J. Phys. Chem. 1987, 91, 5159-5161
5159
OH Formation in the Infrared Multiphoton Decomposition of Jet-Cooled Cyclic Nitramines H. Zuckermann, G. D. Greenblatt, and Y. Haas* Department of Physical Chemistry, The Hebrew University of Jerusalem and The Fritz Haber Center for Molecular Dynamics, Jerusalem 91 904, Israel (Received: June 5, 1987) RDX and HMX, seeded in a supersonic nozzle beam, were dissociated by a pulsed COz laser. OH radicals were observed in both cases. The results support the involvement of a five-membered ring intermediate in the collision-free dissociation of cyclic nitramines. The pressure dependence of the yield indicates that OH is not formed from dimers or higher oligomers of RDX.
Introduction The thermal decomposition of cyclic nitramines, and in particular RDX (1,3,5-trinitro-1,3,5-triazacyclohexane, also called hexogen) and H M X (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, also called octogen) has been extensively studied.’ Most workers concentrated on the solid or liquid and only relatively few investigations were carried out in the gas phase.&* This is partially due to the extremely low vapor pressure on the one hand, and to the relative difficulty involved in handling these sensitive compounds at elevated temperatures on the other. Several primary dissociation processes were s ~ g g e s t e d ,such ’~~~~ as ring cleavage, elimination of nitrogen dioxide, oxygen atom elimination, and OH loss via a five-membered ring intermediate. Recent isotope kinetic studies4s5indicated that the last-named process is an important one. A similar route has been found in the photolysis of aliphatic nitro c o m p o ~ n d s . ~ , ’ ~ Seeded supersonic molecular beams” provide a convenient means to study the properties of molecules under collision-free conditions. One advantage of the method is that it enables the facile study of labile molecules of low volatility and another that it leads to the formation of small aggregates, such as dimers and trimers, which may also be studied in the absence of external interference. In this communication we report the infrared multiphoton decomposition (IRMPD) of RDX and H M X in a supersonic jet. O H radicals are observed by the laser-induced fluorescence method. This result is interpreted as proving that OH loss is a primary process in the unimolecular dissociation of these nitramines. The experiments indicate that nitramine clusters do not form O H upon dissociation. Experimental Section RDX and H M X crystals were heated to 150 and 180 “C, respectively, and their vapor seeded in a helium stream (maintained at 3 atm) that was allowed to expand through a pulsed nozzle into a vacuum chamber maintained at mbar. The apparatus was described in detail earlier1*so that only essential details will be repeated. The molecular beam was crossed at right angles by a pulsed, tunable TEA C 0 2 laserI3 beam, consisting of a 100-ns ( I ) For a review, see: Dubovitski, F. I.; Korsunski, B. L. Russ. Chem. Rev. 1981, 50, 958; and for an extensive survey, see: Boggs, T. L. frog. Astron. Aeron. 1984, 90, 121. (2) Rogers, R. N . Thermochim. Acta 1972, 3, 437. (3) Brill, T. B.; Karpowicz, R. J. J . Phys. Chem. 1982, 86, 4260. (4) Shackelford, S. A.; Coolidge, M. B.; Goshgarian, B. B.; Loving, B. A,; Rogers, R. N.; Janney, J. L.; Ebinger, M. H. J . Phys. Chem. 1985,89, 31 18. (5) Bulusu, N.; Weinstein, D. I.; Autera, J. R.; Anderson, D. A,; Velicky, R. W. Paper presented at the 8th Symposium on Detonation, Albuquerque, NM, July 1985. ( 6 ) Rauch, R. C.; Fanelli, A. J . J . Phys. Chem. 1969, 73, 1604. (7) Rogers, R. N.; Daub, G. W. Anal. Chem. 1973, 45, 596. (8) Cosgrove, J. D.; Owen, A. J. Combust. Flame 1974, 22, 13, 19. (9) Radhakrishnan, G.;Parr, T.; Wittig, C. Chem. Phys. Lett. 1984, I l l ,
TABLE I: OH Radical Yield upon IRMPD of RDX laser parameters
c02
range, fim 10.6
10.2
9.6
9.2
line
re] absorpn“ re1 yieldb
freq, cm-’
P(18)
946.0 944.2 P(20) 942.3 P(22) P(26) 938.7 P(32) 932.9 P(34) 931.0 R(20) 975.9 R(18) 974.6 971.9 R(14) 969.1 R(10) R(8) 967.7 R(4) 964.7 R(28) 1039.3 P(34) 1033.5 P(36) 1031.6 all lines 1065-1080
(A) 0.64 0.68 0.73
0.81 0.96 1 .oo 0.24 0.27 0.29 0.35 0.38 0.39 0.3 1 0.30 0.33
negligible
(B) 977 100 83 55 87
B/A 150 147 113 67 90
88
88
60
250 300 303 248 263 176 216 243
81 88 87 100 69 67 13 39 very weak
118
’Absorption in the gas phase, from ref 16. *Measured by using the intensity of the P,(2) line. “spike” followed by a 1.5-ps “tail”. The CO, laser was arranged to fire in the middle of the 600-ps molecular pulse. A pulsed, frequency-doubled dye laser (Quanta Ray’s PDL-1 pumped by Quanta Ray’s DCR-1A Nd-YAG laser, or Lambda Physik FL3001 pumped by Lambda Physik’s EMG101 MSC excimer laser) was used to monitor OH. The dye laser beam propagated on the same optical axis as the C 0 2 laser beam in the opposite direction. Its timing with respect to the C 0 2laser could be varied, and its pulse duration was about 7 ns. This laser was tuned to excite the A2Z(v=l) X211(u=O) transition near 280 nm. Fluorescence was viewed at right angles to both the molecular and the laser beams, filtered by an interference filter centered around 3 13 nm (fwhm = 15 nm), allowing the observation of the A 2 Z ( v = l ) X211(v=l) transition. The signal generated by a photomultiplier was digitized (Tektronix 2430 transient digitizer, 150 MHz bandwidth) and averaged by a microcomputer until a satisfactory signal to noise was obtained.
-
-
LJ.
Results and Discussion O H radicals were identified by their characteristic fluorescence spectrumI4 which was observed only when the molecular beam and both lasers were on. Blocking either laser or the molecular beam lead to complete elimination of the signal. Timing was important, too. No signal was observed when the dye laser pulse preceded the IR laser pulse. When the dye laser was triggered after the COz laser, a signal appeared, and reached a maximum when the delay between the two pulses was 500 ns. The observed fluoresence decay time, about 800 ns, is a further aid in verifying the assignment of the signal as due to OH radi~a1s.I~
(IO) Greenblatt, G. D.; Zuckermann, H.; Haas, Y . Chem. Phys. Left. 1987, 134, 593. ( 1 1 ) Levy, D. H.; Wharton, L.; Smalley, R. E. In Chemical and Biochemical Applications of Lasers; Vol. 2, Moore, C. B., Ed.; Academic: New York, 1977; p I . (12) Anner, 0.;Haas, Y . Chem. Phys. Lett. 1985, 119, 199.
(13) This laser was described earlier: Ruhman, S.; Anner, 0.;Haas, Y . J . Phys. Chem. 1984, 88, 6397. (14) Dieke, G . H.; Crosswhite, H. M. J . Quant. Spectrosc. Radiat. Transfer 1962. 2, 97.
1 L
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63 1987 American Chemical Society
5160 The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
Letten RDX in 3 o t m He
OH RCTAT IO N A L D I ST Rl B il T IO N IO$
c 3 0'
I
e R D X seeded i n h e l i u m
I
CO, Laser ot 10.2 pm
I
OH monitored a t 282 nm
200
C T e r t n i t r o b u t a n e , bulk photolysis
E
I
=
I
8
0 e
-3 c-
40
d
1
U
I000
500 E,
0
e
-4c-I
If
L 20 30 0
I
I500
lcm-'1
Figure 1. The rotational distribution of OH radicals in the X2TI,u=0 state, observed in the laser dissociation of jet-cooled RDX. See text for
details. Both H M X and RDX yielded appreciable amounts of O H radicals, as observed by the fluorescence signal. Most of the results reported below refer to RDX, which was easier to handle because of its higher vapor pressure. Scanning across the CO, laser tuning range, the signal followed roughly the infrared absorption spectrum of RDX,I6 which was obtained with a much lower resolution (4 cm-' compared to about 0.01 cm-l in this work). A slight red shift could be discerned, in agreement with known properties of the infrared multiphoton dissociation (IRMPD) method." Table I lists the measured yields, normalized to the CO, laser energy. A similar trend was observed with HMX, the gas-phase spectrum of which was not available to us, so that comparison was made with the liquid solution spectrum (cf. ref 18). The rotational distribution of O H radicals was deduced from the intensity of the P,(N) branch lines, I N ,using the relation N N = I N [ A N ( 2 N+ I ) ] where N h is the population of the Nth level and A , the rotational transition probabilities as listed in Table 4 of ref 14. The results for RDX as a function of the rotational energy are presented in Figure 1. The figure shows, for comparison, the population distribution of O H radicals formed in the photolysis of tert-nitrobutane near 282 nm.Io The similarity is striking, possibly indicating a similar reaction mechanism. It is evident that the distribution cannot be represented by a temperature. The slope calculated from the lower N values yields a "temperature" of about 230 K, but higher rotational states are much more abundantly populated than calculated for this temperature. The intensity of the OH signal was found to increase approximately linearly with the CO, laser fluence in the range 30-300 mJ per pulse, corresponding to 6-60 J/cm*. The dependence on the number density of RDX molecules was, however, observed to be nonlinear. Crystals of RDX were heated to a well-controlled temperature ( & I "C) and their vapor seeded into the helium carrier gas (maintained at 3 atm.). By varying the reservoir's temperature, the vapor pressure of RDX could be accurately controlled. The results are plotted in Figure 2, using the vapor pressure data of ref. 19. The fitted line is calculated from a model, discussed below, according to which OH is formed from monomeric RDX only. The results can be interpreted as showing that OH is formed from RDX and HMX under collision-free conditions. The 500-ns interval between the pump (IR) and probe (UV) lasers is too short to allow collisions to interfere. A rough calculation, based on Zare's estimates of the number of collisions in a supersonic jet20 (15) Sutherland, R. A.; Anderson, R. A. J . Chem. Phys. 1973, 58, 1226. (16) Karpowicz, R. J.; Brill, T. B. J . Phys. Chem. 1984, 88, 348. (17) Ben Shaul, A.; Haas, Y.; Kompa, K. L.; Levine, R. D. Lasers and Chemical Change; Springer: Berlin, 1981; p 414. (18) Iqbal, Z.: Bulusu, S.; Autera, J. R. J . Chem. Phys. 1974, 60, 221. Goetz, F.; Brill, T. B.; Ferrearo, J. R. J . Phys. Chem. 1978, 82, 1912. (19) Rosen. J . M.;Dickinson, C. J . Chem. Eng. Data 1969, 14, 120.
10 R D X pressure, mtorr
Figure 2. The yield of OH radicals from RDX upon irradiating it at 10.2 pm as a function of RDX vapor pressure. Data points are shown as filled circles; the smooth curve is calculated from the model described in the text.
indicates that this interval is 100 times shorter than the mean time between collisions with helium. The O H formation mechanism cannot be established solely on the basis of the present results. Hydroxyl radicals may be formed directly, or from nascent H O N O that is formed with sufficient internal energy to undergo secondary dissociation. In either case, the most logical route would be via a five-membered ring. The involvement of such an intermediate is compatible with bulk measurements of the activation energy5 as well as with the unimolecular character of the reaction established in the present experiments. This interpretation is also compatible with the preferential formation of low rotational states: O H eliminated from a five-membered ring is not expected to be subjected to a strong torque. In contrast, simple bond fission of a bent intermediate is likely to result in strong rotational excitation. The small, but clearly observable presence of high N states (cf. Figure 1) may indicate that more than one mechanism is operative. A five-membered ring intermediate was proposed to lead to O H formation in the photolysis of alkyl nitro compound^.^^^^ The close similarity between the rotational distribution obtained for RDX infrared dissociation and tert-nitrobutane (cf. Figure I ) , thus provides further support to the notion that such an intermediate plays a role in nitramine unimolecular dissociation. As usual in pump probe experiments, it is difficult to eliminate the possibility that the UV laser caused the dissociation of vibrationally hot parent molecules.2'~22Since both RDX and HMX absorb at about 280 nm,23the fact that the UV laser alone did not produce an O H signal may be cited as proof that the UV dissociation does not lead to OH formation. It may be argued, though, that the absence of a signal is due to a small absorption coefficient and that hot nitramine molecules simply absorb the UV radiation more efficiently. Although the present data do not allow a conclusive decision, the low activation energy found in bulk experiments5 (43.7 kcal/mol for RDX and 49.8 kcal/mol for HMX) is expected to lead a facile ground-state decomposition upon vibrational excitation. The strong O H signal observed is thus compatible with direct IRMPD of the substrates. The dissociation mechanism of RDX is controversial' and may be drastically different in the gas or solid phases. The pressure dependence depicted in Figure 2 can be construed as indicating a difference between the behavior of the isolated molecule and adduct. Increasing the pressure of the seeded molecules in the supersonic jet source leads to formation of dimers and higher adducts.24 The flattening off of the O H signal intensity is most (20) Lubman, D. M.; Rettner, C. T.; Zare, R. N. J . Phys. Chem. 1982, 86, 1129. (21) Haas, Y.; Reisler, H.; Whittig, C. Chem. Phys. Letr. 1982, 92, 109. (22) Ruhman, S.; Haas, Y . ;Laukemper, J.; Preuss, M.; Stein, H.; Feldmann, D.; Welge, K. H. J . Phys. Chem. 1984, 88, 5152. (23) Suryanarayanan, K.; Bulusu, S. U S . Gouernment Res. Dec. Rep. 1970, 70, 14; Chem. Abstr. 1971, 75, 129462. (24) Hagena, 0. In Molecular Beams and Low Density Gas Dynamics; Wegener, P. P., Ed.; Dekker: New York, 1974; p 93.
J . Phys. Chem. 1987, 91, 5161-5163 easily accounted for by considering this phenomenon, as shown below. The possibility that RDX-helium adducts are involved is not likely since any property relating to them is expected to be affected by the helium pressure, rather than by RDX pressure. Assuming that dimers and higher adducts do not lead to O H formation, the plateau displayed in Figure 2 can be interpreted as representing the decreasing fraction of monomeric RDX in the mixture. Even the simplest model, in which only monomers and dimers are considered, leads to fair agreement with the experimental results. Let the equilibrium 2A A2 be established in the jet, governed by an equilibrium constant K( 7‘). In the presence of a large excess of helium, we assume that the terminal temperature T i s independent of RDX pressure in the range of 0-0.03 Torr, relevant to the experiment. Using a value of K = 100 Torr-’, the calculated curve shown in Figure 2 is obtained. It is seen that the qualitative features of the experimental results are reproduced. The high-pressure portion shows a steeper slope in the calculated curve than in the experimental one. A better fit could be obtained by allowing for the formation of larger clusters (trimers, tetramers, etc.) as the pressure is increased. Although physically quite plausible,24this would necessitate the use of more parameters in the fitting procedure. In the absence of other, independent
5161
measurements (for instance, based on mass spectrometry), the introduction of more free parameters, even if affording a better fit to the experimental results, would not really provide additional physical insight. The flattening off of the O H yield as RDX pressure is increased may be related to cluster formation in several ways. One of them could be that the absorption cross section of IR photons is smaller in the adducts than in the monomers. This possibility is unlikely, since molecular adducts usually absorb better than monomers. A spectral shift may be caused, leading to reduction of the cross section at the irradiation frequency used in the experiment. However, it was found that the O H yield varies only slightly across the tuning range of the C 0 2 laser in the l0.2-pm region. The reduction of O H yield upon RDX pressure rise was observed with many laser frequencies. It is unlikely that dimers show a smaller cross section than the monomer in all these frequencies. In conclusion, it has been shown that O H radicals are formed in the unimolecular decomposition of RDX and HMX. The evidence provided in this work indicates that these radicals are not a primary product in the decomposition of RDX clusters. This result may be indicative of a similar behavior in condensed phases, particularly solids.
Production and Collision-Induced Dissociation of Small Boron Cluster Ions Luke Hanley and Scott L. Anderson* Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11 794-3400 (Received: July I , 1987)
Bonding in small boron cluster cations (B2-st) is probed by measurement of threshold energies and fragmentation patterns for collision-induced dissociation (CID), using a guided-beam tandem mass spectrometer equipped with a novel source of thermalized boron cluster ions. The primary fragmentation channel is loss of B+ in all cases, and the dimer ion is found to be much less strongly bonded than B3-*+. B5+is a particularly abundant cluster in the distribution produced in the source and is also a “magic” fragment in CID of larger boron cluster ions. With the exception of the dimer, boron cluster ions are 2-3 times more strongly bound than the isovalent aluminum cluster ions and yield a different fragmentation pattern.
Introduction Clusters of metal and semiconductor atoms have been the focus of a great deal of research, producing many fascinating examples of the dependence of physical and chemical properties on cluster size and composition.’ A real bottleneck in understanding these effects on a fundamental level is the difficulty in doing accurate electronic structure calculations on complicated, many-electron systems, particularly those involving transition metals. From this standpoint boron, with only three valence electrons and five electrons total, is an attractive element for cluster studies. Boron and boron-rich solids are also interesting from a materials standpoint. As a result of the boron atom’s small size and the “electron deficiency” of its valence shell, boron-rich materials tend to have complex crystal structures involving networks of strongly bound icosahedra.2 These materials are typically quite refractory and chemically stable, with structural, electronic, and thermoelectric properties which can be “tuned” over a wide range by compounding with other elements. Current large scale uses include boron nitride insulators and high tensile strength boron fiber composites. However, development of boron-based high-temperature semiconductor and thermoelectric materials is an active (1) Morse, M. D. Chem. Reu. 1986, 86, 1049. Castleman, A. W. Jr.; Keesee, R. G. Chem. Rev. 1986, 86, 589. Halperin, W. P.Reu. Mod. Phys. 1986, 58, 533. Phillips, J . C. Chem. Reo. 1986, 86, 619. (2) Muetterities, E. L.; Knoth, W. H. Polyhedral Boranes; Marcel Dekker: New York, 1968. Boron, Metallo-Boron Compounds, and Boranes; Adams, R. M., Ed.; Interscience: New York, 1964. Boron Hydride Chemistry; Muetterities, E. L., Ed.; Academic: New York, 1975.
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field of r e ~ e a r c h . ~There is also considerable interest in boron as a high-energy fuel with potential for improved efficiency on either a volume or weight basis compared with high performance hydrocarbon fuels.4 Several experimental and theoretical studies of the electronic structure of the neutral boron dimer have been reported. Douglas and Henbergs identified the spectrum of B2 produced in a discharge and assigned the ground electronic state as 32;. Matrix isolation studies6 and detailed theoretical’ calculations supported this state assignment and provided information on the low-lying excited states. Knudson cell experiments8 have determined the dissociation energy and related thermodynamic parameters for Bz. The highly refractory nature of solid boron has prevented study of larger boron clusters in all but one instance: Berkowitz and Chupka9employed mass spectrometry to study B2-5 ejection from solid boron by focused ruby laser light, more than 20 years prior to the current efforts in laser-produced semiconductor clusters.I0 (3) Emin, D. Phys. Today, 1987, January, 55. Noocel Refractory Semiconductors; Extended Abstracts o f the Materials Research Society, 1987 Spring Meeting. Reisch, M. S. Chem. Eng. News. 1987, Feb. 2, 9. (4) Meinkohn, D. Combust. Flame 1985, 59, 225. (5) Douglas, A. E.; Herzberg, G. Can. J . Res. 1940, 18A, 164. (6) Graham, W. R. M.; Welter, W. Jr. J . Chem. Phys. 1976, 65, 1516. (7) Dupuis, M.; Liu, B. J . Chem. Phys. 1978, 68, 2902 and references therein. (8) Darwent, B . de B. Bond Dissociation Energies in Simple Molecules; NSRDS-NBS 31; National Bureau of Standards: Washington, D.C., 1970. (9) Berkowitz, J.; Chupka, W. A. J . Chem. Phys. 1964, 40, 2735. (IO) Reents, W. D.; Bondybey, V . E. Chem. Phys. Letr. 1986, 125, 324.
0 1987 American Chemical Society
