J. Phys. Chem. 1995, 99, 16276-16283
16276
Effects of Composition, Structure, and H Atom Addition on the Chemistry of Boron Oxide Cluster Ions with HF Jason Smolanoff, Adam Eapicki, Nicole Kline, and Scott L. Anderson* Department of Chemistp, SUNY at Stony Brook, Stony Brook, New York 11 794-3400 Received: May 3, 1995; In Final Form: July 14, 19953
We have studied reactions of thermalized, mass-selected boron oxide (B,?O,,,+,n = 1-3, m = 1-4) and hydrogenated boron oxide (HB,,O,,,+, n = 1, 2, m = 1-3) ions with HF. Cross sections are reported for each product channel for collision energies ranging from 0.1 to 10 eV (center of mass). On the basis of the observed collision energy dependence, product branching, and effects of hydrogenation, we propose a reaction mechanism where facile reaction occurs only at certain sites on the oxide, most likely at terminal -B=O groups. H atom addition greatly decreases reactivity, apparently by blocking the reactive sites. Our data also point out several significant uncertainties in the literature thermochemistry for B/O/F/H species, and we are able to give some limits on AHi for several molecules.
I. Introduction An important issue in the chemistry of clusters is how cluster geometric and electronic structure influence chemical reactivity. Since experimental structures are not available for most cluster systems with more than a few atoms, this type of inquiry requires a combination of experimental and theoretical information. We have focused our efforts on boron, which in principle is relatively easy to deal with computationally. We have reported a series of experimental studies of boron cluster cations'-6 focusing on structure-reactivity relationships. Ab initio structure calculations have been reported by ush and by several theory groups. I One important property of boron is its high volumetric heat of combustion, making it an important additive in high energy density fuels, propellants, and explosives. Achieving high combustion efficiency is difficult, however, and this has recently motivated a concerted effort to understand and boron's chemistry with both oxygen and fluorine-containing oxidizers. As part of that effort, we have focused our boron cluster work on 0- and F-containing reactants. To complement the work on pure boron cluster ions, we have begun to investigate the chemistry of boron oxide cluster ions, examining the effects of cluster stoichiometry and structure on reactivity. The chemistry of boron oxides is of great importance in boron combustion, both because the desired products are oxide polymers and because boron particles are coated by a passivating oxide layer that must be removed before ignition can occur. There has been considerable e ~ p e r i m e n t a l ' ~ - ?and ~ theoretical?"-'* work on gas phase boron oxides and their derivatives. BO, B203, BO?, HBO, and FBO are a few examples of species known to exist in the gas phase. Large polymeric boron oxide cations (larger than B203+) were first observed in sputtering/mass spectrometry experiments by Doyle.?9 On the basis of abundance distributions and collisioninduced dissociation patterns, Doyle classified the oxides into six series. where the members of each series are built up by adding BzO3 units to a core ion, which is different for each series (B+, BO+, B20+, ...). Other than the different core ions, all the B,,O,,,+ were proposed to be similar, with several resonance structures consisting of alternating boron-oxygen
'-
Corresponding author. Current address: Chemistry Department. University of Utah. Salt Lake City, UT 84112. * Abstract published in A d m n c r ACS Ahstrricfs, September 1. 1995.
0022-3654/95/2099- 16276$09.00/0
bonds, terminated by B=O groups. Boron atoms in these oxide networks can have 2- or 3-fold coordination to oxygen atoms. Doyle's structural prototype for all of the boron oxide cluster ions is B304' [O=B-0-B-0-B=O]+. One motivation of our study was to see whether boron oxide cluster ion reactivity is correlated in some way to Doyle's proposed structural series. We show below that the reactions are quite structure sensitive, but reactivity is not obviously related to Doyle classifications. This paper reports a study of the reactions of thermalized, mass-selected boron oxide (B,?O,),-, n = 1-3, nz = 1-4) and hydrogenated boron oxide (HB,,O,,,+,n = 1, 2, m = 1-3) ions with hydrogen fluoride. Cross sections are reported for each product channel for collision energies ranging from 0.1 to 10 eV in the center-of-mass frame. On the basis of the observed collision energy dependence, product branching, and effects of hydrogenation, we propose a reaction mechanism where facile reaction occurs only at certain sites on the oxide. Our data points up several significant uncertainties in the literature thermochemistry for B/O/F/H species, and we are able to give some limits on AHt values for several molecules. To our knowledge, this is the first experimental study focusing on reactivity of size and composition-selected boron oxide clusters.
11. Experimental Approach The cluster ion beam apparatus used for these experiments and the methods of operation, calibration, and data analysis have been described previously,"',?' so only a brief description is given here. Boron oxide cluster ions (B,,O,,,+)are generated using 12 keV argon atom bombardment3? of a thin film of vitreous B203, maintained just above its melting point (-450 "C). The hot nascent cluster ions are thermalized by collecting them with a radio-frequency (rf) octapole ion guide and then storing them in a labyrinthine rf trap where they undergo -10' collisions with 0.01 Torr of helium buffer gas at -350 K. The desired cluster size/composition is selected by passage through a quadrupole mass filter, and then the reactant beam is injected into an octapole ion guide system where the desired collision energy is set. A portion of the guide is surrounded by a collision cell that is filled with 5 x 1 O-i Torr of the neutral reactant. as measured by a capacitance manometer. Product ions and the remaining reactant ions are then collected by the ion guide, mass-analyzed by a second quadrupole mass filter, and counted with a Daly detector?.'-discriminator-scalar combination.
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 44, 1995 16277
Boron Oxide Cluster Ions with HF The only significant change in the instrument for these studies is the homemade quadrupole mass filter,34used to mass select the reactant beam. The ion optics and operating conditions have been carefully optimized so as to minimize perturbations to the kinetic energy distribution of the reactant beam. We are able to simultaneously achieve adequate resolution (MIAM > loo), greater than 50% transmission, with only about a 50 meV perturbation to the energy distribution. For these experiments, the lab frame energy spread from the source is -0.3 eV, corresponding to -70 meV in the CM frame for B304+ +HF, for example. The boron oxide sputtering targets were prepared in similar fashion to that reported by Doyle.35 Isotopically pure (94% IoB, Eagle-Picher) B203crystalline powder was spread on a stainless steel substrate and then melted in a furnace at 650 "C for 3-4 h. During heating, the fumace is purged with 0 2 to keep the environment relatively free of water vapor. The sample is then heated in vacuum (-450 "C) to further remove water. We have found boron oxide to be amazingly hygroscopic. Even in high vacuum (lo-' Torr), our samples begin to show hydrogen contamination (presumably from absorption of trace water35) unless they are maintained near the B2O3 melting point. Despite our best efforts to dehydrate the boron oxide target, some hydrogen-containing cluster ions, HB,O,+, are formed in the sputtering process, and for certain cluster sizes the intensities are high enough to allow studies of their chemistry. This is complicated by the presence of IlB contamination-for example, mass 43 has contributions from both HIoB02+ and from "B02+. Since we can measure the actual 1°B:llB ratio in our sample (94:6) and the intensities of the pure I0BflOm+ peaks, it is straightforward to correct the HB,O,+ signal for I lB contamination. Similarly, by measuring the cross sections for reaction of pure ioBnOmfclusters, the cross sections for products from HB,O,+ reactions can be corrected. The only problem with this situation is that the resulting cross sections are noisier than we would like because, in many cases, the isotope contamination dominates the signal at the masses in question. Since the isotope contamination increases with number of B atoms, we were unable to obtain acceptable data for HB303+ and H B 3 0 4 + .
111. Results Cross sections for all significant product ions from reaction of B,O,+ clusters with HF are given in Figure 1 for collision energies from 0.1 to 10.0 eV (center of mass). Corresponding data for the H,B,O,+ clusters are given in Figure 2. Note that the large range of cross section magnitudes observed for the B,O,+ required a log scale while the data for the HB,O,+ are better shown on a linear scale. Each experiment was done on at least three separate occasions to ensure reproducibility. The curves drawn through the experimental data points are only intended as guides for the eye and have no physical significance. A. The B,O,+ HF System. With one exception, all B,O,+ cluster sizes and stoichiometries react predominantly by elimination of either FBOH or FBO,in either case stripping a BO unit from the parent oxide cluster:
+
B,o,+
+ HF - FBOH+ + B,-~O,-,
-
-
Bn-lOm-l+
€BO+
+ FBOH
(la) (1b)
+ HB,-,O,-,
In each case, the neutral product is inferred, and we list the most energetically favorable possibility.
The only exception to this product branching pattern is BzO+, where the dominant products are BF+ HBO and HBO' BF. For this reactant, reaction 1 or 2 would leave B or BH products, and this is energetically unfavorable. For reaction of BO+, the dominant reaction is (2a), in this case corresponding to H atom elimination from the FBOH+ collision complex. FBOH+ elimination (la) is generally dominant; however, for reaction of B304+ the branching shifts to FBO elimination (2b). The branching between (la) and (lb) and between (2a) and (2b) is presumably controlled by the relative ionization potentials (IP) of FBOH/B,-IO,-Iand FBO/B,-IO,-IH. The fact that, for most reactant clusters, all four product sets are observed to varying extents suggests that the IPS of FBOH and FBO are similar to those for small boron oxides (-13.5 eV). Similarly, the branching between reactions l a and 2b should depend on the relative proton affinities (PAS) of FBO and B,-10,-1. The change in branching observed for B304+ suggests that PA(FB0) = 6.89 eVZ5> PA(B,-IO,-I) for most of the clusters, but not for BzO3. For most of these reactions, the cross sections peak at our lowest collision energies, indicating that the reactions are exoergic and proceed without significant activation barriers. In some cases there also appear to be cross section components that turn on at higher energies, suggesting that there may be more than one channel contributing to some product ions. In reaction of BO+ with HF, FBOH' is observed with an apparent cross section about one-third that of the main FBO+ product. There are several conceivable explanations for this. It could be a metastable collision complex that survives long enough to be detected. A rough RRKM36 calculation using energetics derived from ab initio25and AMlZ4calculations and ab initio vibrational f r e q ~ e n c i e sindicates ~~ that even at our lowest collision energy, the lifetime is too short to survive the -300 ps flight time to the detector. The FBOH+ must therefore be due to a secondary reaction, most likely FBO+ abstracting H from HF, which we estimate to be about 1 eV exoergic. The relative amounts of FBO+ and FBOH+ observed require a secondary reaction cross section of -100 A2, which is entirely reasonable since the velocity of the FBO+ product, and thus the collision energy for secondary reaction, must be substantially lower than the primary collision energy. In fact, the low LAB frame velocity of the primary product seems to be the key factor in allowing the secondary reaction. For the larger reactant clusters, where the mass ratios dictate higher product LAB velocities, no secondary reaction products are observed. Figure 3 summarizes the dependence of total reactivity and product branching (at 0.1 eV collision energy) on reactant cluster size and stoichiometry. Reactivity generally increases with cluster size, while product distributions are not strongly dependent on either size or composition. The chemistry appears to be controlled by elimination of a stable leaving groups such as BO, FBO,FBOH, and B02. B. The HB,O,+ HF System. Figure 2 shows the cross sections for reaction of hydrogenated boron oxide cluster ions with HF. As noted above, these data have been corrected for IlB isotope contaminant, and this, combined with the low reactivity, significantly degrades the signalhoke ratio. The striking feature of this data is the dramatic decrease in total reactivity compared to the corresponding pure oxide clusters. Addition of a single hydrogen atom decreases reactivity by roughly a factor of 10 for reactant clusters containing one boron atom and by a factor of 2-3 for clusters with two boron atoms. The only exception is BO*+, which is the least reactive pure oxide cluster, and shows little effect from monohydrogenation. When doubly hydrogenated, however, its low energy reactivity is reduced by a factor of -20.
+
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Smolanoff et al.
16218 J. Phys. Chem., Val. 99, No. 44,I995
a
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+
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c BO+ v OBO' + FBOH
v
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Collision E n e r g y ( e V )
C o l l i s i o n E n e r g y (eV)
N-
5
y4 v I
11 t
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Collision E n e r g y (eV)
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+ FBOH
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Collision E n e r g y (CME, eV)
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Collision E n e r g y ( C M E , e V )
Figure 1. Cross sections for all products observed in reaction of B,,O,,,+with HF.
10
Boron Oxide Cluster Ions with HF
J. Phys. Chem., Vol. 99, No. 44, 1995 16279
1 x 1
11 -
1 .o
+H + OH
0
FBOH+
A
HBF+
,-0.8 4 v
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+ BF
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FBOH~'
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u)
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Collision E n e r g y (CME, eV) Figure 2. Cross sections for all products observed in reaction of H,B,O,+
with HF.
From the collision energy dependence of their cross sections, it appears that the H,B,O,+ can be divided into two groups. HBO', HB02+, HB202+, and HB203' all show a sharp peak in reactivity at low collision energies, followed by much smaller and nearly energy independent cross sections for energies above -1 eV. For HB2O+ and H2B02' the reactivity peak at low energies is nearly absent. As will be discussed, we believe that the large and varied effects of hydrogenation on reactivity provide clues to the reaction mechanism and cluster structures.
of the species of interest. For example, only Koopmans' theorem IPS are available3 for HBO (13.2 eV) and BOH (9.0 eV), and these suggest that the most stable ion structure is BOH+, unlike the neutral HBO ground state.28 As pointed out in an earlier paper,3 however, Koopman's theorem is suspect for many HB,O, species because there appear to be large changes in electronic structure upon ionization. For other species of interest there are discrepancies between IPScalculated with different theoretical methods. As expected, semiempirical results tend to agree better with the few experimental numbers available for the F/B/O/H system. For B303, B304, HB202, and HB203 there are no experimental or theoretical results available, and we are forced to make estimates. In this context we note several useful facts. Doyle has observed that boron oxide cluster ions tend to fragment by loss of BO. One can imagine, therefore, that B304 is built up by a sequence of BO addition steps: 0 BO2 B203 B304. The energies for the first two steps in this sequence are
VI. Discussion A. Thermochemistry for B/O/H/F Species. The thermochemistry of small B/OiH/F-containing molecules is not well established, and we are only able to estimate the energetics for many reactions of interest. In addition to standard compilation^,^^-^^ we have also used results from ab initio25.26,28,37 and s e m i e m p i r i ~ a l ~calculations. ~,~~ One problem is lack of well-established ionization potentials (IPS)for many
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Smolanoff et al.
16280 J. Phys. Chem., Vol. 99, No. 44, 1995 I
1EC
TABLE 1: Thermochemistry for All Observed Reactions of B,,O,+ with HF Listed in Order of Relative Importance at Low Collision Energief products
estimated AH,,, (eV)
FBO+ H FBOHBFOH HBF0 BHOF B++O+HF HBF' BO BF' t HBO FBOH' + B FBOHt 0 BOt 0 HF BOt HOF FBOH' BO BOH' FBO BOt FBOH BOC BO HF BOt FBO H Bt + BO2 HF FBOH' BO? HOBO- FBO BO+ FBO?H BOFBO OH BOBO? HF B02' FBOH BO>' FBO H FBOH' BzO? FBO' HBzOz HB202- FBO B?Ozf + FBOH Bt BzOi 7 HF HB203' FBO FBOH' BzO? BzO?' FBOH FBO' HB20q BzO:' BO A HF BzO?' + FBO H
-0.6 -7.5 $0.14 no data, obs to be exoergic +2.9
reactant
-
+
+
+ +
+
BO-
B,Ot
BO,+
B,O,*
B,O,-
B,O,-
B,O,*
Figure 3. Dependence on reactant cluster size and composition of the total cross section and product distribution.
5.68 and 5.59 eV. If we assume that the third step is similar, we can estimate AHf(B304) x -330 kcal/mol. Another useful observation is that the IPSfor all small B,O, are within 50 meV of 13.5 eV. The only exception, BO, is only 0.5 eV lower. We therefore assume IP(B3O4) x 13.5 eV, and this results in AHf(B304+) x -18 kcaVmo1. Similar arguments lead to AHf(B303) x -220 kcaymol, AHf(B303+) x +94 kcal/mol, AHf(HB202) * -190 kcaVmo1, AHf(HB202+) x +llO kcaV -4 mol, AHI(HB203) -300 kcaVmo1, and AHr(HB203') kcal/mol. Table 1 summarizes the energetics estimated for the major reactions observed. The one major product molecule for which there seems to be a problem is FBOH. There is no experimental data pertaining to the energetics of this molecule. Nguyen et have reported ab initio calculations on a variety of small boron-containing molecules and their protonated counterparts. For FBOH' they found an optimized geometry that is quasi-linear with FBOH topology and a AHf of 61.4 kcaVmo1. Soto reported37 an ab initio study of the F HBO reaction and gave energetics that result in a AHf of -101.7 kcal/mol for a F(H)BO species of trigonal topology. More recently, she has examined the quasilinear FBOH topology and finds a lower AHf of - 113 kcaV mol. These calculations are still in progress, and this number may decrease further. A more extensive set of calculations by Page42 is currently in progress, but the methods and results are consistent with those of Soto. From the AHf values for the neutral and ion, we calculate IP(FB0H) 7.5 eV. This seems surprising in light of the fact that all known FB,O,H species have IF'S in the 12.5-13.5 eV range and, as noted above, appears to be inconsistent with the branching for reactions l a and 1b that seem to imply an IP near 13.5 eV. This inconsistency would be eliminated if the neutral product in (lb) is not FBOH, but rather FBO H or BO HF, all of which have IPS > 13 eV. This, however, raises another difficulty. The other evidence we have of a problem with FBOH thermochemistry is our observation of reaction 1b with no activation energy. Of course, we only infer the identity of the neutral product, but the lowest energy channel is clearly FBOH elimination. In several cases the "FBOH-loss" cross section appears to have two components: a low-energy component with no activation barrier and high-energy component that turns on above 4 eV. Taking reaction of B203+ as the example, there are several candidates for the high-energy channel, for example BO?+ FBO H, AHrxn% +4.2 eV. For the low-energy
+
-
+
-
+
+
+
+ + + + + + + + + + + + + + + + + + + + + + + +
+
+
+ + +
+3.7 no data, obs to be exoergic no data. obs to be exoergic no data, obs to be exoergic -2.4 f5.2 14.5 -3.4 -4.0 -2.1
+4.2 $2.8 43.3 -2.4 -1.6 no data, obs to be endoergic +5.0 $5.1 f 3 . 5 (obs exoergic) +4.2 -3.3
-2.2 -3.0 f 2 . 7 (obs exoergic) +1.3 -3.0 -2.5 t3.5 (obs exoergic)
-2.2 f5.5 f3.6
" In parentheses we indicate cases where exoergic reactions are observed that are predicted to be substantially endoergic.
+
component, the only candidate is BO>+ FBOH. For this reaction to be exoergic for reaction of B203+, AHdFBOH) must be less than - 194 kcaVmo1, roughly 80 kcal/mol lower than the value given by Soto. The size of the discrepancy raises the possibility that the calculations are not probing the lowest energy structure or state for FBOH. We note, however, that there is also some uncertainty in interpretation of the experiments. The exoergic component for reaction l b is quite small in the case of B203+, raising the possibility that this small component is due to reactions of a higher energy isomer that might be present in our beam at the 1% level. For reaction of B3O3+ and B304+, the cross section for (lb) is far to large to explain this way; however, the thermochemistry for these reactant ions is unknown, as discussed above. If these ions are substantially (3.5 eV) less stable than our estimates, then reaction 1b would be exoergic as observed. This, however, does not explain the large branching observed for (lb). For example in reaction of B304+, reactions l a and 1b have identical branching ratios despite the theoretical prediction that (lb) should be 6 eV less energetically favorable than (la). While it is not impossible that two reactions with such different energetics could have identical branching, it is highly unusual, particularly for reactions of cluster ions at low energies where product distributions tend to be approximately statistical. Experiments are underway to measure the bond energies in B,O,+, and this should resolve this issue. The 6 1.4 kcal/mol AHdFBOH') value calculated by Nguyen et al.25 seems to be about right. FBOH+ production is the
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Boron Oxide Cluster Ions with HF dominant product channel for most of the clusters, suggesting that this reaction must be substantially exoergic in order to compete with other exoergic channels (e.g., FBO elimination). The requirement that reaction l a be exoergic for all reactant clusters sets an upper limit on AH@BOH+) of 5 120 kcal/mol; however, the product branching suggests the true value is much lower. B. Proposed Reaction Mechanism: a Reactive Site Model. To understand the reaction mechanism and its dependence on boron oxide cluster structure and stoichiometry, there are several features of the data to consider along with the possible B,O,+ structures. 1. Collision Energy Dependence. Most of the reactions observed have a somewhat unusual collision energy dependence. Particularly for the larger clusters, the cross sections drop rapidly as collision energy increases, decreasing almost 2 orders of magnitude in the energy range from 0.1 to 1.O eV. This is much faster than the corresponding decline in the ion dipole capture cross thus, the reaction efficiency per collision decreases. Taking B304+ as an example, the efficiency declines from -60% at 0.1 eV to -6% at 1.0 eV. As collision energy 1 eV, the cross sections become nearly is increased above energy independent and in some cases (e.g., FBOH+ from B303+ or B304+) even increase again at high energies. This sharp change in energy dependence suggests that there is a change in reaction mechanism occurring near 1 eV. There are two experimental artifacts to worry about when extremely sharp low-energy cross section peaks are observed. For heavy ion-light target systems at low collision energies, the thermal motion of the target increases the probability of collision compared to the probability calculated assuming a stationary target. When the collision energy is low enough that the ion beam velocity is slow compared to the thermal target motion, this can result in large apparent cross sections. For the extreme case examined here (B304+) this effect accounts for only -2% of the peak observed at Ecol = 0.1 eV. Another potential problem is that slow primary ions can get trapped in the scattering octapole. Since these trapped ions can react with high probability, even a small fraction of trapped ions can contribute a large apparent cross section. Trapping tends to be a problem for light reactant ions, whereas we observe the sharp peaking for the heaviest reactants. For B304+ the lab energy at our lowest collision energy is already > O S eV, and trapping is not likely, especially with a light target such as HF. We conclude that the unusual energy dependence is real and therefore is an important clue regarding the reaction mechanism. We have previously observed similar sharp collision energy dependence for reaction of large boron cluster ions with C022 and N204. We propose the following mechanism. Suppose that reactim can occur with no activation barrier only for attack on certain sites on the oxide cluster. As will be discussed below, the oxide clusters are reasonably coordinatively saturated; thus, it is not unreasonable that there would be preferred binding sites for HF attack. The high reaction efficiency observed at low collision energies can be explained by invoking a HFB,O,+ intermediate complex that allows the reactants time to find a favorable geometry for reaction. The complex would be bound by at least the ion dipole potential, which provides a binding energy of -2 eV at the van der Waals contact distance. As the collision energy is raised, the lifetime of this weakly bound complex drops rapidly, and reaction becomes correspondingly less efficient. At high energies, reaction is direct and occurs mainly in the small fraction of collisions where the impact geometry is correct. This constraint accounts for the nearly energy-
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J. Phys. Chem., Vol. 99, No. 44, 1995 16281 Cluster
Neutral
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BO
.B=O
[B-oj
:B-o-B:
E 8-0-bj'
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[H-8-0-
v 4 )O=B-O-~-O-B=O
[O=B-O-B-O-B=O:'
/EB-O-Hj
s-O-B=Oj
[H-O-b-O-B-O-B=O]'
Figure 4. Likely structures for the B,O,, B,O,+, and HB,O,,+. Note that for many species several resonance structures can be drawn; only one is shown.
independent cross sections observed for Eco~~,s,on > -1 eV. The fact that there are small increases in the some cross sections in the 2-3 eV range suggests that it is possible to collisionally drive reaction in unfavorable geometries. Growth of the B+ or BO+ product ions (BOH+ for the H,B,O,+) at high collision energies is probably due mostly to simple collision-induced fragmentation, which presumably is not strongly dependent on impact geometry. 2. The Efsects of Hydrogen Addition. Addition of hydrogen strongly suppresses the low collision energy component of the reaction cross section. For BO+ and B20+ the suppression is roughly an order of magnitude, while for the larger clusters it is only -50%. For BO>+, a single hydrogen atom has little effect, but H2B02+ is the least reactive species observed. Clearly, this strong and varied effect must be a clue regarding the reaction mechanism and its dependence on the cluster structure. We believe that the H addition effects support our reactive site model. The H atoms are most likely bonded to the oxide clusters at positions that are not already coordinatively saturated, Le., at the reactive sites. We propose that each added H atom effectively blocks one reactive site. For the smaller clusters, a single H atom reduces reactivity by an order of magnitude, suggesting that there is only one reactive site. For the larger clusters, reactivity is only halved by a single H addition, suggesting that there are two reactive sites. The suppression effect varies significantly from cluster to cluster, with particularly large effects for HB2O+, H2B02+, and HB202'. 3. Structure-Reactivity Relationships. Obviously, our proposed reactive site mechanism only makes sense if it is consistent with the cluster structures. Unfortunately, structures for boron oxide clusters, and especially the cations, are not well established. Figure 4 shows a variety of likely structures for the B,O,,, BnOm+,and HB,O,+, based on considerations given below. In the figure we indicate formal bond orders and radical centers; however, in many cases it is possible to draw several resonance structures, and thus the actual electron density distributions are likely to be less clearly defined. The topology for clusters with B/O ratios '1 is reasonably unambiguous: linear OBO, OBOBO, and OBOBOBO. In the first two cases, theory and/or experiments have found linear structures, and for B304 it is difficult to imagine a more likely topology. For cluster ions with B/O 2 1 the structures are less obvious. Even BO is somewhat unusual. In the neutral, &(BO) = 8.28 eV39 consistent with a multiply bonded structure as shown in Figure 4. For the cation the dissociation energy is only -3.7 eV (Le., single bonded), indicating that the electronic structure undergoes a substantial rearrangement upon ionization. The
most stable topology for neutral B202 is generally held2".j4 to be O=B-B=O; however, both STO-3G and semiempirical calculation^^'.^^ suggest that, for the ion, a B-terminated [BOBO]' structure is more stable by 2 1 eV. According to STO-3G and semiempirical calculations,"' BlO is more stable as BOB than as BBO in both the neutral and cation; however, we feel that these calculations are not sufficiently accurate to accept without question. As with BrOz', B3Os+ could assume either BOBOBO+ or OBBOBO'. There is some indirect experimental evidence relating to the structure of our reactant cluster ions. We observe that all the low-energy reaction channels involve loss of BO units as FBO, FBOH, or their cations. This suggests that the reactive sites must allow B-F bond formation, cleaving off a BO group, and that it must be possible for the H atom to bind to either the resulting FBO group or the remaining oxide fragment. We never observe products that require fissioning the reactant cluster, suggesting that reactive sites occur near the ends of the oxide chains. We also note that, with the exception of B02+, reactant clusters with B/O ratios < 1 are significantly more reactive than those with B/O 1 1. If the structures shown in Figure 4 are correct, then this suggests that terminal-B=O groups are more reactive than terminal -0-B' groups despite the presence of a formal radical center. We also note that the suppression from H addition is larger for clusters with B/O 1. This would be consistent with H atom blocking of the -B=O groups, leaving only the less reactive -0-B' site to react with HF. Another indirect clue regarding structure comes from our observation of apparent fragmentation channels turning on at high collision energies. For the clusters with B/O < 1 these fragmentation channels include production of BO', BO, and, in the case of B3O?+, B202+. No B+ or B production is observed. This is just what would be expected for structures of the type OBOBOBO, where cleaving off a BO or BOBO group produces two stable fragments. For clusters with BIO 2 1, a major fragmentation pathway is loss of B+, which seems consistent with B-terminated structures such as BOBO and BOBOBO, rather than OBBO and OBBOBO. For the hydrogenated oxide clusters, the high-energy fragmentation channel is usually HBO+/BOH+ loss. This does not provide much insight, since HBO+/BOH' could be lost from clusters of any of the likely topologies: BOBOH, HBOBO, BOB(H)O, or OBB(H)O. Note that in high-energy collisional activation Doyle observed that all boron cluster cations undergo BO elimination. The more complex fragmentation pattern we observe in lowenergy collisions could reflect the nature of the excitation or possibly indicate that chemical interactions are important even at relatively high collision energies. For comparison it is useful to consider the structure of bulk B2O3. Boron oxide can exist in both crystalline and vitreous forms. Crystalline B203 has been characterized by G u d 5 using X-ray diffraction. Three oxygen atoms surround each boron atom, and each unit cell is interconnected by an infinite oxygen "ribbon". Crystalline B2O3 converts irreversibly to the vitreous form when heated above 450 0C!6 The structure of vitreous B2O3 has been examined by 'OB, "B, ''0 NMR,?' and X-ray d i f f r a c t i ~ nand , ~ ~the structure is proposed to consist of boroxyl rings (B303) interconnected by BO3 units. 4. Proposed Mechanism. On the basis of the considerations outlined above, we propose the following reaction mechanism. Since the structures (geometric and electronic) of the reactants are not well established, the mechanism is speculative. We intend to pursue a combined collision-induced dissociationlab
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Smolanoff et al.
16282 J. Phys. Chem., Vol. 99, No. 44,1995
t
H-F
+
0~0-0--8-0-0=0
step 3
+
r
l+
F-0-C-H
I
o=
+ 0 -0-
0 =o
1
Figure 5. Schematic of proposed mechanism for reaction of boron oxide clusters with HF.
initio study to refine the picture of boron oxide bonding and reactivity. The proposed mechanism is shown in Figure 5 . On the basis of the evidence that the reactive site is near a terminal group, we propose that HF attacks preferentially at terminal -B=O groups. For clusters with terminal B atoms (Le., those with BIO L 1) attack may also occur there; however, as noted above these sites seem less reactive than the -B=O sites. Given that BF bonds are substantially stronger than BH, OH, or OF bonds, it seems likely that the initial attack is of F on the boron center. The two major product channels (FBO t HB,7-10,1-j)* and (FBOH B,2-10,!-l)+ can form by simple H atom migration. Both the final H atom position and the charge state of the products are presumably controlled by the energetics of the four possible product combinations. Our mechanism also needs to rationalize the H addition effects. For B203' a single H atom reduces reactivity by roughly a factor of 2 . This is consistent with blocking half of the two B=O groups. It is not clear whether the H atom attaches to a terminal oxygen or to the boron atom, which has an unpaired electron in the resonance structure shown in Figure 4. We have drawn the 0-bonded structure in analogy with BOW HBO, where BOH- is calculated to be the more stable structure for the cation. For BO?+, which is the least reactive oxide examined, a single H atom slightly increases reactivity, while double H addition greatly decreases reactivity. In this case it seems likely that the first H atom attaches to a oxygen atom as shown in Figure 4, leaving the terminal BO group (and converting the B atom to a formal radical center). The second H atom could attach either to the other oxygen or to the boron atom. For the clusters with B/O ratios 2 1, which we propose to be B-terminated on one end, H could presumably attach at either terminus, though we favor attachment at the 0 terminus. Clearly, this mechanism is speculative, yet we feel that it accounts for our observations and is consistent with what little is known about boron oxide cluster structure. We intend to pursue a combined collision-induced dissociation and a b inirio study aimed at providing better structures for the B,,O,,,+, and this should allow a more firmly based mechanism.
+
Boron Oxide Cluster Ions with HF
Acknowledgment. This work was supported by the Mechanics and Energy Conversion Division of the Office of Naval Research (N00014-92-J-1202). S.L.A. is grateful for support from a Camille and Henry Dreyfus Teacher-Scholar Award. References and Notes (1) Hanley, L.; Anderson, S. L. J . Chem. Phys. 1988, 89, 2848. (2) Ruatta, S. A.; Anderson, S. L. 1. Chem. Phys. 1991, 94, 2833. (3) Hintz, P.; Ruatta. S. A.; Anderson, S. L. J . Chem. Phys. 1990, 92, 292. (4) Hintz, P. A.; Sowa, M. B.; Ruatta, S. A,; Anderson, S. L. J. Chem. Phys. 1991, 94, 6446. (5) Ruatta, S . A.; Hanley, L.; Anderson, S. L. J. Chem. Phys. 1989, 91, 226. (6) Hanley, L.; Whitten, J. L.; Anderson, S. L. J . Phys. Chem. 1988, 92, 5803. (7) Bonacic-Koutecky, V.; Fantucci, P.; Koutecky, J. Chem. Rev. 1991, 91, 1035. (8) Kawai, R.; Weare, J. H. J. Chem. Phys. 1991, 95, 1151. (9) Kawai, R.; Weare, J. H. Chem. Phys. Lett. 1992, 191, 311. (10) Martin, J. M. L.; Francois, J. P.; Gijbels, R. Chem. Phys. Lett. 1992, 189, 529. (1 1) Kato, H.; Yamashita, K.; Morokuma, K. Chem. Phys. Lett. 1992, 190, 361. (12) Ray, A. K.; Howard, I. A.; Kanal, K. M. Phys. Rev. B: Condens. Matter 1992, 45, 14247. (13) Kato, H.; Yamashita, K.; Morokuma, K. Bull. Chem. Soc. Jpn. 1993, 66, 3358. (14) Kato, H.; Tanaka, E. J. Comput. Chem. 1991, 12, 1097. (15) Boustani, I. Int. J . Quantum Chem. 1994, 52, 1081. (16) Brown, R. C.; Kolb, C. E.; Rabitz, H.; Cho, S. Y.; Yetter, R. A,; Dryer, F. L. Int. J . Chem. Kinet. 1991, 23, 957. (17) Yetter, R. A,; Rabitz, H.; Dryer, F. L.; Brown, R. C.; Kolb, C. E. Combust. Flame 1991, 83, 43. (18) Shock initiation of crystalline boron in oxygen and fluorine compounds. Krier, H.; Burton, R. L.; Pireman, S. R. Report UILU-ENG94-4010, Dept. of Mech. and Indust. Eng., Univ. of Ill. (19) Garland, N. L.; Stanton, C. T.; Nelson, H. H.; Page, M. J. Chem. Phvs. 1991, 95, 2511. (20) Kawashima, Y.; Kawaguchi, K.; Endo, Y.; Hirota, E. J. Chem. Phys. 1987, 87, 2006. (21) Saito, S.; Yamamoto, S . ; Kawaguchi, K. J. Chem. Phys. 1987, 86, 3597.
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