Severe sample degradation occurred which prevented analysis. See text. CNotanalyzed.
kn/kD temp. f0.5 K 498.0 508.0
513.0 518.0
5 0 ” run 1.67 f 0.1 b 1.80 0.1 1.51 0.04 1.61 f 0.03 1.67 f 0.05 1.75 f 0.06 b av 1.67 f 0.10
100-me run
*
*
*
E,(NTO) = 87.5 1.8 kcal/mol E,(NTO-d2) = 88.4 f 1.2 kcal/mol
1°1
OComputations are based on data presented in Table I. *Not computed. See Table I.
0
0
50
100
I50
200
250
Reaction Tim (Minutes) Figure 2. HPLC temporal profile for loss of NTO from a neat thermochemical decomposition at 513 K conducted using 50 mg of starting material. The line drawn through data points corresponds to the global rate expression described by eq 2 with the parameters in Table I. Notice that the apparent rate of decomposition is greater in this cast than for the 100-mgdecomposition shown in Figure 1.
0
50
100
150
200
250
3co
Reaction Time (Minutes) Figure 1. HPLC temporal profile for loss of NTO from a neat thermochemical decomposition at 513 K conducted using 100 mg of starting material. The line drawn through data points corrcaponds to the global rate expression described by cq 2 with the parameters in Table I. 0
for which (NTO) and (B) are milligrams of NTO (NTO-d2) and catalytic intermediate/products present at time 1. Upon integration, q 1 yields (NTO) =
which gives the amount of NTO present at various times during
Figure 3. HPLC temporal profile for loss of NTO-d, from a neat thermochemical decomposition at 513 K conducted using 50 mg of starting material. The line drawn through data points corrcaponds to the global rate expression described by eq 2 with the parameters in Table I. A KDIE of 1.67is observed when compared to the decomposition shown in Figure 2.
the decompositions with (NTO)o and (B)o being the amounts of NTO and catalytic intermediates/products present in the earliest stage of the decompositions. The global decomposition rate constants are obtained by performing a nonlinear least-squares fit’2on eq 2 using the experimental data with time and milligrams
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5513
Decomposition of NTO and NTO-d2 T A B U Ilk Sp&"@c
Mixtww
radical species" I 11 111
CbrrrctCriQticafor Radials Observed during Wotochemlal ad Tbermocbemid Decomporltiole of NTO lad hlTo-d~
oe
aN,d oe
an,' Oe
Id
g value (+0.0003)
8.467 8.467 8.467
4.728 4.728 4.633
1.356 1.356 1.256
7.021 7.021 1.090
0.432 0.432 0.350
2.0063 2.0062 2.0063
temp, A0.2 K
aN,b Oe
Id
10.196 10.196 10.196 10.600
a d Oe 2.843 0.670 0.670 2.720
an?
370 370 370 370
a d 0e 3.665 3.665 0.864
(A0.0003) 2.0057 2.0057 2.0057 2.0050
temp, K room room
aN? Oe
room
radical speciesa IV V VI VI1
ON:
oe
1 1.290
g value
0.920 0.950 0.800 0.800
"The radicals corresponding to the numbered entries are referred to in the text. bNitroxide 14Nhfs. eNTO ring position 2 "N hfs. dNTO ring position 5 14N hfs. 'Hydroxyl 'H (2H) hfs. /Line widths are measured as peak-to-peak values from the first-derivative EPR spectra. tortho and para methyl IH (2H)hfs for 3,5-dinitro-4-methylphenylsubunit. Benzyl IH (2H)hfs for 2,4,6-trinitrobenzyl subunit. -14.0
1
P
Hydroxy-235-Triazol~-One Nitroxide
ll-4
limo I
NTO in Acetone-&
-15.0?
-
0
%
P
-16.0-
L 4
; -I7':
-18.0-
22
*-
-I -1.015
-1.005
4.595
0.985
4.975
0965
4
-2
-1IRT (kcallmol)
Figure 4. Arrhenius plot of the rate constants determined from thermochemical decompositions of neat NTO and NTO-d2(Table I). The resulting activation energies are presented in Table 11.
of NTO as independent and dependent variables, respectively. (NTO)o, (B),,, and the rate constant k are taken as parameters in the fitting procedure. The mults of the global kinetics analyses are tabulated in Table I. Global KDIE ratios, kH/kD,determined from the NTO and NTO-dz decompositions appear in Table I1 along with the activation energies determined by an Arrhenius plot (Figure 4) of the global rate constants at each decomposition temperature. The kinetics results also show that differences in sample tube and sample crystal sizes do not affect the temporal profiles or the global decomposition rate constants for experiments conducted using the same amounts of starting material at temperatures of 518 K and below. Thus, chemical reactions that can occur with the reaction tube walls and with the atmosphere over the samples do not appear to significantly contribute to the observed decomposition kinetics. Deflagration is observed, however, in some of the late time samples taken from the 518 K runs (ca. 2-3 h). Several attempts were made to perform the decompositions at temperatures above 518 K;however, Severe sample deflagration occurred which precluded meaningful analysis of the data. In situ flumptic thermometry measurementson samples decomposed at 520 K show an abrupt temperature rise of at least 80 K when the samples undergo deflagration (ca. 10-30 min). Deflagrated samples present in the experimental runs were not included in the analysis of the decomposition kinetics (cf. Table I). Deflagration in our experiments becomes more prevalent with the faster autoacceleration rates present at higher temperatures.'&@ At temperatures above 520 K, the decomposition rates become fast enough that the deflagration process dominates over the thermochemical decomposition process present at temperatures 5 18 (1 2) The nonlinear last-quamfitting mutine d in the kinetics analysis u an implementation of the Ma uardt algorithm by J. A. Menapace, Frank J. Mler Racrrch Laboratory,U% Air Force Academy, CO. For a discussion of tbh algorithm ICC: Maquardt, D. W.1.Soc. I d u s r . Appl. Mark 1963, I I , 431.
I
-305;4~
3350
3360
3370
3380
Field (Gauss) Figure 5. EPR spectrum collected during ultraviolet irradiation of a NTO/acetone-d6sample at room temperature corresponding to radical I: (a) experimental EPR spectrum; (b) simulated EPR spectrum. Spectral parameters are prented in Table 111. Similar EPR spectra are observed for the NTO/acetoned, and NTO-d,/acetone systems (radicals I and 11).
K and below. For our experimental conditions and sample size, an estimate of the critical temperature for deflagration of NTO and NTO-d2 in the vicinity of 520 K is in agreement with the critical temperature of 500 K established for pressed and confined NTO by using the Henkin test.2b At all the temperatures studied, the decompositions take place in the solid state yielding solid polymeric products, or tars, which are insoluble in the solvents used to perform the HPLC analysis. No low molecular weight condensed-phasereaction intermediates or products could be isolated in the HPLC elutions. This most likely results from fast reactions that yield low steady-state concentrations for any stable, low molecular weight intermediates or products that may be present during the course of the decompositions. EPR Identification of Photocbemicd and Thermochemical Decomposition Intermediates. Figures 5 and 6 show the EPR spectra and the corresponding spectral simulations for the radical species present in the NTO (NTO-&)/acetone (acetone-d6) solutions 30 min after the start of ultraviolet irradiation. Of the four NTO/solvent deuteriation combinations studied, only two distinct radical EPR spectra are observed: one corresponding to the EPR spectra collected for NTO/acetone, NTO/acetone-d6, and NTO-d2/acetone decompositions and the other for the NTO-dz/acetoned6 decomposition. Measurement of the hyperfine splittings and the EPR peak intensities in the spectra for the former set of deuteriation combinations identifies the radicals present as species whose molecular structures are consistent with the hydroxy 4-oxo-2,3,5-triazolyl nitroxide, species I, and the hydroxy 4oxo-2,3,5-triazolyl-d2 nitroxide, species 11. The spectra associated with these radicals are composed of hyperfme splittings from three
5514 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 0
Menapace et al.
0
Hydroxy-2,3,.5-Triazol4-One Nitroxide-d, NTO-ii, in Acetone-d,
D,NXN-D
H,NXNeH
'.*;
.
'.4-
.,""I
I
YL0-H
Y-0-H
I
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inequivalent I4N nuclei (two ring and one nitroxide) and one hydroxy 'Hnucleus (Figure 5 and Table 111). No hyperfine splittings are observed for the 'Hor 2H nuclei attached to the ring nitrogens; thus, all three deuteriation combinations result in the observation of a single EPR spectrum for all three systems considered. The g values for these radicals are calculated at 2.0063 f 0.0003. Measurement of the hyperfine splittings and EPR peak intensities for the spectrum observed in the NTO-d2/acetone-d6 decomposition (Figure 6 and Table 111) identifies the radical present as being consistent with hydroxy-d 4-oxo-2,3,5-triazolyl-d2 nitroxide, species 111. In this case,the hydroxy *Hnucleus c a w
'"*0
D-NKN#D
.Q
u9
-100
4
-200
3340
3350
3360
3370
33m
Field (Gams) Figure 6. EPR spectrum collected during ultraviolet irradiation of a NTO-d2/acetoned6sample at room temperature corresponding to radical 111: (a) experimental EPR spectrum; (b) simulated EPR spectrum. Spectral parameters are presented in Table 111.
Hydroxy-3J-Dinitro4-MethylphenylNitroxide AT0 in TNT
YL0-D
111
the EPR spectrum to contract by 6.5 timed3the hyperfine splitting corresponding to that observed in radicals I and I1 which possess a 'Hnucleus in the same position. The g value for this radical is the same as that computed for the other three NTO/acetone systems studied. The low-temperature thermochemical decompositions of 5050 mixtures NTO (NTO-d2) in TNT (TNT-ds) yield four distinct EPR spectra after 1 h of heating at 370 K. These spectra correspond to radical structures consistent with hydroxy 3,S-dinitro-4methylphenyl nitroxide, species IV,hydroxy 3,s-dinitro4methylphenyl-ds nitroxide, species V,hydroxy-d 3,5-dinitro-4methylphenyl-ds nitroxide, species VI, and 2,4,6-trinitrobenzyl 3,5-dinitro-4-methylphenylnitroxide, species VII. The EPR
IV
4004', 3295
I
,
I
'
'
'
#
(
'
8
,
3305
I
I
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,
, ,
,
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, 1
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,
I
I
I
I
I
I
I
3325
I
I
I
I
I
I
3
3
3335
1
Field (Gauss) Figure 7. EPR spectrum collected during thermochemical decomposition of a NTO/TNT mixture at 370 K corresponding to radical IV: (a) experimental EPR spectrum; (b) simulated EPR spectrum. Spectral parameters are presented in Table 111.
Hydr0~-3J-Dinitro4-Methylphenyl Nitroxide-d,
V
hT0 in W - d ,
n
809
VI
VI1
spectra corresponding to radicals IV, V, and VI are shown in Figures 7-9 with the hyperfine splittings and g values tabulated in Table 111. The hyperfine splittings observed in these radicals arise from five equivalent ring 'H (2H) nuclei, a nitroxide I4N nucleus. The spectrum correnucleus, and a hydroxy 'H(zH) sponding to radical VI1 shown in Fi ure 10 is identical with that previously reported by Swanson et a15 The spectrum is composed of EPR peaks arising from hyperfine splittings (Table 111) by five nucleus, and two benzyl 'Hnuclei. ring 'Hnuclei, a nitroxide %95'
IV. Discmion
* '
' ' ' 3305 ' "
'
I
' ' ' ' ! 3315 ' ' " ' ' ' ' ' ' 332s ' ' '
Field (Gauss)
I
'
'
"
' 3335 ' '
'
I
'
HPLC Tbermocbemicd Decompodtlon Kinetics of Nent NTO (NTO-4). The temporal profiles and the kinetics analysis results (eq 2) shown in Figures 1-3 suggest that the loss of NTO
Figure 8. EPR spectrum collected during thermochemical decomposition of a NTOITNT-I, mixture at 370 K corresponding to radical V: (a) experimental EPR spectrum; (b) simulated EPR spectrum. Spectral parameters are presented in Table 111.
(13) Han&ook of Chemistry ond Physics, 59th ed.;CRC Press: West Palm k c h , n, 1978;P ~ 6 9 Factor . of 6.5 ceicuiatcd using- uu~niunlu .. -, - ..for which NN = gN&IN. .
(NTO-d2) proceeds by a global autocatalytic reaction scheme as indicated by the reverse sigmoidal shape of the timehilligram curves. Apparently, both initiation and propagation reactions
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5515
Decomposition of NTO and NTO-d,
Hydroxy-3J-Dinitro4-Methylphenyl Nitroxide-d,
SCHEME I 0
NTO-d2in lWd5
"I
I
awL"'7 33w
"
7
" " " ' " " " ~ " " ' " ' " " " ' '
3310
3320
I
3340
3330
Field (Gauss) F l p e 9. EPR spectrum collacted during thermochemical decomposition of a NTO-dz/TNT-d5mixture at 370 K corresponding to radical V I (a)
experimental EPR spectrum; (b) simulated EPR spectrum. Spectral parameters are presented in Table 111.
2,4,6-Trinitrobenzy1=3>-Dinitro4-Methylphenyl Nitro.de hT0-d.h lW
oj
3285
A
3295
3305
n
3315
3325
3335
3345
3355
Field (Gauss) Figure 10. EPR spectrum collected during thermochemical dccomposition of a NTO-dz/TNT mixture at 370 K corresponding to radical VII: (a) experimental EPR spectrum; (b) simulated EPR spectrum. Spectral parameters are presented in Table 111. contribute significantly to the loss of NTO over the temperature interval studied. This type of reaction scheme is similar to that observed for TATB,6g HMX," and T N T " V ~thermochemical ~ decompositions which are found to proceed via global autocatalytic kinetics. The temporal profiles observed exhibit two characteristic regions that can be identified as acceleratory and decay phases. The acceleratory phase is most noticeable in decompositions accomplished at 51 1-518 K. During this phase, the decomposition rates increase (autoaccelerate) until about a 50% material loss. This phase also contracts in time by about a factor of 2 for every 5 K temperature increase in the temperature region studied. This abrupt contraction in the acceleratory phase suggests that the global activation energy for the overall decomposition is large. The average global activation energy of 88.0 kcal/mol determined from the Arrhenius plot of the kinetic data (Figure 4 and Table 11) not only supports this notion but also experimentally demonstrates,on a small laboratory scale, the inherent thermal stability of the NTO molecule compared to TNT and RDX whose measured global decomposition activation energies are 29-466k and 34-56 kcal/m01,'~ respectively. The temporal behavior of the (1 4) (a) Robertson, A. J. 9. Tram.Faraday Soc. 1949,45,85. (b) Batten, J. J.; Murdic, D. C. A u t . 1.Ckm. 1970,23,749. (c) Rauch, F.C.; Colman, W.P. Studies on Compcuition B, Final Report, Picatinny Arsenal, NY, DDC-AD 869226, March 1970. (d) Rogers, R. N.;Smith, L. C. Thermochlm. Acta 1970, 11, I . (e) Rogers, R. N.; Daub, 0. W.AM/. Chem. 1973, 45, 396. (0 Kirhore, K.Propellanrs Explos. 1977, 2, 7 8 .
decomposition occurring during the decay phase exhibits reaction deceleration with rates nearly mirroring those which are present during the acceleratory phase. A significant KDIE (Table 11) is observed in the decompositions as demonstrated by comparison of Figures 2 and 3. The average KDIE of 1.67 signifies that a normal primary isotope effect is present and signifies that N-H bond rupture occurs along a main decomposition pathway leading to the loss in NTO. Furthermore, the KDIE indicates that N-H bond rupture occurs on or before a mechanistic rate-limiting step in the decomposition." Assuming simple transition-state theory," this value becomes 2.44 at 298 K which is clearly within the region where normal primary isotope effects have been observed in other energetic systems.'bg6 If this KDIE were secondary in nature where bond rupture occurs at a center a or to hydrogen (deuterium), one would expect that its magnitude would be closer to unity at the temperatures where the decompositions are c o n d ~ c t e d . ~ . ~ ~ Even though primary, the observed KDIE is, however, less than one would predict on the basis of theoretical arguments which leads to a KDIE of about 6S4 for N-H bond rupture. Two m n s can be cited that can account for the KDIE being lower than expected. First, the complex nature of the decomposition process makes it possible for numerous reactions to influence the observed rate constant determined from analysis of the kinetics. Reactions that involve N-H bond rupture and those that involve cleavage of other bonds in the system could contribute to the observed global kinetics for the loss of NTO. Contributions to the observed rate constant from these reactions will, in effect, dilute the observed KDIE from the larger theoretical value.'b The magnitude of this departure depends upon the number of reactions involved in the decomposition paths leading to a loss of NTO, the manner in which they participate in the observed rate, and their respective rate constants. Secondly, departure of the transition state for N-H bond rupture from a symmetrical location along the reaction coordinate results in a decrease in the KDIE as contributions from the transition-state vibrational modes begin to become more important. Thus,early or late transition states in individual reactions involving N-H bond cleavage that occur during the decompositions could cause the observed KDIE to be less than e~pected.'~*~ The primary KDIE and the autocatalytic nature of the decompositions suggest that N-H bond rupture is occurring both in decomposition initiation and in reactions of NTO with intermediates generated after the decomposition proceeds. One possible reaction that may be contributing to the observed KDIE in the NTO thermal decomposition is an initiation reaction that arises from hydrogen abstraction by the NOz functionality in an intermolecular process proposed in Scheme I. In this scheme, a normal primary KDIE should be present since the reaction between the two NTO molecules results in N-H bond rupture on one of the NTO molecules. Direct evidence for this reaction type being present is seen in the photochemical and thermochemical decompositions on the NTO/acetone and NTO/TNT systems presented in this work as well as in similar studies on neat TN'f6a.k,7Wand TNB/solvent7' systems. EPR Identification of Photochemical and Thermochemical Decomposition Intermediates. The EPR spectra (Figures 5 and 6 and Table 111) collected during the photochemical decomposition (1 5)
Ullman, E.F.;Call, L.; Osiecki. J. H. J . Org. Chem. 1970,35, 3623.
5516 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
SCHEMEII
Menapace et al. SCHEME1n
0
H,!KNeH
N
4 No2
0
0
H-NX;eH
0
NTO_ H , N X N e H
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produds
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H
of NTO (NTO-dJ in acetone (acetone-d6) exhibit spectral features that support assignment as the hydroxy nitroxide radical adducts of NTO, 1-111. Of the three I4N hyperfine splittings present in the radicals, two can be readily assigned to the N - O (aN= 8.466 Oe) and N-C (aN = 4.633 Oe) sites on the basis of their respective hyperfine splitting magnitude^.'^ The remaining 14N hyperfine splitting (uN = 1.356 Oe), however, can only be tentatively assigned to the 5-position on the NTO subunit of the hydroxy nitroxide radical adducts. Assignment of this hyperfine splitting to the 5-position is made on the basis of the small hyperfine splitting observed. As both the 2- and 5-positions on the NTO subunit possess nitrogens y to the nitroxyl nitrogen, these two positions should exhibit a larger hyperfine splitting than the 3-position which is 6 to the nitroxyl nitrogen if one considers that only the most stable resonance structures dictate the electron spin density distribution. The 2- and 5-positions, however, should possess different hyperfine splittings as the most stable resonance structures for delocalization of the unpaired electron spin density favor resonance including the 2-position relative to resonance including the 5-position. The identical EPR spectra (Figure 5) observed in the NTO/ acetone, NTO-d,/acetone, NTo/aCetOne-d6 decompositions imply that two different mechanistic reaction pathways are operative leading to the formation of radicals 1-111. One reaction pathway involves an intermolecular reaction between two NTO molecules, and the other involves a solvolysis reaction pathway between NTO and acetone. Since radicals I and I1 both contain a hydroxy 'H nucleus and radical 111, which contains the hydroxy 2H nucleus, is only observed in the NTO-d2/acetone-d6 case (Figure 6), a KDIE must be present that alters the dominant mechanistic pathways leading to the formation of the radicals. A proposed reaction scheme outlining these reactions is shown in Scheme 11. The NTO and the acetone radical adducts shown in Scheme I1 are not observed in the decompositions. This most likely results from undetectable steady-state concentrations for the species because of further fast reactions leading to diamagnetic products. Radicals 1-111 are also relatively unstable. In the photochemical decompositions, the EPR spectra associated with these radicals disappear within 1 min after sample irradiation is stopped. Apparently, reactions that destroy 1-111 are fast. Their rates of formation during irradiation are even faster, however, which leads to detectable steady-state concentrations. The KDIE for the NTO/acetone-d6 system results in the intermolecular NTO reaction pathway (Scheme 11) being dominant relative to the acetone solvolysis pathway as evidenced by the presence of the EPR spectrum corresponding to radical I (Figure 5). Here, the KDIE is large enough to allow the reaction between two NTO molecules to be o k d without significant interference from the solvolysis reaction on the time scale used to perform the experiment. This situation reverses when NTO-d2/acetone is decomposed since the same EPR spectrum as observed in the
NTO/a~etOne-d6system results. The spectrum in this case corresponds to radical 11, which can only arise from the solvolysis reaction pathway. If the intermolecularNTO reaction pathway were still dominant in the NTO-d2/acetone system, an EPR spectrum corresponding to radical I11 would result (cf. Figure 6). For the NTO/acetone and NTO-d2/acetone-d6 cases, both mechanistic reaction pathways may be operative as both the solvent and the NTO species possess 'Hor 2H atoms which could react with the NOz group at similar rates yielding EPR spectra consistent with radical I (Figure 5) or radical I11 (Figure 6), respectively. The EPR spectra observed during the low-temperature thermochemical decompositions of NTO (NTO-d2)/TNT (TNT-d5) mixtures are consistent with two different radical species: the aryl hydroxy nitroxide radical adducts IV-VI (Figures 7-9) and the aryl benzyl nitroxide radical VI1 (Figure 10). The 'H and 2Hhyperfine splittings present in radicals IV-VI are divided into two groups. The first group contains five equivalent 'H(2H) nuclei whose hyperfine splittings ( 4 8 = 2.843 Oe and UD = 0.44 Oe) support assignment as aryl substituents. The second group contains a single 'H or 2H nucleus which is assigned to the hydroxy subunit in the radicals. This hyperfine splitting of uH = 3.665 Oe (aD = 0.56 Oe) is smaller than that for the hydroxy 'Hnucleus in radicals I and I1 observed in the photochemical decomposition studies on NTO/acetone solutions. The smaller hyperfine splitting is most likely attributed to larger resonance delocalization present in the TNT hydroxy nitroxide radical adduct which delocalizes the electron spin density throughout the aromatic ring. The delocalization in the NTO hydroxy nitroxide radical, as previously mentioned, is significantlysmaller. The hyperfine splittings present in radical VI1 are consistent with those previously o b ~ e r v e d . ' ~ The two distinct radical spectra observed, like in the NTO/ acetone studies, depend upon the deuteriation sites present in the mixtures. The two distinct EPR spectra arise in these experiments because of the presence of a KDIE which affects the dominant mechanistic reaction pathways which lead to the formation of the aryl hydroxy nitroxide (IV-VI) and the aryl benzyl nitroxide (VII) radical adducts. A possible mechanism involving these two reaction pathways is presented in Scheme 111. In the NTO/TNT, NTO/TNT-dS, and NTO-d2/TNT-d5experiments, the EPR spectra (Figures 7-9 and Table 111) indicate that the reaction pathway leading to the formation of the aryl hydroxy nitroxide radical adducts, IV-VI, dominates the thermal decomposition process. The formation of these radicals suggests that the NO2 groups on TNT preferentially react with the NTO NH groups as in Scheme I11 rather than react with other TNT CH3 suhtituents. In fact, control experiments conducted on neat NTO and TNT samples do not show detectable decomposition at 370 K. Significant decomposition was not found in these cases until about 490 K. The reaction also appears to be more favorable than the reaction between the NOLNm/NHNn, groups observed
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5517
Decomposition of NTO and NTO-d, in the photochemical decomposition experiments (cf. Scheme 11) and more favorable than the N02,m/CH3,mT reaction which is operative in the NTO-dz/TNT decomposition. The observation of the aryl benzyl nitroxide adduct, VII, in the NTO-dz/TNT decompositions implies that the KDIE causes a change in decomposition mechanism. Here, deuteriation of the NTO NH positions makes the NOZmT/NDNTO deuterium abstraction process unfavorable relative to the NOZNTO/CH3,TNT hydrogen abstraction process. The NOz group on NTO most likely reacts with the TNT CH3 group to produce the benzyl radical in a manner similar to that occurring in neat TNT thermochemical decomposition in the vicinity of 520 Km and in photochemical decompositions of TNB in tol~ene.'~The benzyl radical then, most likely, reacts with TNT in a series of steps to produce radical VII. From the radicals observed and the changes in mechanism occurring in the thermochemical decompositions, the reactivities for the hydrogen abstraction reactions present in the NTO/TNT mixtures can be ordered as NOZ,RIT/NHNTO> NOZ,NTO/CH~,TNT > NOZ,NTO/NHNTOI N0Z,TNT/CH3,TNT
The NTO/TNT mixture EPR experiments also provide evidence indicating that the mixtures undergo accelerated decomposition relative to the neat materials as control experiments conducted on both neat NTO and TNT samples show no detectable radical EPR signals at 370 K. The contributions from the cross reactions occurring between NTO and TNT appear to be responsible for this enhancement in decomposition. These reactions suggest that TNT may not be a proper/inert solvent for NTO in melt-cast explosive formulations due to the reactivity of the TNT NOz and CH3 functionalities with the NTO NH and NO, groups under conditions similar to that which may be used in normal processing procedures.
V. Conclusions EPR and HPLC techniques have been used to monitor the decompositions of neat NTO (NTO-d,) and mixed systems containing NTO (NTO-d,) in acetone (acetone-d6) and NTO (NTO-d,) in TNT (TNT-Is) under thermochemical and photochemical conditions. Several important findings that involve the nature of the decompositions, the reaction kinetics and mechanisms observed, and the role of the NH and NOz reaction sites on NTO are summarized as follows. The HPLC global kinetics studies conducted between 498 and 518 K show that the loss of NTO (NTO-dz) during neat thermochemical decomposition involves a solid-phase global autocatalytic reaction scheme. The autocatalytic nature of the decomposition suggests that both initiation and propagation reactions contribute to the overall loss of starting material. The average global activation energy of 88.0 kcal/mol obtained from analysis of the decomposition kinetics demonstrates the inherent thermal stability of NTO. A KDIE of 1.67 (2.44 at 298 K) observed in the decompositions suggests that a normal primary isotope effect is present for which N-H bond rupture occurs along a main decomposition path leading to the loss of NTO. The KDIE also
implies that N-H bond rupture occurs on or before a mechanistic rate-limiting step governing the overall decomposition proccss. The room-temperature EPR photochemical decomposition experiments on NTO (NTO-dz)/acetone (acetone-d,) solutions yield EPR spectra that can be identified with hydroxy 40x02,3,5-triazolyl nitroxide (I), hydroxy 4-oxo-2,3,5-triazolyl-dz nitroxide (II), and hydroxy-d 4-oxo-2,3,5-triazolyl-dz nitroxide (111) radical species. The observation of identical EPR spectra in the NTO/acetone, NTO-d2/acetone, and NTO/acetone-d6 decompositions corresponding to radicals 1-11 implies that two different mechanistic reaction pathways are operative leading to the formation of radicals 1-111. One reaction pathway involves an intermolecular reaction between two NTO molecules, and the other involves a solvolysis reaction pathway between NTO and acetone. These mechanistic pathways appear to involve reactions by which the NOz group on NTO abstracts hydrogen atoms from other NTO species and/or from acetone. The dominant reaction pathway is dictated by the deuteriation sites and the resulting KDIE present. The low-temperature (370 K) EPR thermochemical decompositions performed on the NTO (NTO-d2)/TNT (TNT-d5) mixtures yield EPR spectra of radical intermediates which can be assigned to hydroxy 3,5-dinitro-4-methylphenylnitroxide, hydroxy 3,5-dinitro-4-methylphenyl-dsnitroxide, hydroxy-d 33dinitro-4-methylphenyl-d5nitroxide, and 2,4,6-trinitrobenzyl 3,5-dinitro-4-methylphenylnitroxide species. Like in the NTO/acetone studies, the deuteriation sites and the KDIE present dictate which radicals are formed. In the NTO-dz/TNT decomposition, the KDIE causes a change in mechanism that yields aryl benzyl nitroxide species as dominant intermediates as opposed to the aryl hydroxy nitroxide radicals present in the other decompositions studied. The presence of these different mechanistic pathways and the KDIE effects observed make it possible to order the reactivities for the hydrogen abstraction reactions present in the NTO/TNT mixtures as follows: NOZ,TNT/NHNTO
> NOZ.NTO/CH3.TNT > NOZ,NTO/NHNTO~ N0Z,TNT/CH3,TNT
=
These experiments also provide evidence indicating that the mixtures undergo accelerated decomposition relative to the neat materials due to contributions from cross reactions occurring between NTO and TNT. The presence of these reactions suggests that TNT may not be a proper/inert solvent for NTO in melt-cast explosive formulations due to the reactivity of the TNT NOz and CH3 functionalities with the NTO NH and NO2 groups.
Acknowledgment. The Air Force Office of Scientific Research, Directorate of Chemical and Atmospheric Sciences, and Universal Energy Systems, Inc., are gratefully acknowledged for funding this study under Project 2303-F3-05 and Contract F49620-88C-0053/SB5881-0378. S.A. Shackelford provided helpful scientific discussion and manuscript review. J. L. Pflug conducted the GC/MS and 'H NMR analyses. F. Kibler and G. E. Godec prepared the specialized glassware used for sample preparation, decomposition, and work-up.
