Formation, Structures, and Reactivities of Niobium Oxide Cluster Ions

Aug 8, 1996 - Oxygen Release from Cationic Niobium–Vanadium Oxide Clusters, NbnVmOk, Revealed by Gas Phase Thermal Desorption Spectrometry and Densi...
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J. Phys. Chem. 1996, 100, 13386-13392

Formation, Structures, and Reactivities of Niobium Oxide Cluster Ions H. T. Deng, K. P. Kerns, and A. W. Castleman, Jr.* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: October 20, 1995; In Final Form: May 30, 1996X

Niobium oxide cluster ions are produced by a laser-induced plasma source. The cluster distribution, collisioninduced dissociation (CID), and cluster reactivities are studied using a triple-quadrupole mass spectrometer. CID experiments on the cluster ions Nb3O7-9+, Nb4O9-11+, and Nb5O12+ reveal that their building blocks are Nb2O5, NbO2+, NbO3, Nb3O7+, O, and O2, whereby the cluster stoichiometry is assigned to have the general form (NbO3)m(NbO2)n(O)0-4+. The trends in the ionization potentials of these species are estimated in terms of the CID fragments produced. Nb3O8-9+ and Nb4O11+ cluster ions evidently form via the adsorption of one oxygen atom or molecule onto the cluster surface. Nb3O7+, Nb4O9+, and Nb5O12+ have strong reactivities to abstract an oxygen atom from oxygen-containing molecules and adsorb small hydrocarbons at near thermal energies. In particular, the reactivity of the oxygen atom or molecule in the oxide clusters Nb3O8-9+ and Nb4O11+ is consistent with our suggestions that it has a radical oxygen character.

Introduction Niobium oxide surfaces have been found to exhibit extraordinary catalytic properties which find application in the petrochemical, petroleum, and pollution control industries.1 The preparation, physical-chemical, and catalytic properties of niobium oxide surfaces have been studied using Raman spectroscopy and EXAFS techniques.2-4 These investigations show that the catalytic activity of the niobium oxide surface is dependent on the preparation process and is related to the NbdO bond; in addition, the coordination number of the niobium atoms influences the acid sites and acidity.2 However, due to niobium oxide posing significant complexity in terms of stoichiometries and phase structures,5 a complete understanding of the formation mechanism of these surface oxides and the relationship between the structure and reactivities still has not been achieved. Indeed, studies on gas phase clusters can provide insights regarding the fundamental physical and chemical transformations that occur as matter spans over various states. Due to the importance of transition metal oxides in the field of catalysis, numerous gas phase studies on metal oxides have been conducted during the past 10 years. These studies have concentrated mainly on diatomic transition metal oxides6-15 and high-valence transition metal oxide species, MO1-5.16-20 It is found that the diatomic late transition metal oxides have high reactivities towards the oxidation of benzene and saturated hydrocarbons, and the number of oxygen atoms in MO1-5 influences the reactivities of these molecular ions. For example, OsO2+ and OsO4+ have different reactivities toward the oxidation of methane,16 but there are only a few studies on transition metal oxide clusters.21-24 Studies on titanium oxide clusters show that both metal-rich and oxygen-rich clusters can be formed by using different sources,25 and the metal-rich titanium oxide clusters can further react with oxygen.26 For niobium oxide, the neutral NbO molecule is known to be reactive for the dehydrogenation of saturated hydrocarbons.13 Regarding the oxide anions, NbO2-5- are found to display reactions analogous to condensed phase acid-base interactions with HCl.18 In considering the cations, it is known that the ionization potential of NbO is 7.9 eV.27 It is found that the diatomic niobium oxide cations are not capable of oxidizing X

Abstract published in AdVance ACS Abstracts, July 15, 1996.

S0022-3654(95)03100-5 CCC: $12.00

benzene due to the high bonding energy of Nb+-O, about 7.09 eV.6 Studies on the oxidation of niobium clusters establish the binding energy of Nb+-O2 to be about 7.55 eV.28 Investigations show that variation in the number of oxygen atoms in niobium cluster oxides, NbnOm (n ) 3-20, m ) 1-5), does not significantly change the ionization potential of the niobium clusters29 and that niobium cluster oxide cations, NbnO, can abstract one oxygen atom from carbon dioxide.30 However, to the best of our knowledge, there have not been any reports of studies on niobium oxide clusters with stoichiometries similar to bulk niobium oxide, despite niobium oxide being an important catalyst and one which has been studied intensively in the solid phase.31 In order to understand changes in the physical and chemical properties of niobium oxide as it transforms from the gas phase to the condensed phase, and to provide more information on the catalytic behavior of niobium oxide surfaces, recent work on niobium oxide clusters was conducted in our laboratory using a triple-quadrupole mass spectrometer coupled with a laserinduced plasma reaction source. Plasma reactions of niobium with O2 and N2O give slightly different oxide cluster distributions. Collision-induced dissociation thresholds of Nb3O7+, Nb4O9+, and Nb5O12+ are determined in a triple-quadrupole mass spectrometer using Kr gas as the collision partner and conducting the experiments under near single collision conditions. The formation mechanisms are discussed herein with regard to the cluster building blocks. Ab initio calculations are also used to investigate the optimized structures of Nb2O5 and Nb3O7 and to determine the relative ionization potentials of these species. The reactivities of niobium oxide cluster cations toward oxygen-containing molecules and butadiene are found to be greatly dependent on the number of oxygen atoms in the clusters, and information is gained on the cluster surface adsorbed oxygen species which can oxidize butadiene. The reaction mechanisms and the relationship between niobium oxide clusters and surface niobium oxide catalysts are also discussed. Experimental Section The triple-quadrupole mass spectrometer coupled with a laser vaporization source used in the present studies has been discussed in detail.32 Briefly, the second-harmonic output of a Nd:YAG laser is used to ablate the surface of a rotating niobium © 1996 American Chemical Society

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Figure 2. Plot of collision energy vs fragment intensity of both Nb3O8+ and Nb4O9+.

Figure 1. Distribution of niobium oxide cation clusters produced from a laser-induced plasma reaction of niobium with oxygen.

rod. Niobium is thereby vaporized, and through plasma reactions with oxygen or nitrous oxide gases which are carried in the helium buffer gas, metal oxide clusters are produced. After exiting the source and passing through a 5 mm skimmer, the formed cation clusters are focused and steered by the first group of three electrical ion lenses, and horizontal and vertical deflector plates, into the first quadrupole mass filter. This device is used to select clusters of desired size. Thereafter, the selected clusters are refocused and deflected by a second group of ion lenses and deflectors and then injected into the second quadrupole mass filter. It is operated in the rf-only mode and serves as either a reaction cell or collision cell. The reactant or collision gas is introduced into this cell and monitored by a capacitance manometer (MKS). The length of this cell is about 25 cm. According to the collision cross section calculated by both the Langevin-Gioumousis-Stevenson equation and the hard sphere model, and in previous studies,32 single-collision conditions can be achieved in CID measurements by controlling the pressure of the collision gas, Kr, at lower than 0.1 mTorr. In order to study the cluster reactions at near thermal energies, the entrance voltage of the reaction cell is normally set at a value of +1 V, which can reduce the beam energy so that the clusters will just enter and pass through the cell. After reactions or collisions of these size-selected clusters with the reactant or collision gas, the products drift out of the cell and enter a third quadrupole mass filter, whereby the products are analyzed and detected by a channeltron electron multiplier. When the first quadrupole mass filter is operated in the rf-only mode, the total cluster distribution resulting from the plasma reactions is obtained. In the present experiments, the oxygen or nitrous oxide concentration is maintained at about 10% in the helium carrier gas. The typical pressure of reactant gases or collision gases in the reaction cell is set at a selected value in the range between 0.08 and 2.2 mTorr. Results Cluster Distributions and Dissociation. A typical product distribution of laser induced plasma reactions of niobium with oxygen is exhibited in Figure 1. The cluster distribution starts at NbO2+, with no Nb+ or NbO+ being observed in the mass spectrum. The peaks can be grouped according to the number of niobium atoms, and the mass difference between adjacent peaks in each group corresponds to one oxygen atom. It is also

noticed from this distribution that (1) every group with an odd number of niobium atoms has three peaks, and the first peak has the highest intensity in this group, and (2) every group with an even number of niobium atoms has four or five peaks, and the relative peak intensities are rather different than for the oddniobium species. The cluster distribution can be further represented as (Nb2O5)m(NbO2)1-2(O)0-4+, or alternatively (NbO2)m(NbO3)n(O)0-4+. In order to gain some insight into the formation mechanisms and structures of these niobium oxide clusters, the seven species Nb3O7-9+, Nb4O9-11+, and Nb5O12+, which are displayed in Figure 1, are selected and studied using CID. The findings show that Nb3O7+, Nb4O9-10+, and Nb5O12+ are very stable species, and more than 30 eV (laboratory frame of reference) of collision energy is needed to initiate their dissociations. Under both single- and multiple-collision conditions, both Nb3O7+ and Nb5O12+ give the same CID product, namely NbO2+, by losing masses corresponding to Nb2O5 and Nb4O10, respectively. However, there are two CID channels for Nb4O9-10+, even under single-collision conditions, which lead to the products Nb3O7+ and NbO2+. The CID of Nb4O9+ starts at a collision energy of 30 eV by losing neutral NbO2; the onset of a second dissociation channel corresponding to the loss of neutral Nb3O7 commences at a collision energy around 50 eV. The CID of Nb4O10+ at a collision energy of 40 eV yields both Nb3O7+ and NbO2+ with a branching ratio of 2 to 1. Under higher collision energy (larger than 60 eV) and multiple-collision conditions, oxygen atom loss is also observed for Nb4O10+. It is interesting to contrast these results with the behaviors for Nb3O8-9+ and Nb4O11+. In contrast to the foregoing species, there is a facile loss of one oxygen atom in the case of Nb3O8+ and one oxygen molecule for both Nb3O9+ and Nb4O11+. The intensities of the CID fragments of Nb3O8+ and Nb4O9+ as a function of their translational energies under single-collision conditions are exhibited in Figure 2, which clearly shows that the stabilities of these clusters are significantly different. A summary of the CID products of these clusters is displayed in Table 1, where they are seen to be NbO2+, Nb2O5, Nb3O7+, NbO3, O and O2. By combining the CID results and the cluster ion distributions, we can find that the cluster stoichiometry can be well represented as (NbO2)m(NbO3)n(O)0-4+. During the course of this work we determined the CID cross sections versus the ion translational energies under singlecollision conditions for CID of Nb3O7-8+, Nb4O9-10+, and Nb5O12+. The minimum collision energy needed for initiating the dissociation can be determined by using the commonly accepted formula33,34 as follows:

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Deng et al.

TABLE 1: Collision-Induced Dissociation Products of NbxOy+ NbxOy+ 3, 7 3, 8 3, 9 4, 9 4, 10 4, 11 5, 12

CID product(s) (cations)

neutral(s) lost

center-of-mass energy (eV)a

1, 2 3, 7 3, 7 3, 7 1, 2 3, 7 1, 2 4, 9 4, 9 1, 2

2, 5 0, 1 0, 2 1, 2 3, 7 1, 3 3, 8 0, 1 0, 2 4, 10

5.1 0.9 d 4.2 b 5.3 5.3 c d 4.0

Figure 3. Optimized structures of Nb2O5 and Nb3O7 by ab initio calculations.

a Measured CID thresholds as the center-of-mass energy. The absolute error may be as high as (10%, but the relative error is lower than 5%. b This channel appears at high collision energy. c This channel occurs at high collision energy and under multicollision conditions. d Thermal conditions.

σ ) σ0(E - E0)n/E where E is ion translational energy, E0 is the threshold energy, and σ0 is an energy-independent scaling factor. By using a curve-fitting method, E0 is determined and can be transformed to the center-of-mass collision energy by using the following formula:

Ec ) E0Mn/(Mn + Mp) Ec denotes the center-of-mass collision energy, Mn is the mass of the neutral collision gas, Kr, in the present experiments, and Mp is the mass of the selected ions. The calculated center-ofmass dissociation thresholds are also listed in Table 1. In principle, the absolute values of the CID thresholds of clusters can be influenced by their internal energy and the kinetic shift. Although direct measurements of the internal energy are not conducted, several observations imply that this factor does not have significant influence on the CID measurements in the present work. Under similar experimental conditions, such as laser power, backing pressure, and reactant concentration, clusters have been observed exiting the source with attached N2 and CH4. The presence of these weakly bound ligands is evidence that the cluster temperatures are quite low. Studies in our laboratory show that varying the experimental conditions does not lead to a change of CID thresholds of Met-Cars and other metal-carbon species.32 Regarding the potential influence of the kinetic shift, it should not affect our ability to discern differences in the relative thresholds of proximate species which is the intent of the present study. In our experimental setup, the shortest time for the ions to reach the third quadrupole after collision with neutral gases is 50-60 µs. According to the RRKM model,36 a rather similar kinetic shift for Nb3O7+ and Nb3O9+ should be obtained. However, we found that Nb3O9+ can be dissociated even at thermal energies while a center-ofmass energy of 5.1 eV is required for Nb3O7+. Additionally, as shown in Table 1, the dissociation threshold of Nb5O12+ is lower than Nb3O7+ and about the same as Nb4O9+. As discussed below, the relative CID thresholds for species of proximate size are valuable in the present work to provide insight into the bonding properties of the clusters which we investigated. Cluster Structures and Properties by ab Initio Calculations. Ab initio calculations are performed for Nb2O50/+ and Nb3O70/+ using Spartan software35 on an IBM RISC 6000 Model 550 computer in our laboratory. The cluster structures are optimized at the Hartree-Fock level using a 3-21G* polarization

Figure 4. Mass spectrum of Nb3O7+ reacting with 1.24 mTorr of N2O at near thermal energies.

basis set, and these structures are displayed in Figure 3. The bond lengths of Nb2O5 are 1.71 Å for the terminal Nb-O bond and 1.94 Å for the bridged Nb-O bond. For Nb3O7, the central niobium atom has a similar bond length of 1.85 Å for both the terminal Nb-O bond and the bridged Nb-O bond. The other two niobium atoms have bond lengths of 1.85 Å for the terminal Nb-O bond and 2.05 Å for the bridged Nb-O bond. It is also noticed that the Mulliken charge for the central niobium atom of Nb3O7+ (1.2) is larger than that of the other niobium atoms (0.88). The ionization potentials of Nb2O5, NbO2, and Nb3O7 are calculated to be 10.8, 10.6, and 9.1 eV, respectively. Detailed discussion of the structures, stability, and ionization potentials of these niobium oxide clusters are given in the following Discussion section. Reactivities with Acetone and Butadiene. The reactivities of Nb3O7-9+, Nb4O9-11+, and Nb5O12+ toward oxygen-containing molecules, such as N2O and acetone, and hydrocarbon gases such as butadiene, are also investigated using a triple-quadrupole mass spectrometer at near thermal energies in order to provide information on the reactivity of niobium oxide. The pressure of the reactant gas was varied in the range of 0.1-2.0 mTorr in the present work. It is found that there are two reaction channels when Nb3O7+, Nb4O9-10+, and Nb5O12+ react with oxygen-containing molecules: molecule addition and oxygen abstraction. Typical results are presented in Figure 4 for the reaction of Nb3O7+ with N2O. The reaction processes can be represented as follows:

M + A-O

f MOA f MO + A

M stands for Nb3O7+, Nb4O9-10+, and Nb5O12+. A-O stands for a neutral oxygen-containing molecule, and a similar distribution has been found for acetone. It is found that the branching

Niobium Oxide Cluster Ions

Figure 5. Mass spectrum of Nb3O7+ reacting with 2.2 mTorr of benzene.

Figure 6. (a) Mass spectrum of Nb3O8+ reacting with 1.0 mTorr of butadiene at near thermal energies. (b) Mass spectrum of Nb3O8+ reacting with 0.16 mTorr of acetone at near thermal energies.

ratio of M-O-A compared to M-O increases with the pressure of the neutral reactant gases. At low reactant gas pressures ( IP(NbO2), and IP(Nb4O10) > IP(NbO2). These results indicate that oxygen-rich oxide clusters generally have higher IP’s than metal-rich oxide clusters in comparison to the normal stoichiometry of Nb2O5. From Table 1, it is found that Nb4O10+ has the highest dissociation threshold and the stability order of these clusters is Nb4O10+ g Nb3O7+ > Nb4O9+ > Nb5O12+. The high stability of the Nb4O10 species may be due to the fact that Nb4O10+ has the same stoichiometry as its counterpart in the solid phase. It is recognized that the chemical reactivities of materials are governed by their electronic and/or geometric structures. Briefly, Nb3O7+, Nb4O9+, and Nb5O12+ all have a positive charge center and somewhat more metal-rich character in comparison to the normal stoichiometry of Nb2O5. Thus, it would be reasonable to think that these clusters might exhibit properties somewhat similar to those of the reduced metal oxide surface and could be further oxidized. These expected chemical properties for Nb3O7+, Nb4O9+, and Nb5O12+ are verified in the present experiments, in which oxygen abstraction from acetone is observed to occur with each of these three species. The reactions probably occur on the central niobium atom for Nb3O7+, Nb4O9-10+, and Nb5O12+. The central niobium atom, which has a higher charge density than other niobium atoms according to the calculations, has the potential to accept an electron pair and hence function as a Lewis acid. The bonding between niobium and the oxygen atom of oxygen-containing molecules is strong, and the energy released from the formation of Nb-O bond(s) might contribute to the breaking of bonds in oxygen-containing molecules. However, collisions with a third body could serve to stabilize the intermediates involved and lead to the formation of an addition product. This is proven by the fact that the ratio of the oxygen abstraction product to the molecular addition product decreases with the reactant gas

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concentration. Investigations of the adsorption of hydrocarbon molecules onto Nb3O7+, Nb4O9+, and Nb5O12+ are still underway. On the other hand, Nb3O8-9+ and Nb4O11+ have oxygenrich character in comparison with normal oxide stoichiometry. CID studies show that there are significantly different CID behaviors for Nb3O8-9+ and Nb4O11+ from those of Nb3O7+, Nb4O9+, and Nb5O12+, in which loss of one oxygen atom for Nb3O8+, or one oxygen molecule for Nb3O9+ and Nb4O11+, is the only dissociation channel. A significant reactivity difference between metal-rich and oxygen-rich niobium oxide clusters is also observed. Nb3O8-9+ and Nb4O11+ cannot abstract an oxygen atom from acetone. Indeed, the main reaction channel for Nb3O8-9+ and Nb4O11+ with acetone and butadiene molecules is loss of one oxygen atom from the oxide clusters. It should be stressed that the mechanisms for oxygen(s) being lost in the CID and reaction processes are quite different as would be expected. For example, no CID fragments are observed when Nb3O8+ collides with 0.1 mTorr of Kr gas, and a collision energy of 5.0 eV (lab frame) is needed for Nb3O8+ to lose one oxygen atom at this Kr pressure. But virtually no collision energy is needed for the loss of one oxygen atom when Nb3O8+ reacts with butadiene or acetone molecules at a gas pressure of 0.1 mTorr. Apparently, the reaction of Nb3O8+ with butadiene or acetone molecules is exothermic, and the energy released by the reaction is higher than the bond strength of Nb3O7+-O. This result implies an exothermic reaction leading to a newly formed neutral product. As mentioned before, it can be thought that Nb3O8+ comes from the addition of one oxygen atom onto Nb3O7+. This adsorbed oxygen atom, which is not weakly bound to Nb3O7+ as evidenced by the high collision energy required for its loss in the CID experiments, evidently has a radical character, which can oxidize acetone or butadiene. The possible reaction of Nb3O8+ with acetone can be represented as follows:

Nb3O8+ + CH3COCH3 f Nb3O7+-O-CO(CH3)2 f Nb3O7+ + CH3COOCH3 It is also noticed that CID of Nb3O9+ and Nb4O11+ results only in loss of one oxygen molecule, but the main reaction channel of Nb3O9+ and Nb4O11+ with acetone is by losing one oxygen atom, as shown for reactions of Nb3O9+ in Figure 7b. As mentioned before, Nb3O9+ and Nb4O11+ involve the weak bonding of O2 onto Nb3O7+ and Nb4O9+, as shown in Figure 7a and Table 1. It is believed that this adsorbed O2 is activated by the niobium center and also has a radical oxygen character, like a peroxy acid. Therefore, it can oxidize acetone by providing an oxygen atom. The reactions of Nb3O9+ with acetone are represented as follows:

Nb3O9+ + CH3COCH3 f Nb3O7+-O-O-CO(CH3)2 f Nb3O8+ + CH3COOCH3 Although the neutral organic products of Nb3O8-9+ and Nb4O11+ reacting with butadiene are not detected in the present experiments, it is expected that the neutral product will be an epoxide for reactions of Nb3O8-9+ and Nb4O11+ with butadiene, similar to the case of peroxy acids reacting with alkenes.37 Conclusions Niobium oxide cluster ions are produced by the laser-induced plasma reactions of niobium with O2. The clusters have a formula of (NbO2)m(NbO3)n(O)0-4+. CID studies on Nb3O7-9+,

Nb4O9-11+, and Nb5O12+ show that Nb3O7+, Nb4O9-10+, and Nb5O12+ are very stable species with high dissociation energies, and Nb3O8-9+ and Nb4O11+ are species comprised of the addition of an oxygen atom or oxygen molecule to Nb3O7+ and Nb4O9+. The ionization potentials for these clusters increase with the ratio of oxygen to niobium and have the order IP(NbO3) > IP (Nb3O7), IP(Nb3O8) > IP(NbO2), and IP(Nb4O10) > IP(NbO2). The equilibrium structures of Nb2O5 and Nb3O7 are optimized using ab initio calculations, and the structures of other niobium clusters are suggested, in which Nb3O7+ is the core for the formation of large clusters. Nb3O7+, Nb4O9-10+, and Nb5O12+ are able to abstract one oxygen atom from oxygencontaining molecules and also adsorb hydrocarbon molecules. The adsorbed oxygen atom or molecule in Nb3O8-9+ and Nb4O11+ evidently has a radical oxygen character and can oxidize acetone or butadiene. Taken together, the present findings reveal the insight which can be gained through cluster research into the mechanisms of surface reactions and catalytic processes. Acknowledgment. Financial support by The Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research of the U.S. Department of Energy, Grant DEFGO2-92ER14258, and the DuPont Company, through an unrestricted grant, is gratefully acknowledged. The authors thank Dr. David Thorn for helpful discussions during the course of this work. References and Notes (1) Wachs, I. E. Proc. Int. Conf. Niobium Tantalum 1989, 679. (2) Jehng, J.-M.; Wachs, I. E. Catal. Today 1990, 8, 37. (3) Yoshida, S.; Nishimura, Y.; Tanaka, T.; Kanai, H.; Funabiki, T. Catal. Today 1990, 8, 67. (4) Tanabe, K. Catal. Today 1993, 16, 333. (5) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford Science Publications: Oxford, 1991. (6) Ryan, M. F.; Sto¨ckigt, D.; Schwarz, H. J. Am. Chem. Soc. 1994, 116, 9565. (7) Stevens, A. E.; Beauchamp, J. L. J. Am. Chem. Soc. 1979, 101, 6450. (8) Kang, H.; Beauchamp, J. L. J. Am. Chem. Soc. 1986, 108, 7502. (9) Jackson, T. C.; Carlin, T. J.; Freiser, B. S. J. Am. Chem. Soc. 1986, 108, 1120. (10) Fiedler, A.; Schro¨der, D.; Shaik, S.; Schwarz, H. J. Am. Chem. Soc. 1994, 116, 10734. (11) Chen, Y. M.; Clemmer, D. E.; Armentrout, P. B. J. Am. Chem. Soc. 1994, 116, 7815. (12) Clemmer, D. E.; Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1993, 97, 544. (13) Parnis, J. M.; Lafleur, R. D.; Rayner, D. M. J. Phys. Chem. 1995, 99, 673. (14) Ryan, M. F.; Fiedler, A.; Schro¨der, D.; Schwarz, H. Organometallics 1994, 13, 4072. (15) Jackson, T. C.; Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1984, 106, 1252. (16) Irikura, K. K.; Beauchamp, J. L. J. Am. Chem. Soc. 1989, 111, 75. (17) Keesee, R. G.; Chen, B.; Harms, A. C.; Castleman, A. W., Jr. Int. J. Mass Spectrom. Ion Processes 1993, 123, 225. (18) Sigsworth, S. W.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1992, 114, 10471. (19) Fokkens, A. W.; Gregor, I. K.; Nibbering, N. M. M. Rapid Commun. Mass Spectrom. 1991, 5, 368. (20) Schro¨der, D.; Fiedler, A.; Schwarz, J.; Schwarz, H. Inorg. Chem. 1994, 33, 5094. (21) Maleknia, S.; Brodbelt, J.; Pope, K. J. Am. Soc. Mass Spectrom. 1991, 2, 212. (22) Cassady, C. J.; Weil, D. A.; McElvany, S. W. J. Chem. Phys. 96, 691 (1992). (23) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1986, 108, 27. (24) Freas, R. B.; Campana, J. E. J. Am. Chem. Soc. 1986, 108, 4659. (25) Yu, W.; Freas, R. B. J. Am. Chem. Soc. 1990, 112, 7126. (26) Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. Int. J. Mass Spectrom. Ion Processes 1992, 117, 129. (27) Balducci, G.; Gigli, G.; Guido, M. J. Chem. Phys. 1986, 85, 5955. (28) Radi, P. P.; von Helden, G.; Hsu, M. T.; Kemper, P. R.; Bowers, M. T. Int. J. Mass Spectrom. Ion Processes. 1991, 109, 49.

13392 J. Phys. Chem., Vol. 100, No. 32, 1996 (29) Athanassenas, K.; Kreisle, D.; Collings, B. A.; Rayner, D. M.; Hackett, P. A. Chem. Phys. Lett. 1993, 213, 105. (30) Song, L.; Eychmu¨ller, A.; St. Pierre, A.; El-Sayed, M. A. J. Phys. Chem. 1989, 93, 2485. (31) Kung, H. H. Transition Metal Oxides, Surface Chemistry and Catalysis; Elsevier: New York, 1989. (32) (a) Kerns, K. P.; Guo, B. C.; Deng, H. T.; Castleman, A. W., Jr. J. Chem. Phys. 1994, 101, 8529; (b) CID of Vanadium-Carbon Cluster Cations. J. Phys. Chem., submitted. (33) Schultz, R. H.; Crellin, K. C.; Armentrout, P. B. J. Am. Chem. Soc. 1991, 113, 8590.

Deng et al. (34) Anderson, S. G.; Blades, A. T.; Klassen, J.; Kebarle, P. Int. J. Mass Spectrom. Ion Processes 1995, 141, 217. (35) Hehre, W. J.; Burke, L. D.; Shusterman, A. J. Spartan Software, Wavefunction, Inc.: Irvine, CA, 1994. (36) Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; Wiley: New York, (1972). (37) Solomons, T. W. G. Organic Chemistry, 4th ed.; Wiley: New York, 1988.

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