J. phys. Chem. 1082, 86, 1657-1661
1657
X-ray Photoelectron Study of Electronic Structure and Ultraviolet and Isothermal DecompositIon of 1,3,5-Trlamlno-2,4,6-t rlnitrobenzene J. Sharma,+W. L. Garrett,* F. J. Owens,' and V. L. Vogel Energetic Meterials Division, Large Caliber Weepon Systems Laboratory, United States Army Amment Research and Development Command, Dover, New Jersey 07801 (Received: September 29, 1981; I n Final F m : December 15, 1981)
The valence and core electronic levels of triaminotrinitrobenzene (TATB) have been investigated by X-ray photoelectronic spectroscopy. The nitrogen and oxygen core spectra show unusual high binding energy satellite structure. The separation of the satellites from the main core lines decreased with the successive removal of the donor NH2groups forming diaminotrinitrobenzene(DATB),picramide, and trinitrobenzene. The core-level spectra of the nitrogen and the oxygen decreased when TATB was subjected to isothermal and photolytic decomposition, indicating severing of the C-N02 bond.
Introduction Triaminotrinitrobenzene (TATB), C6(NH2)3(N02)3, is an interesting material from a number of points of view. TATB can undergo detonation when subjected to heat or shock but requires substantially more energy to cause detonation than other energetic materials. This property has generated considerable interest in TATB in the munitions industry because of the reduced vulnerability and hazard associated with its use. TATB is also of interest for research purposes because it is almost a two-dimensional material having a layered structure analogous to graphite.' The distance between the layers, which are made up of planar molecules, is 3.3 A, twice that of a van der Waals radius, indicating that bonding between the planes is weak. The carbon ring of the molecule is larger than that of benzene. The C-C bond distances in TATB are 1.435 and 1.450 A compared with 1.39 A in benzene. This work reports the results of X-ray photoelectron spectroscopy (XPS) studies of the electronic structure in both the core and valence regions. In the core region, shake-up peaks are observed, which is unusual for aromatic compounds. Such peaks at separations of 1-2 eV from the principle peaks have been reported by Pignataro2p3for nitroanilines, but in TATB the separation is unusually large. These shake-up peaks are interpreted in terms of charge-transfer processes of a positive kind. The core and satellite XPS spectra of C6H(NH2),(N02),(DATB) and C6H2(NH2)(N02),(picramide) are also reported in order to assist with interpretation of the satellite structure. CNDO molecular orbital calculations of TATB are employed to assist in interpreting the XPS spectra. The XPS technique is also used to study the photolytic and thermal decomposition of TATB. By monitoring the core-level peaks of the different atoms of the molecule as a function of heating and UV irradiation, one can identify specific bonds broken. Some preliminary results of the effect of shock impact and UV irradiation on TATB have been previously reported.* Experimental Section
The XPS measurements were made on a Varian IEE-15 photoelectron spectrometer and a Kratos (AEI) DS300 instrument, using Mg K lines for excitation. The specimen of TATB used was obtained from Los Alamos Scientific Naval Surface Weapons Center, White Oak, Silver Springs, MD 20910.
* Bell Telephone Laboratories, Holmdel, NJ
07974.
TABLE I : Binding Energies and Line Widths of Core-Level XPS Spectra of TATB, DATB, Picramide, and Trinitrobenzene peak halfposition, width, orbital eV eV C6(NH,),(N0,), (TATBI
C(1s) N(lsI
285.0 399.0 404.6 407.4 531.6 535.4 284.5 399.0 405.1 407.2 531.2 534.0 286.0 399.1 405.1 406.1 531.0 532.6 284.6 405.4 532.4
3.55 1.75 1.75 1.90 1.75 1.8 3.6 2.04 2.0 2.3 2.6 3.0 1.7 2.35 1.0 2.1 1.0 2.8 2.1 2.3
comments amine nitro satellite satellite amine nitro satellite satellite amine nitro satellite satellite nitro
Laboratory, Blend 73-16, and contained about 0.52% of chlorine as contamination. In most of the study, the specimen was mounted on a double-stick scotch tape. In some cases, such as in the thermal decomposition studies, the sample was pressed onto the roughened copper surface of the sample holder. The position of the core levels was calibrated relative to the gold 4f,(, line (assumed to be 83.8 eV). The thermal decomposition studies were carried out by heating the sample in the analyzer chamber of the instrument. For photolysis studies, a low-pressure mercury quartz lamp was installed in the reaction chamber of the Varian instrument, and, since the lamp was in the vacuum system, the 3663-, 2536-, 1942-, and 1842-Alines were effective. The valence band spectra were measured with the Kratos instrument because better signal-to-noise ratio and resolution could be obtained. The X-ray beams of the instrument did cause some decomposition of the TATB molecule, but the degree (1)H.Cady and A. C. Larson, Acta Crystallogr., 18, 485 (1965). (2)S.Pignataro and G. Distafano, J.Electron Spectrosc. Relat. Phenom., 2, 171 (1973). (3) S. Pignataro, R. D. Marino, and G. Distefano,J.Electron Spectroec. Relatd. Phenom., 4,90 (1974). (4)J. Sharma and F. J. Owens, Chem. Phys. Lett., 61, 280 (1979).
This article not subject to U S . Copyright. Published 1982 by the American Chemical Society
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Sharma et al.
The Journal of Physical Chemisrv, Vol. 86, No. 9, 1982 25 000 -
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Figure 1. Core spectrum of carbon Is 1s of TATB. The vertical axis Is counts per second. AM1NE.N
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of decomposition was estimated to be about 1% in a scan of 400-s duration, so that this decomposition could easily be subtracted from photolytic and thermal decomposition.
Results The XPS spectra of the core levels of TATB are shown in Figures 1-3 and the energies of the peaks are given in Table I as well as the line widths. The line width of the carbon 1s spectrum is larger than the other peaks and may be a result of some hydrocarbon contamination which is usually present unless the sample is cleaned by sputtering. For TATB it is not possible to sputter without decomposing the sample. The line width would also be expected to increase with addition of nitro groups to the benzene ring. This is demonstrated by the narrowing of the carbon Is, which occurs when the nitro groups are removed by photolysis. Figure 2 shows the two nitrogen 1s peaks, one associated with the amine nitrogen a t 399.0 eV and the main nitro nitrogen at 404.6 eV. The nitro nitrogen consists of two peaks with a smaller peak about the intensity of the larger peak, but shifted to higher binding energy by 2.88 eV. The intensity ratio of these peaks did not change during photolysis when some of the nitro groups were removed from the ring. The oxygen 1s peak, shown in Figure 3, also shows a satellite line shifted by 3.6 eV to higher binding energy, and having an intensity about lI6 of the main line. However, the intensity of the oxygen line may be a result of oxygen contamination, which is common, so this ratio may be actually larger. Since the
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Flgure 4. (a) Nitrogen core spectrum of TATB after isothermal decomposition at 260 OC. (b) Nitrogen core spectrum of molecule when NH, is replaced by Br,. (c) Nitrogen core spectrum of TATB after UV photoiysls.
nitro nitrogen 1s peak decreases on photolysis and the ratio of the satellite to the nitro remained constant during this process, the satellite is associated with the nitro group rather than the amine group. Shake-up plasmon spectra associated with the 1s peaks of carbon, nitrogen, and oxygen were observed to the high-energy side of the core peaks. These ranged 30 eV toward the high binding energy side and were typical of the broad plasmon spectra observed in most solids. In order to assist in the interpretation of the TATB core spectra, the 1s core spectra of diaminotrinitrobenzene (C6H(NHJ2(NO2)J and picramide (C6H2(NH2)(N02)3) have been measured. The amine and nitro nitrogen 1s peaks are listed in Table I as well as the position of the satellite line of the core spectra. It is interesting that the separation between the main nitro peak and the satellite decreases as the number of amine groups on the ring decreases. No satellite is observed in trinitrobenzene, where there is no amine group on the ring. The same is true of the oxygen satellite. For reasons that will be discussed below, an NH2 group on TATB was replaced by a bromine and the core-level spectra were recorded. As seen in Figure 4b, the satellite line of the nitro 1s peak shifted closer to the main line. The X P S spectrum obtained in the valence band region is shown in Figure 5, where the background has been
~
XPS Study of 1,3,5-Triamino-2,4,6-trinitrobenzene
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982
1659
0
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Flgure 5. XPS spectrum In valence band region of TATB.
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Figure 7. Auger spectra of carbon, nitrogen, and oxygen TATB.
However, a definitive determination of the nature of this decomposition will require further work. The Auger spectra excited by X-rays arising from creation of 1s holes on carbon, nitrogen, and oxygen are shown in Figure 7. The combined structure of the carbon, nitrogen, and oxygen Auger spectra might be expected to resemble the valence band spectra because Auger spectra result from filling of core levels from valence levels and emission of valence electrons.
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Flgure 6. Effect of UV irradiation on valence band spectra of TATB showing a decrease in some peaks.
subtracted out. Six prominent peaks are evident at 7.5, 14.0, 19.0, 27.0, 33.0, and 55.0 eV. As will be discussed below, the valence band of the TATB molecule consists of 48 molecular orbitals, meaning that the six observed peaks in the valence band are a result of considerable superimposition of lines. When TATB is exposed to UV light for 18 h, the 1s peak of the nitro group nitrogen decreases by 1/3 as does the satellite. However, the satellite does broaden somewhat. The amine nitrogen peak shows no change in intensity or energy. This result clearly indicates that photolysis in the solid results in removal of the NO2 group from the ring. The oxygen 1s peak shows a similar decrease, consistent with the removal of NOz, but the intensity of the satellite line relative to main line decreases, and the satellite line shifts closer to the main line. All peaks in the valence band except at 27 eV showed some decrease with UV irradiation, as shown in Figure 6. When TATB is heated to 260 “C, the peak due to the 1s of nitrogen decreases in intensity as in photolysis. The amine nitrogen peak also decreases but at a slower rate than the nitro peak, as shown in Figure 4a; at the same time that this is occurring, a new peak is growing at 397.8 eV. In fact, the decrease of amine peak correlates to the emergence of the new peak. One explanation of this latter behavior is that the amine is losing a hydrogen and the new peak at 397.8 eV is due to the nitrogen in a N-H group. Another poeeibility could involve some chemical interaction between the NH2 and groups on adjacent molecules.
Discussion Core Levels. A major feature of the nitrogen 1s spectrum is the large separation between the amine nitrogen peak and the nitro peak. This is qualitatively understandable because the presence of oxygen in the nitro group will shift the nitro peak to higher binding energies. There have been a number of efforts to quantitatively predict chemical shifts in the core levels using quantummechanical molecular orbital calculation^.^^^ Understandably ab initio calculations have been most successful. However, for large molecules less costly approximate methods such as CNDO/2 or INDO have been employed and can assist in interpreting data. Chemical shifts in the 1s levels are assumed to be proportional to the 2s and 2p electron charge density on the atom in the molecule and the electrostatic potential at the atom due to the charges on the other atoms of the molecule. The chemical shift is given by E = -K0Aq20 - KpAqZp- AV (1) where Aq, is the difference in the charge density localized in the 2s atomic orbital of the atom of the moelcule and the 2s orbital of the atomic orbital in a reference molecule, and Aq2 is the same for the 2p charge density. The A V is the difference in the electrostatic point charge potential at the atom due to the other atoms of the molecule considered as point charges and that of the reference molecule
The R,i is the distance between the nuclei and the other nuclei of the moelcule. The parameters K are empirical parameters. Using the CNDO MO method and the crystal geometry of the TATB molecule, we obtained q%, qa, and Qn for each atom of the molecule. These results were then (5)F. 0. Ellison and L. L. Larcom, Chem. Phys. Lett., 10,580 (1971). (6) F. 0.Ellison and L. L. Larcom, Chem. Phys. Lett., 13,399 (1972). (7) J. A. Pople, D. L. Beveridege, and P. A. Dobosh, J.Chem. Phys., 47, 2026 (1967).
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The Journal of phvsicel Chemistty, Vol. 86,No. 9, 1982
used in eq 1and 2 to calculate the separation between the nitro nitrogen 1s and the amine nitrogen 1s peaks. The calculated separation between the amine nitrogen and the nitro nitrogen is 2.32 eV compared with the experimental separation of 5.6 eV. It should be pointed out that the model has been paramaterized for gas-phase molecules and the data reported here are for the solid state. It is therefore only used here for calculation of relative shifts between the different nitrogens in the same molecule, and even in that should only be considered semiquantitative. The nature of the satellite lines on the high binding energy side of the nitrogen core peaks is of some interest. One possibility is that the chemical shifts of the different nitro groups of the molecule are slightly different because of the slight geometric asymmetry of the TATB molecule.' However, the MO calculation of qb and qQ on the nitrogen of each nitro group and the calculated electrostatic potential at each nitrogen when used in eq l and 2 showed no appreciable difference in chemical shifts between the different nitro nitrogens, indicating that the satellite lines are not due to geometric inequivalencies in the molecule. The possibility that the satellite lines are due to an ionized TATB molecule has also been considered. Molecular orbital calculations of the +1ion of TATB assuming the geometry is the same as that of the un-ionized TATB were performed, and the resulting 2s and 2p charge densities and the total charge on each atom were obtained. These were then used in conjunction with eq 1and 2 to calculate the chemical shift. The results showed that the chemical shift of the nitro nitrogen of the ionized TATB would be considerably larger than the separation of satellite line from main line. Satellite spectra have been observed in nitro nitrogen core spectra in nitroaniline~.~,~ The satellite structure observed in TATB is similar to that observed in the nitroanilines except the separations are somewhat larger. In p-nitroaniline this structure was explained in terms of a "charge transfer of the negative kind".8p9 Essentially a T* unoccupied level of the molecule is believed to be shifted below the top filled level of the molecule due to the electrostatic potential of the core hole. This reduction of the electronic energy of the molecule is gained by the emitted electron, and the emitted electron has an apparent lower binding energy, meaning that the larger peak at the lower binding energy is actually the shake-up spectra. The XPS spectra of the 1s of the nitro nitrogen in trinitrobenzene showed no satellite structure. When NH2 groups replace hydrogen atoms formin picramide, DATB, and TATB, satellite structure is observed. The separation between the satellite lines and the larger line increases with the number of NHz groups. Replacement of an NH2 group on TATB by a chlorine or a bromine caused the satellite structure to move closer to the main line. It is suggested that the satellite lines here are due to the more commonly observed shake-up spectra of the positive kind resulting from the perturbation of the core hole exciting outer valence electrons causing an apparent increase in the binding energy of the emitted core electron. However, further theoretical work such a s molecular orbital calculations are necessary to verify this conclusion. It is interesting that in p-nitroaniline, where lower binding energy lines are attributed to the satellites, addition of CH, donor groups reduced the satellite-main line separation whereas here addition of NHz donor groups increased it., (8) R. Nakagari,D. C. Frost, and C. A. McDowell, J . Electron Spectrosc. Relat. Phenom., 19, 355 (1980). (9)W. Domcke, L.S. Cedarbaum, J. Schirmer, and W. von Nieasen, Phys. Reu. Lett., 42,1237 (1979).
%
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Flgure 8. Valence band spectrum of TATB simulated from molecular orbitals calculated by CNDO.
Valence Band. CNDO and other moelcular orbital methods have been shown to be useful in interpreting valence band spectra in solids which have molecular subunits.'+12 The relative intensities of the peaks in the valence band region will be proportional to the photoionization cross section Ijof the jth molecular orbital from which the electron comes. This can be approximated from the calculated molecular orbital wave functions by Ij = CCCjAa2u,A
(3)
a X
where a labels the atoms of the molecule, CjAgis the molecular orbital coefficient of the Xth atomic orbital making up the jth MO on atom a. The ueAvalues are cross sections for emission of electrons relative to the carbon 2p cross section. The values obtained by G e l i d 3 are used. The C.;, values were obtained from the CNDO calculation of $ATB. The XPS spectra were simulated by placing at each calculated energy level a Lorentezian-shaped line of 2.5 eV and of height proportional to the calculated Ijfrom eq 3. The widths of the lines were adjusted from 0.5 to 6.0 eV. The simulation using a line width of 2.5 eV shown in Figure 8 is not in total agreement with the experimental valence band spectra shown in Figure 6, where the energy scale is the same. The relative separation of peaks 1-3 in the simulation agree with the separation of the three lowest binding energy peaks in the measured spectra. The separation of the remaining peaks of the simulation and the relative intensities is in poor agreement with experiment. In view of the fact that the valence band consists of 48 molecular orbitals spanning an energy of 60 eV and because CNDO is not an accurate predictor of energy-level separation, this disagreement is not surprising. Nevertheless, the calculation can provide some information concerning the nature of the valence band peaks, and in particular the contribution of atomic orbitals to the peaks may be identified. The molecular orbital calculation indicates that the lowest binding energy peak of the valence band arises from a molecular orbital primarily localized on the NOz groups and made up of oxygen 2p atomic orbitals. This is consistent with the fact that this peak is (10)P.M. Plash, J. Sharma,S. Bdusu, and G. F. A h , J. Electron Spectrosc. Relat. Phenom., 6,429 (1975). (11)J. Sharma, D. S. Downs, Z. Iqbal, and F. J. Owens, J. Chem. Phys., 67,3045 (1977). (12) F. J. Owens and J. Sharma, Chem. Phys. Lett., 74,72 (1980). (13)V. Gelius,P.F. Heden, J. Hedman, B.J. Lindber, R. Manne, R. Nordberg, and K.Siegbahn, Phys. Scr., 2,70 (1970).
J. Phys. Chem. 1982, 86, 1661-1669
not observed in the valence band of benzene, which has no nitro groups, and with the fact that this peak decreases with UV irradiation (see Figure 6) along with the decrease in the nitrogen and oxygen 1s core peaks of the NO2group, indicating removal of NO2 from the ring. The Auger spectra also show that the lowest binding energy peak will be associated with the oxygen atoms. The CNDO calculations also indicate that the peaks at 14 and 1.90 eV in the experimental valence band spectra arise from molecular orbitals localized on the NO2 group, and indeed these also decrease with exposure to UV light. The Auger spectra of the oxygen suggest that the peak at 55 eV in the valence band spectra may be associated with an oxygen level. Thermal Decomposition and Photolysis. The marked decrease in the core spectra of the oxygen and the nitrogen of the nitro group when TATB is subject to UV light clearly shows that one of the effects of photolysis of TATB is a removal of the NO2 group from the ring. Of course, there could be other bonds breaking which may not be evidenced in the XPS experiment. In the valence band region a decrease in peaks 7.5, 14.0, and 19.0 eV was observed, indicating as discussed above that these are associated with molecular orbitals localized on the nitro groups. When TATB is thermally decomposed, the core peak of the nitro nitrogen decreases, indicating that NO2 is severed from the ring. However, the amine nitrogen peak also decreases but more slowly. The decrease in amine nitrogen peak is accompanied by the growth of a new peak at 397.8 eV. The shift of lower binding energy indicates an increase in negative charge on the amine nitrogen which might, for example, accompany the removal of H+ from the NH2 group, or perhaps some change in the chemical
1001
interaction between the NH2 group and the adjacent molecule in the lattice. Mass spectrometry has been used to study the thermal decomposition of TATB, and NO2 was detected as a decomposition product.14 However, the NO2 was not believed to be a primary decomposition product, as the data indicated an initial production of large molecular fragments resulting from breaking of ring carbon-carbon bonds. I t is difficult to compare mass-spectrometric thermal decomposition results with X P S because mass spectrometry generally measures gas-phase decomposition, whereas the XPS studies here are measuring decomposition in surface layers of the solid. Summary The XPS core spectra of carbon, nitrogen, and oxygen of the homologous series TATB, DATB, picramide, and TNB displayed satellite structure on the high binding energy side. The separation of the satellites from the main core peaks increased as the number of donor, NH2, groups increased on the benzene ring. These unusual satellite lines are attributed to shake-up spectra of the positive kind. The XPS valence band spectra of TATB were recorded. CNDO molecular orbital calculations proved to be of limited value in interpreting the valence band structure, but, when used in conjunction with other data such as the effect of UV irradiation and the Auger spectra, some information about valence band peaks can be determined. The XPS of UV-irradiated and thermally decomposed TATB showed evidence for the removal of the NO2 group from the ring. (14)
M. Farber and R. D. Srivaetava, Combust. Flume, 42,165 (1981).
Reaction Rate Constant for OH 4- HOON02 ---* Products over the Temperature Range 246 to 324 K Paula L. Trevor, Graham Black, and John R. Barker' phvslcal Sdences Dlvlslon, SRI Internationel, Menlo Park, Califomla 94025 (Recelved: February 12, 1981; In Flnal F m : November 23, 198 1)
Absolute bimolecular reaction rate constanta for the title reaction have been determined for temperatures ranging from 246 to 324 K. The laser flash-photolysisresonance-fluorescence(LFPRF) technique was used to generate O(lD) which reacted with Hz and/or HzO to produce OH radicals. The bimolecular rate constants for the title reaction showed no dependence on total (He) pressure over the range -3 to 15 torr, and they did not depend upon initial [OH] or upon ita mode of formation. The H202impurity was explicitly measured in all experiments, and the rate constanta were corrected for ita contribution. A weighted least-squares analysis of the data obtained at nine temperatures (226 data points) gave the Arrhenius expression (k f la) = (8.05 f 5.69) X exp(-193 f 194/7') cm3s-l with covariance 1.098 X lo4. A simple weighted average (temperature independent) fits the data just as well, and when the effects of systematic errors are taken into account, our recommended rate constant is (k f 2a) = (4.0 f 1.6) X cm3 s-l.
Introduction Both HOz and NO2 play crucial roles in the chemistry of the upper atmosphere. For several years, these two species were thought to react via a radical disproportionation reaction, although it was suggested1 that a longerlived complex could be formed according to reaction 1. (1)R.Simonaitis and J. Heicklen, J. Phys. Chem., 80, 1 (1976). 0022-3654/02/2086-1661$01.25/0
HO2 + NO2 + M s HOONO2 + M
(1) The importance of reaction 1 has been verified by direct observation of HOzN02using Fourier transform infrared spectroscopy.2 Moreover, the rate of reaction 1has been measured,38and its reverse reaction (-1) has been studied! (2) H. Niki, P. D. Maker, C. M. Savage,and L. P. Breitenbach,Chem. Phys. Lett., 45, 564 (1977).
0 1982 American Chemical Society
