J. Phys. Ckem. 1988,92, 5803-5812 table are the values for these ratios that have been obtained by others using the water loss method. The ratios obtained for the numbers of Lewis sites to hydroxyl groups at temperatures from 300 to 750 "C on y-AlZ0, (Table I) are lower than the values obtairred by the water loss method.However, our values reflect only the numbers of hydroxyl groups and Lewis sites that are accessible to reaction with trimethylgallium. Additionally, reactions of a single TMG molecule with more than one hydroxyl species (for example, reaction with adjacent AlOH groups to produce a bridged A10Ga(CH3)0Al species) were not considered (and there was no spectroscopic
5803
evidence for the formation of such species). In spite of these deficiencies, the data in Table I show the general trend expected when y-alumina is dehydroxylated and illustrate how different reactions with the same probe molecule can be used to determine the proportions of Lewis sites to hydroxyl sites as a function of the temperature of activation.
Acknowledgment. We are grateful to the NSERC of Canada for financial support and for the award of a postgraduate scholarship to R.A.M. Registry No. A1203,1344-28-1; Me,Ga, 1445-79-0.
Collision- Induced Dissociation and ab Initio Studies of Boron Cluster Ions: Determination of Structures and Stabilities Luke Hanley, Jerry L. Whitten, and Scott L. Anderson* Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York I 1 794-3400 (Received: March 10, 1988) Bonding in small boron cluster cations (BZTl3+) is examined by measurement of appearance potentials and fragmentation patterns for collision-induced dissociation (CID) with Xe. Cluster stabilities are generally found to increase with increasing cluster size; however, there are large fluctuations from the overall trend. The lowest energy fragmentation channel for all size cluster ions is loss of a single atom. Clusters smaller than six atoms preferentially lose,'B while for the larger clusters the charge remains on the BW1+fragment. The results are used to estimate cluster ionization potentials and geometries. Comparison of measured stabilities with "magic" numbers in the cluster ion size distribution and with total CID cross sections shows that neither is a reliable indicator of stability. We also report on ab initio calculations for both neutral and ionic BI4. The results include cluster geometries, ionization potentials, charge distributions, dissociation energies, and bonding character. The results for IPS, geometries, and De)s are compared with experiment.
Introduction In the past several years, much effort has gone into examining the physical and chemical properties of clusters of metal and semiconductor atoms.'Sz Cluster chemistry has been studied for both i o n i ~ and ~ - ~neutral6 clusters using a variety of techniques including flow tubes, ion beams, and ICR's. Chemistry has frequently been observed to depend strongly on cluster size, presumably due to changes in geometrical and electronic structure. Spectroscopic problems have limited detailed studies of metal cluster structure to diatomic2 and a few triatomic' species. Detailed structural information for larger clusters is only available for systems such as carbon*, silicon9, and aluminurnlo which have been the subject of accurate theoretical work. With only five electrons per atom, boron clusters are particularly amenable to a b initio theoretical calculations, and boron is the lightest element (1) Castleman, A. W. Jr.; Keesee, R. G. Chem. Rev. 1986, 86, 589. Halperin, W. P. Rev. Mod. Phys. 1986, 58, 533. Phillips, J. C. Chem. Rev. 1986, 86, 619. (2) Morse, M. D. Chem. Rev. 1986,86, 1049. (3) Mandich, M. L.; Bondybey, V. E.; Rents, W. D. J. Chem. Phys. 1987, 86, 4245. Alford, J. M.; Williams, P. E.; Trevor, D. J.; Smalley, R. E. Int. J . Mass Spectrom. Ion Processes 1986, 72, 33. McElvaney, S. W.; Creasy, W.R.; O'Keefe, A. J . Chem. Phys. 1986,85,632. Zakin, M. R.; Brickman,
R. 0.; Cox, D. M.; Kaldor, A. J . Chem. Phys., to be published. Jarrold, M. F. In Modern Inorganic Chemistry: Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; to be published. (4) Ruatta, S.A.; Anderson, S. L. J . Chem. Phys. 1988,89,273. Ruatta, S.A.; Hanley, L.; Anderson, S . L. Chem. Phys. Lett. 1987, 137, 5. (5) Jarrold, M. F.; Bower, J. E. J . Chem. Phys. 1987, 87, 5728. (6) Riley, S.J.; Parks, E. K.; Nieman, G. C.; Pobo, L. G.; Wexler, S.J . Chem. Phys. 1984, 80, 1360. Geusic, M. E.; Morse, M. D.; Smalley, R. E. J . Chem. Phys. 1985,83,2293. Trevor, D. J.; Whetten, R. L.; Cox, D. M.; Kaldor, A. J . Am. Chem. SOC.1985, 207, 518. (7) Morse, M. D.; Hopkins, J. B.; Langridge-Smith, P. R. R.; Smalley, R. E. J. Chem. Phys. 1983, 79, 5316. Fu, Z.; Lemire, G. W.; Hamrick, Y. M.; Taylor, S.;Shui, J.-C.; Morse, M. D. J. Chem. Phys., to be published. (8) Raghavachari, K.; Binkley, J. S. J . Chem. Phys. 1987,87, 2191. (9) Raghavachari, K.; Rohlfing, C. M. Chem. Phys. Lett. 1988,143,428. Raghavachari, K. J. Chem. Phys. 1986, 84, 5672 and references within. (10) Pettersson, L.G. M.; Bauschlicher, C. W. Jr.; Halicioglu, T. J. Chem. Phys. 1987,87, 2205. Upton, T. H. J . Chem. Phys. 1987,86, 7054.
which forms chemically bound clusters and is not prohibitively toxic to handle. We have begun a series of experimental and theoretical studies of boron cluster ions. Our goal is to examine the relationship between cluster electronic and geometrical structure and chemical reactivity at a level not possible with more complex systems. Boron's small covalent radius and electron deficiency result in formation of materials involving strong bonds delocalized over several nuclei. Boron and boron-rich solids tend to have complex crystal structures involving networks of strongly bound Blz icosahedra." The boron-rich solids are refractory and chemically stable, with structural, electronic, and thermoelectric properties which can be varied over a wide range by doping with other elements.'* These properties have resulted in several important technological applications including thermally and chemically stable insulators, and high modulus boron fiber composite^.'^ Application of boron-rich materials as high-temperature semiconductor14 and thermoelectric materialslZis currently an active area of research. There has also been considerable research in the use of boron-rich materials as high energy density f ~ e 1 s . l ~ In this paper, we report a combined experimental and theoretical study of bonding and structure in boron cluster ions. Measurements were made of absolute collision-induced dissociation (CID) cross sections, fragment appearance potentials, and fragmentation branching ratios. This data is interpreted to obtain stabilities, and qualitative structural information ionization potentials (IPS), (11) Boron Hydride Chemistry; Muetterties, E. L., Ed.; Academic: New York, 1975. Muetterties, E. L.; Knoth, W. H. Polyhedral Boranes; Dekker: New York, 1968. Boron, Metallic-Boron Compounds, and Boranes; Adams, R. M., Ed.; Interscience: New York, 1964. (12) Emin, D. Phys. Today 1987, Jan., 55. Novel Refractory Semiconductors; Extended Abstracts of Materials Research Society, 1987 Spring
Meeting.
(13) Reisch, M. S.Chem. Eng. News. 1987, Feb. 2,9. Technical Bulletin, Avco Textron Division. (14) Mishima, 0.; Tanaka, J.; Yamaoka, S.; Fukunaga, 0.Science 1987, 238, 181. (15) Meinkohn, D. Combust. Flame 1985,59, 225.
0022-3654/88/2092-5803$01.50/0 0 1988 American Chemical Society
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on the boron cluster ions. Self-consistent field configuration interaction (SCF-CI) calculations for positive and neutral boron clusters up to six atoms in size are also described. Detailed cluster geometries, electronic structures, and dissociation energies (0,’s) are reported and compared with experiment. Several theoretical and experimental studies of the electronic structure, bond length, and bond strength of the neutral boron dimer have been reported. The ground electronic state of B2 was assigned by Douglas and Herzberg16 as 32[ from discharge emission spectra. Matrix isolation studies” and detailed theoreticalls calculations supported this assignment and provided further data on other low-lying electronic states. Bz is frequently mentioned in quantum mechanical studies on changes in the bonding properties as a function of period number for the fist-row elements.19 Knudson cell experiments% where Bz(g) is produced under equilibrium conditions have given a dimer dissociation energy (Doz98)of 3.1 f 0.2 eV. Boron’s 2300 K melting point makes the production of clusters from oven sources rather difficult and generation of boron clusters larger than two atoms has been reported in only a few instances. Berkowitz and Chupkazl produced B2-5 by ablation of solid boron with focused ruby laser light in 1961. Gole and co-workerszzhave prepared a distribution of small boron clusters in an oven source and observed intense chemiluminescence from their reactions with N 2 0 and NOz. Only a few theoretical studies on small boron clusters larger than two atoms have been performed. Whitesidez3determined ground electronic states, geometries, and atomization energies for Bz-4 and B6 using self-consistent field configuration interaction (SCF-CI) calculations. He used unrestricted Hartree-Fock wave functions with STO-3G, 6-31G (double f), and 6-31G* (double { with d-functions) basis sets, and fourth-order Moller-Plesset theory to calculate the configuration interaction. The lowest energy geometries for each cluster size were determined by optimizing the bond lengths for several initial symmetric geometries (Le,, for B4: linear, square, rhombus, and tetrahedron). Each geometry was then optimized with progressively larger basis sets, and the lowest energy configuration at the highest level of theory was chosen as the equilibrium structure. We have used Whiteside’s results on the neutral clusters as a starting point for our own theoretical work on the properties of boron cluster ions. Another theoretical study of the neutral tetramerz4 generally agrees with Whiteside’s results. Several studiesz5 have also probed the properties of the capped pentagon structure for B6 but have not attempted to optimize the geometry. BIZhas been the subject of several theoretical s t ~ d i ewhich s ~ ~ have ~ ~ been ~ cited below to help explain the bonding and structures of the larger boron cluster ions. Experimental Details Only a brief description of the experimental method will be given since a previous paperz8has already detailed the construction (16) Douglas, A. E.; Herzberg, G. Can. J . Res. 1940, 18A, 164. (17) Graham, W. R. M.; Welter, W. Jr. J . Chem. Phys. 1976,65, 1516. (18) Dupuis, M.; Liu, B. J . Chem. Phys. 1978,68, 2902 and references therein. (19) See for example: Averill, F. W.; Painter, G. S. Phys. Rev. B 1986, 34,2088. Pathak, R. K.; Sharma, B. S.;Thakkar, A. J. J. Chem. Phys. 1986, 85, 958. Becke, A. D. Phys. Rev. A 1986, 33, 2786. (20) Darwent, B. de B. Bond Dissociation Energies in Simple Molecules; NSRDS-NBS 3 1; National Bureau Standards: Washington, DC, 1970. (21) Berkowitz, J.; Chupka, W. A. J . Chem. Phys. 1964, 40, 2735. (22) Woodward, R.; Le, P. N.; Temmen, M.; Gole, J. L. J . Phys. Chem. 1987, 91, 2637. (23) Whiteside, R. A., Ph.D. Thesis, Carnegie Mellon University, Pittsburgh, 1981. (24) Koutecky, J.; Pacchioni, G.; Jeung, G. H.; H a s , E. C. Su$. Sci. 1985, 156, 651. (25) Helmstreet, L. A. Gw. Rep. Announce. Index (US.)1976, 76(1), 88. Sykja, B.; Lunell, S. Surf.Sci. 1984, 141, 199. (26) Longuet-Higgins, .~ H. C.; Roberts,M. de V. Proc. R.Soc. London Ser. A 1955, 236 110. (27) Hoffmann, R.; Gouterman, M. J . Chem. Phys. 1962, 36, 2189. (28) Hanley, L.; Ruatta, S. A.; Anderson, S. L. J . Chem. Phys. 1987, 87, 260.
Figure 1. Schematic diagram of laser ablation source for boron cluster ions. B,’ clusters produced by laser ablation of solid boron are trapped by the radio frequency fields of the octapole ion trap, entrained in the H e gas pulse from the pulsed nozzle, and accelerated into the RF cooling
1, 5
~
10
ATOHS PER CLUSTER
Figure 2. Typical mass distribution of BZ from the laser ablation source.
and operation of the cluster ion beam machine and the data analysis techniques. In summary, boron cluster ions produced by laser ablation are cooled in a convoluted rf trap, mass selected by a Wien filter, and injected into a set of octapole ion guides where collisions occur inside a scattering cell filled with 0.10 mTorr of Xe. All fragment ions together with remaining parent ions are collected by the rf field of the octapole, accelerated into a quadrupole mass spectrometer, and counted. Boron cluster ions are produced by laser ablation29of an isotopically pure boron target (Eagle-Pitcher, IlB, 98.6%) inside a specially designed radio frequency (rf) ion trap. A schematic diagram of the laser ablation source is shown in Figure 1. About 10 mJ of the 1064-nm fundamental of a Nd:YAG laser (Quantel) is tightly focused onto a piece of solid boron. Before the laser is fired, a valve backed by 40 psi of H e is pulsed open such that the gas pulse crosses the laser pulse over the boron sample. The laser ablation process occurs inside a rectangular 8-pole rf trap with dc front and back plates. Once past the source region, the rectangular rf trap is deformed to an octapolar, cylindrically symmetric array which in turn couples with the cooling trap. Comparison of CID and reactions with and without the additional helium buffer gas contributed by the pulsed valve suggest that there are no metastable B,+ ions in the thermalized ion beam. Theoretical Details Our calculations took Whiteside’sz3 lowest energy geometries for the neutral boron clusters and reoptimized them for the cations with the 6-3 1G basis set. Bz4+ were reoptimized again with the 6-31G* basis and total energies were obtained by configuration interaction. In the case B5+, we took the linear, symmetric pentagon, square pyramid, and trigonal bipyramid structures as starting geometries for optimization. The B6+ geometry was derived by optimizing the bond lengths of Whiteside’s neutral B6 structure for B6+. The SCF-CI calculations were run on a VAX 8600 with programs using restricted Hartree-Fock wave functions (29) Hanley, L.; Anderson, S. L. J. Phys. Chem. 1987, 91, 5161.
Structure and Stabilities of Boron Cluster Ions
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5805
1.2 -
Product K e y
12
1.0
-
.8
-
i
B3+ e3@
.6 -
O-B,'-1 2
.o
.2
4
6
8
10
12
14
i
.5 .6
.4
-
.3
-
Collision Energy (CM, eV) Figure 3. Cross sections for CID of B2-,+. Data are plotted as points and the best deconvoluted fits to the data are plotted as curves.
developed by one of the authors (J.L.W.30). The configuration interaction employed single and double excitations from all ( m ) occupied orbitals to (24 - m ) unoccupied orbitals of the 6-31G* basis. The geometry optimizations of the pentamer and hexamer clusters only considered structures and deformations of high symmetry and it is possible that we have missed structural isomers. Other theoretical techniques can more easily obtain structures for the larger clusters. Hopefully the experimental and computational results presented here will stimulate further theoretical work on boron clusters.
Experimental Results Figure 2 shows a typical mass spectrum of boron cluster ions emanating from the source, recorded by allowing all cluster ions to pass through the primary mass selector, and then sweeping the second mass selector. The peak in the cluster size distribution is dependent on experimental conditions and the observed fine structure is partially due to instabilities in the laser ablation source. There are, however, consistent "magic numbers": cluster ions with (30) Whitten, J. L. Acc. Chem. Res. 1973,6, 238 and references within.
5, 10, 11, and 13 atoms always appear with anomalous intensity, while 2, 3,4, 12, and 14 atom clusters are less intense. In most cases the 15-atom cluster ion appears with higher intensity than average, while the 16-atom cluster is weak. Intensity fluctuations such as these in cluster size distributions from various types of sources are frequently interpreted as indication that the more abundant size clusters have enhanced stability.31 In fact, we find only limited correlation between the magic numbers in the source distribution and actual cluster stability as measured in the CID experiments described below. For example, the CID data indicates that the magic size cluster ions B5+and BI3+are indeed particularly stable. However, while Blo+ and BI1+are intense and Biz+ is weak in the source distribution, the CID data indicates that their stabilities are identical. The same is true of B7++; 7- and 8-atom clusters are intense in the distribution while the 9-atom cluster is weak, yet the CID data indicates that all three have comparable stability. The point is that magic numbers alone are not a reliable indication of stability (31) Cohen, M.L.; Chou, M. Y.;Knight, W. D.; der Heer, W. A. J. Phys. Chem. 1987, 92, 3141 and references therein.
5806 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988
Hanley et al. 9 -
1 6 1 14 12
B8+
B,+
8:
t
7 -
t
6 -
t
P
4
5 -
i 6 4
14 n
-
30 28 26 2 4 22 c 20 c 18 c 16 14 12
0
c
10
i
Product K e y
.8 -
0-BA-1
.6 -
000
$-B'
Collision Energy (CM,eV) Figure 4. Cross sections for CID of B8+
Data are plotted as points and the best deconvoluted fits to the data are plotted as curves.
unless, as is rarely the case, the cluster source is operating near equilibrium conditions. The absolute cross sections for production of all fragment ions in CID of B2-ls+ with Xe are plotted as a function of collision energy in Figures 3 and 4. The points are the experimental data for the major channels ( u ( E ) , 2 0.1 A2).The measured collision energy dependence of the cross sections is distorted by the translational energy spread of the reagent ion beam, and by thermal motion of the target gas.32 We can correct the threshold behavior of the cross sections and derive the "true" appearance potentials for each fragment, using a standard deconvolution fitting technique developed by Lifschitz et and detailed in ref 28. The corrected appearance potentials are given along with error estimates in Table I, and the best fits for the threshold region of each cross section are given in Figures 3 and 4 as smooth curves. The deconvolution procedure is only accurate when the cross sections rise rapidly from threshold, as is true for cluster ions up to BI2+. For B13+the threshold is very broad, and the AP's given (32) Chantry, P. J. J . Chem. Phys. 1971, 55, 2746. (33) Lifschitz, C.; Wu, R. L. C.; Tiernan, T. 0.; Terwilliger, D. T. J . Chem. Phys. 1978, 68, 247.
-
TABLE I: Apoearance Potentials for B.+ + Xe B+,B,,+ product reagent product reagent B.+ B+, eV B,,+, eV B.' B+, eV Be,+, eV 2 0.8 f 0.6 8 6.2 f 0.7 4.2 f 0.5 3 2.3 f 0.6 4.3 f 0.7 9 4.3 f 0.7 4.0 f 0.5 10 5.6 f 1.0 5.4 f 0.5 4 2.4 f 0.6 8.0 f 1.5" 5 3.6 f 0.6 7.1 f 0.6 11 6.5 f 0.8 5.6 f 0.8 6 3.2 f 0.7 2.7 f 0.6 12 7.0 f 1.5 5.5 f 0.5 7 4.8 f 0.5 5.5 f 0.7 13 7.8 f 0.9' 8.0 f 1 . 5 O
"These AP's estimated by visual examination, due to poor quality of data.
-
for it were estimated visually. Due to low B4+intensity from the cluster source, the B4+ B3+ threshold may be artificially high, which is reflected in the larger error bars for this AP. Fragmentation channels which are too noisy to be fit accurately (Le. BI3+ BI2+)do not have curves plotted over the data. The lowest energy AP for each cluster ion gives an upper limit on its stability. These are plotted as a function of cluster ion size in Figure 5. Low reagent intensity coupled with insufficient Wien filter resolution has prevented measurement of AP's for clusters
-
Structure and Stabilities of Boron Cluster Ions
The Journal of Physical Chemisfry, Vol. 92, No. 20, 1988 5807
Clusfer Size
Figure 5. Cluster ion stabilities and total CID cross sections (at 5 , IO, and 20 eV) plotted as a function of cluster size. Stability refers to the lowest energy fragmentation appearance potential for each cluster, containing more than I3 atoms. A new cluster ion beam machine is under construction which should allow work with boron cluster ions containing >IO0 atoms. Total C I D cross sections at several collision energies are also plotted as a function of cluster ion size in Figure 5 , along with the lowest energy AP's. At 5.0-eV collision energy, only B,+ and B9+ are above threshold with significant fragmentation cross sections. As the energy increases to 10.0 eV, CID cross sections for all the cluster ions except Bii+ and Big+ increase and are closc to their maxima. Between 10.0- and 20.0-eV collision energy, the fragmentation cross sections for most of the cluster ions are almost constant, with large increases seen only for Bii+ and BI1+, and a large decrease for BIZ+. It is interesting to compare the trends in o, with cluster size with the trends in measured cluster stabilities (AP's). AP's generally increase with cluster ion size: however, there are large fluctuations from the overall trend. The C I D cross sections (at energies well above threshold) are relatively independent of cluster size. This presumably reflects two offsetting effects: the increase in physical size results in larger collision cross sections, but the clusters become more difficult to fragment as size increases. In many cases the fluctuations in stability are reflected in the total cross sections, which agrees with the intuitive idea that more stable species should be harder to fragment. This relation of low C I D cross sections with high stability has been used by Jarrold and co-workersg' as an experimentally simple method for determining relative cluster stabilities from measurements of CID a t a single collision energy. In some cases this approach can be misleading, as shown by comparison in Figure 5 of am's and measured AP's for Bio+-Big+. With cross sections a t 10 eV one would conclude that Bii+ and Blgc are anomalously stable and that B,z+ is unstable. With data a t 20 eV one would conclude that stability is relatively independent of size and that BIl+ is less stable than BIZ+. Of course with data a t enough energies it is likely that the correct relative stabilities would be obtained, hut that is nearly as time consuming as measuring the fragmentation thresholds directly. CID product branching ratios change with both the number of atoms in the parent cluster ion and the collision energy. The pattern is clearly seen in Figure 6, which give the branching ratios for 10.0- and 20.0-eV collision energies. For all parent clusters, the only significant fragmentation channel a t the lower energy is loss of a single boron atom. For the smaller cluster ions the atom usually carries the charge (B+ production), while for the larger cluster ions the charge usually remains on the cluster fragment (Bpi* production). The crossover occurs a t B6+,which dissociates to both B+ and Bpi* fragments in roughly equal proportions. At high collision energies, loss of two or three neutral atoms begins to be important, particularly for the larger parent clusters. At 20 eV there is sufficient energy for these products to reflect either B, or B, loss, or sequential loss of B atoms. The uncertainty in the absolute magnitudes of the cross sections is estimated to be
