J. Phys. Chem. 1988,92, 7067-7074 1b2 orbital is also empty; however, it crosses the 2al* orbital evolving from the metal 4s. If the symmetry of the reaction is reduced from C , to C,, both the 2al* and 1b2 MO's have a' symmetry and this crossing is avoided. This may provide a lower energy pathway for reaction along a side-on approach of Ca+ and H2, although the collinear geometry is still probably favored. In the case of Zn+, this secondary interaction cannot occur because the 3d orbitals are occupied and because they are much too low in energy. This difference can explain the very large difference in the reaction behavior of Ca+ and Zn+. While both prefer a collinear geometry, deviations from collinear lead rapidly to a very repulsive surface for Zn+ and hence the reaction is very inefficient at threshold. For Ca+, other reaction geometries still allow reaction to occur. This secondary 3d7r-uu* interaction can also help explain the difference in reactivity between Ca+ and transition-metal ions. For the latter, the 3d orbitals lie below the 4s and are occupied. Thus, if the in-plane 3 d r is occupied, this leads to a more attractive situation in C, symmetry for the transition-metal ions than for Ca+. This can explain why the reaction of Ca+ is less efficient
7067
than the transition-metal-ion reactions, why it rises more slowly than is generally observed in these reactions, and why it has some impulsive character. Overall, the results of this study are in basic agreement with the findings of Elkind and Armentrout.' They clearly demonstrate the importance of the occupation and relative energy of the 4s and 3d orbitals in determining metal ion reactivity with H2. The reactivity of ground-state Ca' and Zn+ can be understood and predicted by using MO concepts if the interactions are considered in sufficient detail. This success suggests that the activation of molecular hydrogen can be similarly understood for more complicated transition-metal systems such as metal complexes, metal clusters, and metal surfaces. Continued work in our laboratories is attempting to address these specific issues in the former two cases.
Acknowledgment. This research was supported by the National Science Foundation under Grant No. CHE-8796289. Registry No. Ca+, 14102-48-8;Zn', 15176-26-8;H2,1333-74-0;D2, 7782-39-0;HD,13983-20-5.
Translational and Electronic Energy Dependence of Chromium Ion Reactions with Methane R. Georgiadis Department of Chemistry, University of California, Berkeley, California 94720
and P. B. Armentrout*vt Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: April 14, 1988)
The reaction of Cr+ with methane is studied as a function of translational and electronic energy in a guided ion beam tandem mass spectrometer. The electronic energy of the Cr+ ion is varied by altering the ionization technique. The ground-state Cr+('%) is found to react endothermically with CH4 to produce CrH', CrCH2+,and CrCH3+. Only the former product has been observed previously for the reaction of ground-state Cr'. Excited states (4D and possibly 4G) of Cr+ react with much higher efficiency to form the same products, while Cr+(6D), the first excited state, exhibits no evidence of reaction. The results are interpreted by using simple molecular orbital concepts that have been developed previously for reactions of transition-metal ions. This model is shown to explain the observation by Ridge and co-workers that the quartet excited states of Cr+ are efficiently quenched by methane in a spin-forbidden process.
Introduction Gas-phase ion beam experiments have provided a succession of insights into the reactivity of transition metals with small molecules. The electronic structure of the ion strongly effects the reaction mechanism and efficiency as illustrated recently for reactions of the first-row atomic transition-metal ions with Hz, HD, and D2.I Basic chemical concepts such as conservation of spin and molecular orbital correlation from reactants to products have been extraordinarily useful in understanding and predicting the behavior of endothermic reactions. Most of these arguments have been developed and applied for the simplest atom-diatom reactions such as M+ + Hz.A logical extension of the molecular hydrogen studies is to study C-H bond activation by investigating reactions with methane. A considerable number of metal ion reactions with CH4 have been examined both by ion beam techniques and by ion cyclotron resonance (ICR) mass spectrometry. Ion beam studies include Sc+? Ti+,3V+$ Cr+: Fe+,6.7CO+,*and Ni+.7 Using the available thermodynamic data, the reactions of ground-state M+ with methane are predicted to be endothermic. Not surprisingly, ICR
studies of transition-metal ions generally report no reaction with methane, as in the case of Ti+: V+,l0 Fe+,I1 and Rh+.I2 An exception is the case of Cr+, where the observed reactivity has (1) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1987,91,2037-2045.
(2)Sunderlin, L. S.;Armentrout, P. B., submitted for publication in J. Am. Chem. Soc. Armentrout, P. B. In StructurelReactivity and Thermochemistry of Ions; Ausloos, P., Lias, S . G., Eds.; Reidel: Dordrecht, 1987;pp 97-164. Armentrout, P. B. In "Gas Phase Inorganic Chemistry" Modern Inorganic Chemistry; Russell, D. H., Ed.; Plenum: New York, 1988;in press. (3)Sunderlin, L.S.;Armentrout, P. B. J. Phys. Chem. 1988,92, 1209. 14) Aristov. N.:Armentrout. P. B. J. Phvs. Chem. 1987. 91.6178-6188. (5j Halle, L. F.'; Armentrout, P. B.; Beaichamp, J. L.J . h i . Chem. SOC. 1981, 103,962-963. (6) Schultz, R. H.;Elkind, J. L.; Armentrout, P. B. J. Am. Chem. SOC. 1988,110.41 1-423. (7)Halle, L. F.; Armentrout, P.B.; Beauchamp, J. L. Organometallics 1982,I, 963-968. (8)Armentrout, P. B.; Beauchamp, J. L.J . Am. Chem. SOC.1981,103, 784-791. (9)Byrd, G.D.; Burnier, R. C.; Freiser, B. S . J. Am. Chem. SOC.1982, 104, 3565-3569. (10) Jackson, T.C.;Carlin, T. J.; Freiser, B. S . J . Am. Chem. SOC.1986, 108., 1120-1126. (11) Jackson, T. C.; Jacobsen, D. B.; Freiser, B. S . J . Am. Chem. SOC. 1984,106, 1252-1257. (12)Byrd, G. D.; Freiser, B. S . J. Am. Chem. Soc. 1982,104,5944-5950. ~
TNSF Presidential Young Investigator, 1984-1989. Alfred P. Sloan Fellow. Camille and Henry Dreyfus Teacher-Scholar, 1987-1992.
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0 1988 American Chemical Society
Georgiadis and Armentrout
7068 The Journal of Physical Chemistry, Vol, 92, No. 25, 1988
been shown to be due to electronically excited states of the metal ion For the case of Ta+,I4reactivity is also observed; however, it is not known whether it is due to electronically excited states. In previous ion beam studies of ground-state Cr+ ions with methane,5 the only endothermic product detected was CrH+ and it was postulated to be formed via a stripping mechanism. In the same study, excited states of Cr+ were found to produce three products, CrCH2+, CrH+, and CrCH3+,in exothermic reactions. This dramatic increase in reactivity was attributed to the ability of electronic but not translational energy to promote the dehydrogenation reaction. In a complementary ICR study,I3 Ridge and co-workers also showed that excited states of Cr+ react with alkanes and, in particular, that they dehydrogenate methane. In this study, it was also nicely demonstrated that these excited states of Cr+ are quenched quite efficiently by methane with a rate constant equal to (6.1 f 2.7) X cm3/s (about 57 f 25% of the collision rate), even though the reaction is spin forbidden.I3 Further studies of the reactions of ground-state Cr+ with larger alkanes, alkenes, and cycloalkanes find none of the exothermic reactions which are typical of most other first-row transition-metal ions.I5 This complete lack of reactivity led Schilling and Beauchamp to ask "What is wrong with gas-phase chromium?" The Cr+-methane system is also of interest since the ionic products of the reaction have seen more theoretical study than any other transition metal. The electronic and geometric structures of CrH+,I6l8 CrCH+,18 CrCH2+,18,19 and CrCH3+18,20 have been predicted and their bond energies have been calculated by ab initio methods. The ground state of CrCH2+(4B1)is found to contain a chromium carbon double bond. CrH+ and CrCH3+ are both respectively. The low-lying singly bonded quintets, 5Z+and electronic states of chromium carbene ions have been probed using high-resolution translational energy loss spectroscopy21and the experimental results are in good agreement with the theoretical predictions.Ig The translational energy loss features in the spectrum are attributed to spin-forbidden electronic transitions. Motivated by the experimental observations of Ridge and Beauchamp, we undertood the present detailed examination of the reactions of ground and excited state Cr+ with methane as a function of translational energy. With the increased sensitivity of guided ion beam mass spectrometry, we observe product channels below the experimental detection limit of previous experiments. Thus, we observe three endothermic products, CrCH2+, CrCH3+, and CrH+, for the ground-state reaction. We follow these reactions as the electronic energy of the metal ion is increased, in order to study the effect of electronic state on reactivity. An electronic state correlation diagram is generated and allows an integrated explanation of our results and those of previous researchers.
Experimental Section General. The ion beam apparatus used in these experiments has been described earlier.22 Briefly, ions are extracted from the ion source region with a series of lenses and then accelerated and (13) Freas, R. B.;Ridge, D. P. J . A m . Chem.Soc. 1980,102,7131-7132. Reents, W. D.; Strobel, F.; Freas, R.B.; Wronka, J.; Ridge, D. P. J . Phys. Chem. 1985,89, 5666-5670. (14) Wise, M. B.; Jacobsen, D. B.; Freiser, B. S . J. Am. Chem. SOC.1985, 107, 1590-1595, 6744. (15) Schilling, J. B.; Beauchamp, J. L. Organomefallics 1988, 7, 194-199. (16) Schilling, J. B.; Goddard, W. A.; Beauchamp, J. L. J. Am. Chem. Soc. 1986, 108, 582-584; J . Phys. Chem. 1987, 91, 5616-5623. (17) Pettersson, L. G. M.; Bauschlicher, C. W.; Langhoff, S. R.;Partridge, H. J . Chem. Phys. 1987,87,481-492. (18) Alvarado-Swaisgood, A. E.; Allison, J.; Harrison, J. F. J . Phys. Chem. 1985, 89, 2517-2525. (19) Carter, E. A,; Goddard, W. A,, 111 J . Phys. Chem. 1984, 88, 1485-1490. . . - - - .- .. (20) Schilling, J. B.; Goddard, W. A.; Beauchamp, J . L. J . Am. Chem. Soc. 1987, 109, 5573-5580. (21) Hanratty, M. A.; Carter, E. A,; Beauchamp, J. L.; Goddard, W. A.; Illies, A. I.; Bowers, M. T. Chem. Phys. Leff.1986, 123, 239-242. (22) Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1985,83, 166-189.
TABLE I: Low-Lvine Electronic States of Cr+ E", eV population,b % state confign
a6S a6D a4D a4G a4P b4D a21
3d5 4s3d4 4s3d4
b4P
4s3d4
3d5
3d5 3d5 3d5
0.0
99.83 f 0.06 0.17 f 0.06
1.52 2.46 2.56
