15688
J. Phys. Chem. 1996, 100, 15688-15694
Gas-Phase Organolanthanide (Ln) Chemistry: Formation of Ln+-{Benzene} and Ln+-{Benzyne} Complexes by Reactions of Laser-Ablated Ln+ with Cyclic Hydrocarbons John K. Gibson Chemical and Analytical Sciences DiVision, Oak Ridge National Laboratory, P.O. Box 2008, Building 5505, Oak Ridge, Tennessee 37831-6375 ReceiVed: March 22, 1996X
Nascent laser-ablated lanthanide metal ions, Ln+, were reacted with cyclohexacarbons, C6H6+2n (n ) 0, 1, 2, or 3), and the resulting organometallic complex ions, {Ln+}-{CpHq}, were identified by time-of-flight mass spectrometry. Cyclohexane and cyclohexadiene were especially reactive, primarily undergoing one or more dehydrogenations to produce adduct ions corresponding to the benzene and benzyne complexes, {Ln+}{C6H6} and {Ln+}-{C6H4}. Also identified as minor products were the “sandwich” complexes, {C6H6}{Ln+}-{C6H6}. Carbon-carbon bond activation was generally an unimportant reaction channel, with small yields of {Ln+}-{CpHq} for p e 5. Significant differences were observed in product yields and distributions between the several Ln+ studied, providing the following comparative reactivities: Ce+ J Tb+ J Gd+ ≈ Pr+ J Ho+ J Dy+ J Lu+ > [Sm+/Tm+/Eu+/Yb+ unreactive]. These differences are explained by the metal ion ground state electronic configurations (typically 4fn-1 6s1) and promotion energies (PE) for excitation of a (nonbonding) 4f electron to a valence 5d orbital. For example, upon reaction with 1,3-cyclohexadiene, Eu+ (PE ) 394 kJ mol-1) was virtually inert while Tb+ (PE ) 38 kJ mol-1) produced abundant Tb+‚{C6H6}. The distinctive Ln+ reactivities indicate that most ablated Ln+ were in the ground or a low-lying (j0.3 eV) electronic state. Contrasting the reactivities of two or more Ln+ co-ablated from a multicomponent target circumvented effects of experimental variables and provided especially reliable comparative reactivities. In addition to the primary chemical effects, variations in product abundances with ion velocity indicated enhanced H2 loss for high-energy ion-molecule collisions.
Introduction Gas-phase reactions between transition metal ions (M+) and neutral organic molecules (R) probe fundamental organometallic chemistry and reaction mechanisms, elucidating condensedphase processes such as catalysis.1 Although early investigations of metal ion-hydrocarbon reactions focused primarily on first-row transition metals,2 recent studies have proceeded to examine lanthanide ions (Ln+).3-8 Whereas condensed-phase organolanthanide chemistry is largely similar across the series,9 substantial systematic discrepancies have been revealed in the corresponding gas-phase Ln+ chemistries.7 Some of these distinctions are explained by the Ln+ ground state electronic configurations and variations in the energy required to promote a nonbonding 4f electron to a valence 5d or 6s reactive orbital.3,7 Improving basic understanding of C-H and H-H bond activation by lanthanides as well as the nature of bonding in their complexes with organic ligands is a timely pursuit in view of the increasing importance of organolanthanides due to catalytic and other applications.10 The remarkably strong quasi-electrostatic bonding between metal cations and benzene is central to prevalent biochemical systems.11 The cohesion in the gas-phase adducts is similarly strong, with the Sc+‚{benzene} bond energy, for example, having been determined as 220 kJ mol-1 12 and that for V+‚{benzene} as 260 kJ mol-1.13 A theoretical treatment of bonding in transition metal ion-benzene complexes by Bauschlicher et al.14 attributed the enhancement in bonding beyond that anticipated for pure electrostatic interaction to electron donation from d-orbitals of M+ to a π* ligand orbital. Armentrout et al.15 determined the bond dissociation energies (BDE) for the first-row transition metal-benzene complexes, C6H6X
Abstract published in AdVance ACS Abstracts, September 15, 1996.
M+ (BDE-1) and C6H6-{M+‚C6H6} (BDE-2). The irregular variation in BDE-1 across the series was correlated with the M+ promotion energy to a low-spin state suitable for complexation. The drop in BDE-2 across the series was attributed to a decrease from tridentate bonding (i.e., η6-C6H6) to lower hapticity C6H6 in accord with the 18-electron rule; e.g., both Mn+-{η6-C6H6}2 and {η4-C6H6}-Co+-{η6-C6H6} are 18electron complexes. Gas phase M+‚{benzene} adducts have been prepared for transition metals by direct association of M+ with C6H616 and by dehydrogenation of cyclohexacarbons3,7,17-20 according to the following:
M+ + C6H6+2n f M+‚{C6H6+2n-2e} + eH2 (n ) 0, 1, 2, 3; e ) 0, 1, 2, 3) (1) In eq 1, n ) e for benzene complex formation and n ) e ) 0 for the particular case of intact benzene attachment. Also prepared have been sandwich complexes: {Ar}‚M+‚{Ar} where Ar is benzene21 or 1,3,5,-tri-tert-butylbenzene.5 Cleavage of C-C bonds to fragment the C6-ring additionally produced adduct ions such as M+‚{C3H6}.20 Loss of an additional H2 from a benzene complex, i.e., e ) n + 1 in eq 1, gives the benzyne complex M+‚{C6H4}. Although benzyne is not isolable, it has been stabilized in the condensed phase by transition metal complexation.22 Metal ion-benzyne bonding in the gas phase is stronger than that with benzene, with the Sc+‚{C6H4} dissociation energy estimated at J 340 kJ mol-1.23 In analogy to heterogeneous catalysis on solid surfaces, Berg et al.24 have compared the formation of metal ion-benzene adducts to physisorption and dehydrogenation complexation to chemisorption. In general, the propensities of different Ln+ to form arene and other complexes from reactions such as represented in eq
S0022-3654(96)00882-9 This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society
Gas-Phase Organolanthanide Chemistry 1 coherently distinguish their chemistries, bond activation mechanisms, and electronic structures.5,7 Two primary experimental approaches for studying metal ion-molecule reactions are Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS)1,5,7,12 and guided ion beam mass spectrometry.3,4 In addition, Sato et al. have described the laser ablation-molecular beam (LAMB) technique25,26 and applied it to a variety of M+ + R reactions27 including some with R ) benzene.16,21 The LAMB technique entails injecting neutral reactants into the path of nascent laserablated M+ and characterizing the collision product ions with a mass spectrometer. A potential complication is that laser ablated metal ions can be produced with a broad distribution of translational and internal energies; the kinetic energy (e.g., ∼1-10 eV) is generally substantially greater than the internal energy (e.g., 0.1-1 eV).28 The abundant energy of ablated species can diminish state and element specificity in subsequent photo or chemical processes;29 in particular, endothermic reactions may become accessible with the augmentation of ion kinetic energy.30 Ion beam experiments have demonstrated effects of kinetic energy on the course of M+ + hydrocarbon reactions; e.g., H2 loss may be dominant at low energy while significant C-C bond activation appears only at high energy.4,31 Similarly, LAMB studies of M+ + benzene revealed substantial intact arene adducts, M+‚{C6H6}, only for low collision energies while dehydrogenation to the benzyne complex, M+‚{C6H4}, proceeded irrespective of ion energy; this suggested that H2 loss dissipated excess internal energy to avert fragmentation of the M+-ligand bond.16 Similar behavior was reported from an FTICR-MS study of the reactions of benzene with “cold” Pt+smajor product ) Pt+‚{C6H6}svs “hot” Pt+smajor product ) Pt+‚{C6H4}.32 Effects of ion energy on M+ + hydrocarbon reaction pathways are analogous to the excitation/fragmentation of M+‚{CpHq} complexes by photoexcitation33 or collision induced dissociation.34 In addition to the demonstrated effects of kinetic energy, the gas-phase chemistry of an electronically excited metal ion can differ substantially from that of the ground state species by accessing reaction pathways or endothermic reaction channels.35 We have developed a variation of the LAMB technique to study metal ion-molecule reactions and previously investigated the formation of metal fluorides, MFn+, from M+ + C14F24 reactions.36 In the work reported here, Ln+ were reacted with cyclohexacarbons of varying degrees of hydrogen saturation, C6H6+2n with n ) 0 (benzene), n ) 1 (1,3-cyclohexadiene), n ) 2 (cyclohexene), or n ) 3 (cyclohexane). Variations in yields of Ln+-{benzene} and Ln+-{benzyne} illuminated formation mechanisms and stabilities of these complexes and probed the metal ion electronic structures and chemistries. A primary goal was to expand application of the LAMB technique and demonstrate it as a reliable probe of f-block transition metal ion chemistries; a central consideration is the role of excited state metal ions. Sensitivity and specificity in differentiating metal ion chemistries were enhanced by simultaneously determining the comparative reactivities of two or more metal ions co-ablated from a multicomponent target. The effects of collision energy were assessed through variations in product abundances as a function of ion velocity. Experimental Section The laser ablation mass spectrometer has been described previously.36-38 The experimental configuration is shown in Figure 1; alternatively, locating the target inside of the Knudsen cell described in ref 36 provided higher reactant pressure. The pressure of the continuously leaked reactant gas in the vicinity
J. Phys. Chem., Vol. 100, No. 39, 1996 15689
Figure 1. Schematic of the laser-ablation-molecular-beam experiment (unenclosed target configuration). The distance between the target and mass spectrometer (RTOF-MS) ion source is ∼3 cm.
of the target can only be characterized as significantly greater than the ∼3 × 10-4 Pa pressure increase measured downstream. A +200 V repeller pulse (∼3 cm from the target) injected positive ions traversing the mass spectrometer source region into the -2 kV flight tube, sampling a roughly cylindrical volume of ∼6 mm diameter out of the ion plume. Varying the time delay, td, between the laser and injection pulses characterized gradients in ion abundances; tds of 20-40 µs provided optimal sensitivity to most product ions. The XeCl excimer laser (λ ) 308 nm; pulse width ≈15 ns) was attenuated to ∼1 mJ focused onto a spot of ∼0.5 mm2 to provide a nominal irradiance of ∼107 W cm-2. Mass spectra were averaged for 100-200 laser shots on a fixed spot. The solid ablation targets were prepared from commercial materials with purities of at least 99.9%. Pieces of ytterbium metal (Yb°) and sintered CeSi2 were used as supplied. A pellet containing Eu and Tb was prepared by compacting a mixture of Cu° + Eu2O3 + Tb2O3 powders to give the following atomic composition: 94% Cu/3.3% Eu/2.7% Tb (the copper binder produced only minor Cu+). Alloy targets were prepared by arcmelting the respective pure metals36 to give the following aggregate compositions: Tb50Ho50; Dy65Tm35; Gd64Lu36; Cu86Pr6.5Sm7.5; and Nb48Ta52. Thulium contamination of the GdLu alloy during preparation provided considerable Tm+ along with the Gd+ and Lu+. The Nb-Ta alloy was contaminated with Tb and Ho, resulting in substantial Tb+ and Ho+. The benzene (C6H6), cyclohexane (C6H12), and toluene (C6H5CH3) reactants were standard analytical grade reagents. The other reactants were stock commercial products (Aldrich), 98% 1,3-cyclohexadiene (C6H8), and 99% cyclohexene (C6H10). A small ( Sm+ J Tm+ ≈ Eu+ J Yb+ For those Ln+ studied simultaneously in the present work, the results match those from ref 7. For the other Ln+, the reactivity trends are consistent with those from ref 7 with some minor discrepancies in the specific ordering; for example, we con-
Sm+ Eu+ Tm+ Yb+
4fn-25d16s1
4fn-25d2
4f96s1 4f36s1 4f75d16s1 4f116s1 4f106s1 4f146s2
29 39 94 0 ? 127 142
0 107 70 48 ? 234 353
Unreactive Ln+ 4f66s1 4f76s1 4f136s1 4f146s1
258 362 199 321
? 440 370 540
ground state configuration 4f15d2
a Values in kJ mol-1 from ref 47 for lowest energy term and J value. Ln+ are listed in approximate order from most (Ce+) to least (Lu+) reactive; unreactive Ln+ are ordered by increasing atomic number. b
cluded that Lu+ and Dy+ were similarly reactive whereas the FTICR results indicated Lu+ to be slightly more reactive. In contrast to our results with 1,3-cyclohexadiene, Sm+, Tm+, and Eu+ did activate 1,4-cyclohexadiene though they were the least reactive Ln+sonly Yb+ was entirely unreactive. This discrepancy may reflect the longer time scale of FTICR experiments (e.g., g10-2 vs e10-4 s here20). The present results are in accord with the requirement of two chemically active metal ion valence electrons for C-H bond activation.3 Spin-orbit coupling in heavy atoms such as the lanthanides sufficiently degrades spin as a valid quantum number that strict compliance with spin conservation is not anticipated for these insertions.4,46 Since the electrons in the contracted 4f orbitals are chemically inert and the closed-shell 6s2 orbital is nonbonding, a triplet 5d16s1 or 5d2 configuration is presumed requisite for hydrocarbon activation. Such a reactive configuration may be attained from the ground state by discrete promotion prior to insertion or by “curve crossing” of energy levels during activation,7 both of which should be facilitated by the availability of a suitable low-lying state. The ground state configurations and relevant free ion promotion energies are given in Table 5 for the Ln+ studied here; the lanthanide ions are listed in order of decreasing reactivity as determined in the present work (Ce+ ) most reactive). It is evident that the reactivities correlate with the energy required for promotion to a divalent electronic configuration suitable for insertion into a C-H bond. The comparative reactivities were not sufficiently resolved to assess whether a particular divalent state, e.g., 5d16s1 or 5d2, is more effective in C-H activation. The low reactivity of ground state 6s2 Lu+ confirms its alkaline earthlike closed-shell behavior.4 The greater reactivity of Lu+ compared with Tm+ indicated 5d16s1 as the important excited state configuration for Lu+. The successful correlation of the observed reactivities with the promotion energies suggests that the ground or low-lying electronic states were dominant for ablated Ln+. Specifically, the observed distinctions between Ln+ imply that most were produced with internal energies of j30 kJ mol-1 (j0.3 eV). The dominance of ground or low-lying excited state ions is required for the application of the LAMB method to discerning M+ chemistries. The present results demonstrate differentiation between M+ with monovalent-to-divalent promotion energies which differ by at least ∼30 kJ mol-1 (e.g., Ce+ vs Tb+). It is emphasized that excited electronic states at j30 kJ mol-1 may be substantially populated in nascent laser ablated metal ions but that more highly excited M+ are apparently minor constituents.
Gas-Phase Organolanthanide Chemistry Ion Kinetic Energy. Varying the delay td between the laser pulse and ion ejection selected ions according to their velocity so that changes in ion abundances with td reveal kinetic energy dependencies of reaction pathways. Ions sampled ∼3 cm from the target at time td traveled with an apparent net velocity, Vnet ≈ 3 cm/td. For product ions, this apparent velocity is comprised of those of the precursor M+ and its product (adduct) ion. However, the mass of the projectile metal ion (m[Ln+] ≈ 150 amu) was typically about twice that of the quasi-stationary target (m[C6H6+2n] ≈ 80 amu) and consequently the final adduct velocity should have been only ∼30% less than that of the precursor M+ (V[M+‚R] ≈ V[M+] × {m[M+]/(m[M+] + m[R])}); accordingly, the average velocity (Vnet) provides a valid estimate of comparative collision energies. The center-of-mass kinetic energy available to the M+ + R collision is approximated from the laboratory frame energy by48
E[CM] ≈ {m[R]/(m[R] + m[M+])}{KE[Lab]} (KE[Lab] ≈ (1/2)(m[M+])(Vnet)2) For a 150 amu M+ sampled at td ) 25 µs, Vnet ≈ 1200 m s-1 and KE[Lab] ≈ 110 kJ mol-1 (∼13 000 K); collision of this M+ with neutral R (m[R] ) 80 amu) gives KE[CM] ≈ 38 kJ mol-1. For product ions at td ) 100 µs, the corresponding values are KE[Lab] ≈ 7 kJ mol-1 and KE[CM] ≈ 2.3 kJ mol-1. It has been demonstrated that energies (KE[CM]) as low as ∼0.1 eV (∼10 kJ mol-1) can significantly affect Ln+ + hydrocarbon reaction cross sections.4 Substantial variations of product ion abundances vs td were found, as illustrated by the results in Figure 4 and Table 2. The abundances of the directly ablated ions, Ln2+, Ln+, and LnO+, decreased more rapidly with increasing td than did those of reaction products. The results for Tb+/Ho+ + toluene exemplify the kinetic energy dependence of arene attachment vs dehydrogenation. Whereas the dehydrogenation complex, Tb+‚{C6H3CH3}, was dominant at short td (Figure 4, top) the intact toluene adduct, Tb+‚{C6H5CH3}, became dominant for long td (Figure 4, bottom). The endothermic H2-elimination process was apparently enhanced for more energetic ion-molecule collisions while survival of intact adduct was favored at low energies where fragmentation is less likely. In analogy with this result, enhancement in H2 loss from M+ + C6H6 reactions to yield M+‚{C6H4} has been demonstrated at high energy for M+ ) Pt+32 and Nb+.16 The dominant reactions under the present experimental conditions are attributed to the collision of a M+ with a single neutral molecule. However, multiple collisions were possible, as evidenced by small yields of the “sandwich” complexes, Ln+‚{C6H6}2. The observed relative enhancement in the Ln+‚{C6H6}2 (vs Ln+‚{C6H6}) at long td (Table 2) may be partly attributed to a greater probability for multiple ion-neutral encounters for slower Ln+. As with the monoadducts, comparison of the td dependence of the bis(arene) ion abundances for different organic reactants accords with basic energetics. Specifically, the abundance maximum for Ln+‚{C6H6}2 was at td ≈ 90 µs for the exothermic dehydrogenation of two C6H8, but at td ≈ 45 µs, i.e., at higher energy, for the endothermic dehydrogenation of two C6H10.
J. Phys. Chem., Vol. 100, No. 39, 1996 15693 electronic structures. We have demonstrated a similar sensitivity using nascent laser-ablated ions and a shorter reaction time scale. Specifically, the yields of lanthanide benzene and benzyne complex ions, Ln+-{C6H6} and Ln+-{C6H4}, formed via dehydrogenation reactions exhibited distinct variations across the series understandable in terms of the promotion energy necessary to achieve the divalent bonding configuration of Ln+ presumed prerequisite for initial C-H bond activation. The relationship between observed reactivities and promotion energies suggests that the ablated Ln+ were primarily in ground or low-lying (j30 kJ mol-1) electronic states. That the ion kinetic energies were apparently substantially greater than their internal (equilibrium) energies is consistent with previously elaborated models of the ablation process.49. The key point is that although low-lying electronic states may have been substantially populated, the variations in the relevant Ln+ promotion energies are sufficiently large that coherent differentiation in chemistries was retained. Comparative Ln+ reactivities were consistent regardless of the reactant (e.g., C6H8 vs C6H12) or complex (e.g., Ln+{C6H6} vs Ln+-{C6H4}) considered, consistent with a universal rate-controlling initial step involving insertion of (divalent) Ln+ into a C-H bond. The reaction efficiency and product composition, particularly the amount of benzene vs benzyne adduct, varied coherently with the degree of hydrogen saturation of the reactant. Single- and double-dehydrogenation of C6H8 and C6H10 were typically the most efficient processes. Whereas the internal energy of laser-ablated Ln+ was apparently insufficient to dramatically affect reactivities, there were discernible effects of ion kinetic energies. Faster, highenergy Ln+ induced a greater average degree of dehydrogenation in the process of forming a stable (low internal energy) adduct, consistent with a “cooling” effect of H2 loss. Uncertainties in apparent reactivities due to variations in experimental parameters such as ion kinetic energy were circumvented by comparing reactions of different metal ions concurrently ablated from a multicomponent target. The elucidation of metal ion electronic structures and interactions with organic molecules should extend to gas-phase organoactinide chemistry, where scarcity and radioactivity hinder the study of the conventional condensed phase chemistry. The incentive to investigate the gas-phase reactivities of actinide ions (An+) has been highlighted by recent results for Th+ and U+ which suggest distinctive C-H and C-C activation mechanisms.50,51 The role of 5f electrons should be clarified by comparing the behaviors of heavier actinides where the 5f orbitals become contracted and chemically inert. Extrapolation to An+ of the C-H activation mechanism proposed for Ln+ would require electron promotion to the lowest-lying An+ triplet configuration with at least two non-f valence electrons. The large variations in these promotion energies52 predict substantial differences among An+. For example, Am+ (PE ) 245 kJ mol-1) should be about as inert as Sm+, while Cm+ (PE ) 48 kJ mol-1) should behave similarly to Tb+. However, the greater radial extension and bonding interactions of 5f vs 4f orbitals may relax the requirement for non-f valence electrons to achieve C-H activation, especially for the lighter actinides. In the specific case of Pu+ it should be possible to discern if the ∼100 kJ mol-1 5f f 6d promotion energy is required for bond activation or rather if a 5f electron can participate directly in C-H bond activation.
Conclusions Products of reactions of laser-ablated Ln+ with cyclic-C6 hydrocarbons were monitored by time-of-flight mass spectrometry. Previous FTICR-MS studies of related reactions using cooled metal ions showed a marked dependence on Ln+
Acknowledgment. This work was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.
15694 J. Phys. Chem., Vol. 100, No. 39, 1996 References and Notes (1) Eller, K.; Schwarz, H. Chem. ReV. 1991, 91, 1121. (2) Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum Press: New York, 1989. (3) Schilling, J. B.; Beauchamp, J. L. J. Am. Chem. Soc. 1988, 110, 15. (4) Sunderlin, L. S.; Armentrout, P. B. J. Am. Chem. Soc. 1989, 111, 3845. (5) Yin, W. W.; Marshall, A. G.; Marcalo, J.; Pires de Matos, A. J. Am. Chem. Soc. 1994, 116, 8666. (6) Heinemann, C.; Schroder, D.; Schwarz, H. Chem. Ber. 1994, 127, 1807. (7) Cornehl, H. H.; Heinemann, C.; Schroder, D.; Schwarz, H. Organometallics 1995, 14, 992. (8) Marcalo, J.; Pires de Matos, A.; Evans, W. J. Organometallics 1996, 15, 345. (9) Schumann, H.; Genthe, W. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A. Jr., Eyring, L., Eds.; Elsevier: New York, 1984; Vol. 7, p 445. (10) (a) Watson, P. L.; Parshall, G. W. Acc. Chem. Res., 1985, 18, 51. (b) Edelmann, F. T. Angew. Chem., Int. Ed. Engl. 1995, 34, 2466. (11) Dougherty, D. A. Science 1996, 271, 163. (12) Lech, L. M.; Freiser, B. S. Organometallics 1988, 7, 1948. (13) Hettich, R. L.; Jackson, T. C.; Stanko, E. M.; Freiser, B. S. J. Am. Chem. Soc. 1986, 108, 5086. (14) Bauschlicher, C. W. Jr.; Partridge, H.; Langhoff, S. R. J. Phys. Chem. 1992, 96 3273. (15) Meyer, F.; Khan, F. A.; Armentrout, P. B. J. Am. Chem. Soc. 1995, 117, 9740. (16) Higashide, H.; Oka, T.; Kasatani, K.; Shinohara, H.; Sato, H. Chem. Phys. Lett. 1989, 163, 485. (17) Armentrout, P. B.; Beauchamp, J. L. J. Am. Chem. Soc. 1981, 103, 6628. (18) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1983, 105, 7492. (19) Huang, Y.; Wise, M. B.; Jacobson, D. B.; Freiser, B. S. Organometallics 1987, 6, 346. (20) Seemeyer, K.; Schroder, D.; Kempf, M.; Lettau, O.; Muller, J.; Schwarz, H. Organometallics 1995, 14, 4465. (21) Higashide, H.; Kaya, T.; Kobayashi, M.; Shinohara H.; Sato, H. Chem. Phys. Lett. 1990, 171, 297. (22) Bennett, M. A.; Wenger, E. Organometallics 1995, 14, 1267. (23) Bjarnason, A. Organometallics 1991, 10, 1244. (24) Berg, C.; Beyer, M.; Schindler, T.; Niedner-Schatteburg, G.; Bondybey, V. E. J. Chem. Phys. 1996, 104, 7940. (25) Toya, K.; Kawasaki, M; Sato, H. Jpn. J. Appl. Phys. 1988, 27, 962. (26) Oka, T.; Toya, K.; Kasatani, K.; Kawasaki, M.; Sato, H. Chem. Lett. 1988, 1865.
Gibson (27) Sato, H. Res. Chem. Intermed. 1993, 19, 67. (28) Otis, C. E.; Goodwin, P. M. J. Appl. Phys., 1993, 73, 1957. (29) Rothschopf, G.; Zoller, J.; Lewis, R.; Grant, C.; Schur, R.; Estler, R. Int. J. Mass Spectrom Ion Processes 1995, 151, 167. (30) Armentrout, P. B.; Georgiadis, R. Polyhedron 1988, 16/17, 1573. (31) Sunderlin, L. S.; Armentrout, P. B. Organometallics 1990, 9, 1248. (32) El-Nakat, J. H.; Dance, I. G.; Fisher, K. J.; Willett, G. D. Polyhedron 1993, 12, 2477. (33) Dunbar, R. C. In Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum Press: New York, 1989; p 323. (34) MacMillan, D. K.; Gross, M. L. In Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum Press: New York, 1989; p 369. (35) Freiser, B. S. Acc. Chem. Res. 1994, 27,353. (36) Gibson, J. K. J. Fluorine Chem. 1996, 78, 65. (37) Gibson, J. K. J. Phys. Chem. 1994, 98, 6063. (38) Gibson, J. K. J. Vac. Sci. Technol. A 1995, 13, 1945. (39) Chandrasekharaiah, M. S.; Gingerich, K. A. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A. Jr., Eyring, L., Eds.; North-Holland: New York, 1989; Vol. 12, p 409. (40) Jackson, T. C.; Carlin, T. J; Freiser, B. S. J. Am. Chem. Soc., 1986, 108, 1120. (41) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17 (Suppl. 1). (42) Wenthold, P. G.; Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1991, 113, 7414. (43) McMillen, D. F.; Golden, D. M. Annu. ReV. Physical Chem. 1982, 33, 493. (44) Armentrout, P. B. In Gas Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum Press: New York, 1989; p 1. (45) Rothwell, I. P. Polyhedron 1985, 4, 177. (46) Weisshaar, J. C. In Laser Chemistry of Organometallics; Chaiken, J., Ed.; American Chemical Society: Washington, DC, 1993; p 208. (47) Martin, W. C.; Zalubas, R.; Hagan, L. Atomic Energy LeVelssThe Rare-Earth Elements; NSRDS-NBS 60; National Bureau of Standards (NIST): Washington, DC, 1978. (48) McLuckey, S. A. J. Am. Soc. Mass Spectrom. 1992, 3, 599. (49) Kang, H.; Beauchamp, J. L. J. Phys. Chem. 1985, 89, 3364. (50) Marcalo, J.; Leal, J. P.; Pires de Matos, A. Proceedings of the 43rd ASMS Conference on Mass Spectrometry and Allied Topics; American Society for Mass Spectrometry: Santa Fe, NM, 1995; p 704. (51) Heinemann, C.; Cornehl, H. H.; Schwarz, H. J. Organomet. Chem. 1995, 501, 201. (52) Brewer, L. J. Opt. Soc. Am. 1971, 61, 1666.
JP960882H
