5135
J. Phys. Chem. 1986, 90, 5135-5140
Collision- Induced Dissociation of Vanadium Monoxide Ion N. Aristov and P. B. Armentrout*+ Department of Chemistry, University of California, Berkeley, California 94720 (Received: April 1, 1986)
The collision-induced dissociation (CID) of VO+ by rare gases has been studied by using a guided ion beam tandem mass spectrometer. CID by xenon yields @(V+-O) = 6.00f 0.35 eV, in excellent agreement with a value derived from the literature. However, when the collision gas is Ne, Ar, or Kr, CID is inefficient until 1.0 h 0.2 eV above the thermodynamic threshold. Possible origins for this behavior are discussed. Production of vanadium-rare gas molecular ions is observed in the Ar, Kr, and Xe systems. No reactions, including CID, are observed when helium is the collision gas. In light of this discussion, the utility of threshold CID measurements as a means of determining bond dissociation energies is assessed.
Introduction Interest in gas-phase ionic transition-metal species is very keen as evidenced by a number of recent experimental' and theoretical studies2 In this work, we are interested in whether low-energy collision-induced dissociation (CID) can be used to measure the bond dissociation energies of some of these novel species. Relatively few studies of this application of C I D can be found in the literature., In contrast, the use of high-energy CID to find structures of molecules (primarily organic and organ~metallic)~ and the dynamics of CID processess have been thoroughly studied. Potentially, measurement of the excitation function for low-energy CID of organometallic species such as ML,' provides a direct means of obtaining individual metal-ligand bond energies. No knowledge about the thermochemistry of any of the other participating species (ML,,', MLPI+, L) is required. One need only determine the cross section threshold, which is reasonably straightforward since models for the threshold behavior of CID cross sections are well-developed.6 Indeed, Parks et a1.6ahave shown that the thermochemistry derived from CID studies can compete in accuracy with light particie impact and spectroscopic measurements. Unfortunately, experimental difficulties have made routine application of low-energy CID experiments uncommon. These difficulties include the need for intense sources of internally cold molecular ions, accurate measurement of CID cross sections near threshold, potential problems with kinetic shifts and activation barrier^,^ and poor energy resolution compared with photons or electrons. Two developments in our laboratory have improved this situation. We have constructed an ion source capable of producing thermalized molecular ions. The utility of this source is documented in this study. Guided ion beam techniques provide excellent experimental sensitivity in the critical threshold region of cross sections. These experimental advances allow a reassessment of the ability of a low-energy CID experiment to provide bond energies. To test the accuracy of thermochemical values derived from CID experiments, a reasonably well-characterized transition-metal system is required. Here, the determination of the VO+ bond dissociation energy is chosen as the test case. @(V+-O) can be derived from five reoent experiments. First, Kappes and Staley,6 using ICR mass spectrometry, found that V+ reacts exothermically with SO2to form VO+ + SO, but does not react with NO to make VO' + N . This brackets the V+-0 bond energy between 5.72 and 6.55 eV.9 Second, Flesch and SvecIo used electron impact mass spectrometry to dissociatively ionize VOC1, and VOF, and measured appearance potentials (AP) of the ionic products. From the A P of VO+ + 3X (X = C1, F) and the heats of formation of VOX, and X, one can obtain AHf(VO+). The V+-0 bond strength is then easily derived as 5.70 f 0.10 eV. The difference between the AP's of VO+ + 3X and V+ + 0 + 3X should also yield Do(V+-0), but a much higher value is found, 6.96 f 0.45 eV. Flesch and Svec believe that the production of V+ + 0 3X may require surmounting an activation barrier, producing
+
'NSF Presidential Young Investigator 1984-1989; Alfred P. Sloan Research Fellow.
0022-3654/86/2090-5135$01.50/0
electronically or kinetically excited products. Third, a measurement of the threshold energy of reaction l in our laboratory1'
v+ + co
+
vo++ c
(1)
gave @(V+-O) = 5.68 f 0.22 eV, in good agreement with Kappes and Staley and the first value determined by Flesch and Svec. Fourth, DO(V-0) can be derived from measurements of the ionization potential (IP) of VO. Balducci et a1.,I2using electron impact mass spectrometry, found IP(V0) = 8.4 f 0.5 eV. Using eq 2, this IP can be combined with the known neutral VO bond Do(V+-0) = DO(V0)
+ IP(V) - IP(V0)
(2)
energy, D"(V0) = 6.49 f 0.09 eV,12J3and the ionization potential
(1) Some recent works by authors in the field: Jacobson, D. B.; Freiser, B. S. J. Am. Chem. SOC.1985, 107, 7399-7407. Cassady, C. J.; Freiser, B. S.; McElvany, S. W.; Allison, J.; Zbid. 1984, 106, 6125-6135. Babinec, S. J.; Allison, J. Zbid. 1984, 106, 7718-7720. Tolbert, M. A.; Beauchamp, J. L. Zbid. 1984, 106, 8117-8122. Halle, L. F.; Klein, F. S.; Beauchamp, J. L. Ibid. 1984,106,2543-2549. Armentrout, P. B.; Loh, S. K.; Ervin, K. M. Zbid. 1984,106, 1161-1163. Rents, Jr., W. D.; Strobel, F.; Frease, R. B.; Wronka, J.; Ridge, D. P. J . Phys. Chem. 1985,89, 5666-5670. Larsen, B. S.; Freas 111, R. B.; Ridge, D. P. Zbid. 1984, 88, 6014-6018. Dyke, J. M.; Gravenor, B. W. J.; Josland, G. D.; Lewis, R. A.; Morris, A. Mol. Phys. 1984, 53, 465-477. Dyke, J. M.; Gravenor, B. W. J.; Lewis, R. A.; Morris, A. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 1083-1088. Brucat, P. J.; Zheng, L.-S.; Pettiette, C. L.; Yang, S.; Smalley, R. E. J. Chem. Phys. 1986,84, 3078-3088. Ibid. 1985, 83, 4273-4274. Delacretaz, G.; Fayet, P.; Woste, L. Ber. Bunsenges. Phys. Chem. 1984,88,284-287. Hanley, L.; Anderson, s. L. Chem. Phys. Lett. 1985,122,410-415. Leopold, D. G.; Miller, T. M.; Lineberger, W. C. J. Am. Chem. SOC.1986, 108, 178. Miller, A. E. S.; Feigerle, C. S.; Lineberger, W. C. J . Chem. Phys. 1986, 84, 4127-4131. (2) Alvarado-Swaisgood,A. E.; Allison, J.; Harrison, J. F. J. Phys. Chem. 1985, 89,2517. Alvarado-Swaisgood, A. E.; Harrison, J. F. Zbid. 1985, 62, 5198. Carter, E. A.; Goddard 111, W. A. Zbid. 1984, 88, 1485. Schilling, J. B.; Goddard, W. A.; Beauchamp, J. L. J. Am. Chem. SOC. 1986, 108, 582-584. Vincent, M. A.; Ycshioka, Y.; Schaefer, H. F. J . Phys. Chem. 1982, 86, 3905-3906. Rappe, A., private communication. Dupuis, M.; Hammond, B. L.; Lester, W. A., private communication. (3) (a) Armentrout, P. B.; Beauchamp, J. L. Chem. Phys. 1980,50,21-25. (b) Ervin, K.; Loh, S. K.; Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1983, 87,3593-3596. (c) Parks, E. K.; Wexler, S. Zbid. 1984,88,4492-4494. (d) Semo, N. M.; Koski, W. S. Ibid. 1984, 88, 5320-5324. (4) Levsen, K.; Schwarz, H. Mass Specirom. Rev. 1983,2,77. Cooks, R. G.; Ed. Collision Spectroscopy; Plenum: New York, 1978. ( 5 ) For a biblioarauhv of seminal works on CID see the chaDters bv: Diestler, D. J.; Ku&, P. j.In Atom-Molecule Collision Theory; Bernsteii, R. B., Ed. Plenum: New York, 1979. (6) (a) Parks, E. K.; Wagner, A.; Wexler, S . J . Chem. Phys. 1973, 58, 5502-5513. (b) Rebick, C.; Levine, R. D. Zbid. 1973, 58, 3942. (7) Aristov, N.; Armentrout, P. B. J . Am. Chem. SOC.1986, 108, 1806-18 19. (8) Kappes, M. M.; Staley, R. H. J . Phys. Chem. 1981, 85, 942-944. Narl. Bur. (9) Stull, D. R.; Prophet, H. Natl. Stand. Ref:Data Ser., (US., Stand.) 1971, No. 37. (10) Flesch, G. D.; Svec. H. J. Znorg. Chem. 1975, 14, 1817-1822. ( 1 1) Aristov, N.; Armentrout, P. B. J . Am. Chem. SOC.1984, 106,4065. (12) Balducci, G.; Gigli, G.; Guido, M. J . Chem. Phys. 1983, 79, 56 16-5622. (13) Coppens, P.; Smoes, S.; Drowart, J. Tram. Faraday SOC.1967, 63, 2140. Jones, R. W.; Gole, J. L. J . Chem. Phys. 1976, 65, 3800.
0 1986 American Chemical Society
5136
The Journal ofphysical Chemistry, Vol. 90, No. 21, 1986
of a vanadium atom, IP(V) = 6.74 eV,I4 to yield Do(V'-O) = 4.8 f 0.5 eV, a value in sharp disagreement with all the others. The most recent experiment resolves this discrepancy. Dyke et al.,15who used photoelectron spectroscopy, measured the ionization potential of VO to the ground state, 32-.as 7.25 f 0.01 eV and to the first excited state, 3A, as 8.42 f 0.01 eV. Their spectrum showed that the cross section for ionization to the 3A state is -6 times larger than to the ground state. In light of this result, it is likely that Balducci's experiment was sensitive only to the ionization to the first excited state. From Dyke's value for the IP of VO in eq 2, the bond strength for VO+ is found to be 5.98 f 0.10 eV. This is clearly the most definitive value. In this study, we have measured the excitation function of reaction 3. CID of VO', where A is a rare gas. In the absence VO+
+A
-
Vf + 0 + A
(3)
of activation barriers, the threshold energy, AE, for this reaction should correspond directly to Do(V+-O). We have used several rare gases in this study. This allows an examination of the dynamics of the dissociation process by studying mass effects and effects due to the polarizability of the target gas. It also provides a check on any systematic errors in the determination of the threshold. In addition to reaction 3, process 4 is also observed when A is argon, krypton, and xenon. To the best of our knowledge, this VOc
+A
+
VAf
+0
(4)
is the first observation of a rare gas-transition metal ion species. Similar types of species, CsA' and TlA', have been observed in the collision-induced ion-pair production of Cs and TI halides.6aj16
Aristov and Armentrout reaction of V+ with the O2 bath gas, process 5 ( A H , = -0.72 f 0.10 eV). A weak electric field (1.5 V/cm) draws the ions to
v+ + 0,
- vo++
0
the drift cell exit where they enter the beam apparatus. Under these conditions, the ions' dwell time in the cell, 150 ps, is such that the number of collisions they undergo with 0, molecules is lo3. Experimental results indicate that this should be sufficient to thermalize both the internal and translational motion of the VOf. In the SI source, VOC1, effuses into the source chamber toward a resistively heated rhenium filament. Dissociation occurs and species with low ionization potentials are boiled off. Surface ionization ordinarily produces few molecular ions. However, because of the low ionization potential of VO and the high bond strength of VO', appreciable amounts of VO' are generated. At the estimated filament temperature, 1900 & 100 K, the average vibrational energy in the ion beam is 0.11 eV ( k T = 0.16 eV). This is calculated by using the vibrational frequency of VO', 1060 f 40 cm-' measured by Dyke et al.15 No corrections for anharmonicity are made. This translates to an ion beam which is -55% in u = 0, -25% in L' = 1, -11% in u = 2, -5% in L: = 3, -2% in u = 4, and -1% in u = 5 (at 0.66 eV). Electronic excitation is also possible. The only excited state for which the excitation energy is known is the 3A at 1.17 eV.15 Spectroscopic evidenceI9 suggests that there are l A and 'Z states close to the 'A state. The calculated population of the 3A state is less than 0.2%, however, so that electronic excitation is probably unimportant. Data Analysis and Derivation of Bond Energies. Reaction cross sections, u, are calculated from the data via eq 6, where n is the
-
-
I = Io exp(-nul)
Experimental Section
Apparatus and Ion Source. The guided ion beam apparatus and data reduction procedures have been described in detail previ0us1y.l~ Briefly, ions are prepared in an ion source (described below), extracted, and focused into a magnetic mass spectrometer. Ions of a selected mass, here VO+, are then decelerated to a specific translational energy, and directed into the interaction region where they encounter the reactant rare gas. The interaction region is surrounded by an rf octopole ion guide. This type of device has excellent product collection capabilit allowing meaand avoiding surement of very small cross sections, dynamic biases. The reactant gas is kept at a pressure low enough that multiple collisions are improbable (
