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.
0022-3654/87/2091-5161$01.50/0
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
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The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
>
t
II1
(I
Letter4
I
NUMBER OF ATOMS PER CLUSTER
Figure 1. Typical boron cluster ion size distribution produced by laser
Collision Energy (CM,eV)
ablation. Perhaps due to the lack of experimental data, only a few theoretical studies on small boron clusters exist. Ground electronic states, stable geometries, and atomization energies were determined by configuration interaction calculations" for B,, B4,and B,. Bonding properties of the tetramer'* and the hexamerf3 for restricted geometries have been examined by similar methods.
Experimental Details The design of the cluster ion beam apparatus has been described in detail previously14and is only summarized here. Unlike in our previous work, the cluster ions for these experiments are produced by laser ablation of an isotopically pure boron target ("B, Eagle-Pitcher, 98.6%) in a specially designed 1 I-pole ion trap which will be described in detail in a future publication. The trapped cluster ions are guided by the rf field into a second, maze-shaped rf trapI4 which is filled with several millitorr of helium. In diffusing through the -60 cm long, gas-filled maze, the clusters undergo several thousand collisions and are thermalized at the gas temperature (-400 K). After exiting the cooling maze, the cluster ions are mass selected by a Wien filter and injected into a final set of octapole ion guides where the desired center of mass collision energy is set. While inside this octapole, the clusters pass through a scattering cell filled Torr of the neutral target gas. Ionic products with 1.0 X and unreacted cluster ions are collected by the octapole and guided to the entrance of a quadrupole mass filter where they are mass analyzed and counted. Cross sections for collision-induced dissociation at center of mass energies ranging from 0.5 to 20.0 eV were determined by using the procedure described previ~usly.'~ All fragmentation channels with cross sections greater than 0.1 A2 were examined. The accuracy of the absolute magnitude of the cross sections is estimated to be f50% (due primarily to uncertainity in the effective length of the scattering cell). The errors in relative cross sections are estimated to be the larger of &20% or f0.20 A*. Results and Discussion A typical size distribution of thermalized boron cluster ions produced in our source is shown in Figure 1. The size range of the distribution can be shifted to larger or smaller clusters by varying the buffer gas, its density, and the electrical potentials on the ion cooling trap. The sharp oscillations of intensity with cluster size are also somewhat dependent on experimental conditions and in most cases are unrelated to cluster stabilities. However, comparisons of many such size distributions do show some consistent features; B5+ is typically an especially abundant cluster, and B9+ and BIZ+are generally anomalously weak. The low abundance of Biz+ is interesting in light of the ubiquity of ( 1 1) Whitesides, R. A . Ph.D. Thesis, Carnegie Mellon Universit). Pittsburgh, PA, 1981. (12) Koutecky, J.; Pacchioni. G.: Jeung, G. H.; Hass, E. C. Surf. Sci. 1985, 156, 65 I . (13) Helmstreet, L. A. Goc. Rep. Announce. Index ( U S . ) 1976, 76(1), 88. (14) Hanley, L.; Ruatta, S.;Anderson, S. L. J . Chem. Phys. 1987, 87, 260.
-
Figure 2. Cross sections as a function of center-of-masscollision energy for B2-s+ Xe B+.
+
TABLE I: Cross Sections (in A*) for B2-s+ + Xe at 5.0, 10.0, and 20.0 eV
parent
product
5.0 eV
10.0 eV
total
2.5 0.6 a 0.6
total
a 0.7
3.3 1.2 0.4 1.6 3.8 a 3.8 I .I
0.7
0.3
a total
total
total
0.3 2.0
0.3 0.4 a a 0.4 a
2.7 a
0.6 3.4 2.6
a 0.5 a 0.5
a I .4 0.3 4.3 0.6 0.2 a n.a.
a 0.1
a total
n.a. 0.1
0.9
20.0 e V
3.4
n.a.h n.a. n.a. 3.8 0.3 4.1 3.7 0.2 3.8 3.0 0.2 0.4 3.6 4.2 0.2 0.6 a
5.0 2.7 0.5
0.5 n.a. 3.8
"Denotes fragments with o ( E ) 5 0.1. All values are rounded to the nearest 0.1 A2. bn.a. denotes a channel which was not monitored experimentally. strongly bound BIZicosahedra in virtually all polymorphs of boron and boron-rich solids.2 In Figure 2, we plot the cross sections as a function of collision energy for the process B2-5+ + Xe
-
B+
the major (and lowest energy) collision-induced-dissociation channel for small boron cluster ions. Examination of the figure shows that B3+, B4+, B,+ have similar CID thresholds and cross sections, while fragmentation of B,+ has a considerably lower threshold and larger cross section. While the experimental cross sections have not been deconvoluted to correct for the effect of ion beam energy spread and target Doppler spreadI5 on the thresholds, comparison of the excitation functions with previous work on CID of aluminum cluster ionsI4allows us to estimate the "true experimental" thresholds: B2+ = 1.2 f 0.5 eV, B3 3.0 f 0.8 eV, B4+ = 2.9 f 0.8 eV, and B5+= 3.7 f 0.9 eV. Preliminary CID data for larger boron cluster ions indicate that dissociation thresholds for B6+ and B7+ are comparable to or slightly greater than that for B5+. (15) Chantry, P. J. J . Chem. Phys. 1971, 55, 2746.
Letters Table I lists the cross sections for all major CID channels for B,-*+ Xe at 5.0, 10.0, and 20.0 eV collision energies. For the cluster ions studied, the major product at all collision energies is B+; for B2+, B4+, and B5+ reagent clusters, this is the only significant channel. At higher collision energies, roughly 25% of the total product signal for the B3+ reagent is due to B2+,and for the larger cluster ions (Bbs+), the B5+product channel is IO-30% of u(tota1). The appearance of the pentamer as an important fragment ties in well with the high abundance of B5+ in the size distribution of cluster ions emanating from our source as shown in Figure 1. As the hot clusters produced by laser ablation are cooled by collisions in the rf trap, many of them may fragment to produce B5+, thus increasing the intensity of B5+ relative to other clusters in the mass spectrum. It is interesting to compare the boron cluster dissociation thresholds with those observed in our CID study14of the isovalent aluminum cluster ions. Like boron, aluminum cluster ion dissociation thresholds generally increase with increasing cluster size. However, the threshold energies are 2-3 times smaller for aluminum cluster ions, ranging from 0.90eV for AI2+to 1.30eV for AIS+. The increase in binding energy of boron relative to aluminum is not unexpected in light of the bulk roperties of the two elements (AHf:i(B) = 139 kcal/mol, AHfi(A1) = 78.8 kcal/ moll6), and presumably reflects less diffuse bonding orbitals and more efficient hybridization for the boron n = 2 valence shell. Unlike B2+,the aluminum dimer ion does not appear to be especially weakly bonded; however, both A14+ and B4+ are more weakly bound than the adjacent trimers and pentamers. We can rationalize the weak bonding observed in B2+ using a simple molecular orbital picture (yielding a net bond order of one-half); however, the full description of the electronic structure of boron cluster ions is certainly more complex. Unfortunately, while detailed calculations7 are available for B2, the larger clusters and cluster ions have not been thoroughly treated. The CID product distributions observed for aluminum and boron cluster ions at first glance appear quite similar; however, on closer examination significant differences appear which suggest that the structure of the boron clusters and/or their fragmentation mechanisms are different from those of aluminum. For small aluminum cluster ions, the two dominant fragmentation channels both involve loss of a single atom (Al+ and AIpl+ products), Al' production having the lowest threshold energy and largest cross section.I4 At higher collision energies, varying amounts of all possible product ions are observed. Taking advantage of existing ab initio theoryI7 for neutral aluminum clusters, the following picture was developed to explain these results. Aluminum cluster ions have compact, high coordinate structures with delocalized bonding electrons (metallic bonding). Loss of a single atom breaks the fewest bonds and is thus the lowest energy channel. The
+
(1 6 ) CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton,
FL, 1981; D-46. (17) Upton, T. J . Chem. Phys. 1987, 86, 7054.
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5163 branching between AI+ and Alp,+ products is determined by the differences in ionization potential between A1 and AI,, , favoring AI+ for the small clusters. By analogy with aluminum, the dominance of B+ product in CID of boron cluster ions, and the fact that the CID threshold energies generally increase with cluster size, may be taken as evidence that boron cluster ions have compact, high coordinate geometries. One must then explain, however, the near-total absence of other fragment channels, particularly the B,-,+ channel. This can be partially understood if the ionization potential of the boron atom is assumed to be much lower that those of the clusters (as in aluminumI4 and carbon18), thus lowering the energy of B+ relative to larger fragment ions. It would seem though that if energetics alone determine the product branching, then at high collision energies a wide variety of fragment ions should be produced, which is not the case. Two further anomalies in the CID product distributions bear brief discussion. The first is the appearance of B5+as the only significant fragment ion other than B+ in CID of the larger cluster ions, and as a "magic number" in the cluster ion distribution from our source. It is tempting to suggest that this is evidence that B5+ is a particularly stable ion; however, our preliminary CID measurements for B6+ and B7+ suggest that the five-, six-, and seven-atom cluster ions are all of comparable stability. Other possible explanations for the bias toward B5+production include structures for the larger clusters involving a B5 unit, or an anomalously low B, ionization potential. Further speculation is probably not warranted in view of our plans for continued experimental and theoretical work. Finally, it is interesting to compare the CID behavior of B3+ with that of AI3+. The trimer is the only small aluminum cluster ion for which the dominant product ion is A12+instead of AI+ (in a roughly 3:l ratio). This effect is seen to a lesser extent in B3+, which fragments primarily to B+, with B2+as a significant product channel. In aluminum, it seems likely that this effect is due to spin c o n ~ e r v a t i o n . ' ~ Applying ~'~ the same arguments to boron would predict that B3+has a triplet ground state; again, electronic structure calculations are needed. This letter reports the first in a series of studies of boron cluster ions from our group. Further work on CID of larger clusters and on the chemistry of boron cluster ions is under way. We also plan ab initio electronic structure calculations, with the hope that this concerted approach will provide insight into the relationships between cluster size, electronic structure, bonding, and chemical properties for boron and, indirectly, for other materials.
Acknowledgment. This work is supported by the US Office of Naval Research under contract no. N00014-85-K-0678. (18) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules: Van Nostrand: New York, 1979: p 112. (19) Jarrold, M. F.: Bower, J. E.; Kraus, J. S. J . Chem. Phys. 1987, 86, 3816.
