High-Energy Processes in Organometallic Chemistry - ACS Publications

Chapter 2

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Gas-Phase Organometallic Ion Chemistry J. L. Beauchamp Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125

Gas phase studies of the reactions of small molecules at transition metal centers have provided a rich harvest of highly speculative reaction mechanisms. Traditional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of of alkanes. Ion beam techniques, ion cyclotron resonance spectroscopy, and laser activation methods have been employed to characterize the energetics and mechanisms of gas phase organometallic reactions in considerable detail. For example, product kinetic energy distributions reveal important features of the potential energy surfaces associated with the formation and rupture of H-H, C-H and C-C bonds at transition metal centers. In complex systems where several products are formed competitively, IR multiphoton activation of stable reaction intermediates allows the lowest energy pathway to be identified. Several applications of these techniques in studies of the reactions of alkenes and alkanes with atomic transition metal ions are presented to illustrate endeavors in the growing field of gas phase organometallic ion chemistry.

An impressive array of experimental methods are available which permit the use of extreme conditions to promote chemical reactions. In studying the reactions of ionic species with small molecules in the gas phase, it is possible to vary reagent translational energy over a wide range. In addition, the possibility of trapping ions in appropriate configurations of electromagnetic fields facilitates the use of single and multiphoton excitation processes to effect specific chemical reactions. These activation techniques can provide relative translational and internal energies which correspond to stellar temperatures. Such studies would be a mere curiosity if extreme conditions were always necessary to promote interesting reactions. Fortunately this is not the case, and the observation of processes 0097-6156/87/0333-0011 $09.00/0 © 1987 American Chemical Society

Suslick; High-Energy Processes in Organometallic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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HIGH-ENERGY

P R O C E S S E S IN O R G A N O M E T A L L I C

CHEMISTRY

such as the conversion of cyclohexane to benzene in a single bimolecular encounter with an atomic transition-metal ion at room temperature (1) clearly demonstrate that it is not necessary to "nuke" the system to identify interesting candidates for detailed mechanistic and dynamic studies. Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. The purpose of this article is to review some of the current endeavors in this developing field. To maintain brevity, the focus is on recent studies carried out in our own laboratory and in conjunction with Professor M.T. Bowers at the University of California at Santa Barbara, with emphasis on the use of kinetic energy release distributions and infrared laser multiphoton excitation to probe potential energy surfaces for the reactions of atomic metal ions with alkenes and alkanes. Experimental Methods The majority of studies in our laboratory of the reactions of atomic transition metal ions with small molecules have been conducted using the ion beam apparatus shown in Figure 1 (8). The key to the success of this apparatus has been the use of a surface ionization source shown in the inset of Figure 1. Metal halide salts are evaporated from a heated tube furnace onto a resistively heated rhenium ribbon (1800 - 2500 °K) where they thermally decompose and the resulting metal atoms are surface ionized. Atomic metal ions are extracted, mass analyzed in a magnetic sector, and allowed to interact with neutral molecules in a collision cell with relative kinetic energies in the range 0.2-10.0 eV. Products scattered in the forward direction are mass analyzed using a quadrupole mass spectrometer and counted with a channeltron detector. Armentrout and co-workers have built an improved version of this apparatus with the important difference being the substitution of an octopole ion guide for the collision chamber to improve product ion collection efficiencies (9). With the surface ionization source it is generally assumed that the reactant ion internal state distribution is characterized by the source temperature and that the majority of the reactant ions are in their ground electronic state. This contrasts with the uncertainty in reactant state distributions when transition metal ions are generated by electron impact fragmentation of volatile organometallic precursors (10) or by laser evaporation and ionization of solid metal targets (1JL). Many examples



2.

BEAUCHAMP

Gas-Phase Organometallic Ion Chemistry

13

have now been recorded of modified excited state reactivities of atomic transition metal ions (12). The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes (16) . In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments (17) . A very powerful technique for obtaining information relating to potential energy surfaces for organometallic reactions involves the determination of kinetic energy release distributions for product ions (7). Experiments of this type have been conducted in collaboration with Professor M.T. Bowers and his students at the University of California at Santa Barbara. The instrument used for these studies, a V G ZAB-2F reversed geometry double focusing mass spectrometer, is shown schematically in Figure 2. Adducts of atomic metal ions with neutral reactants are extracted from a high pressure ion source and mass analyzed using the magnetic sector. Translational energies of products formed by adduct dissociation in the second field free region between the magnetic and electric sectors are determined from an analysis of the metastable peak shape obtained by scanning the electric sector (18). In addition to the experimental methods pertinent to the work described herein, Farrar and co-workers have recently published a crossed beam investigation of the decarbonylation of acetaldehyde by atomic iron ions (19). Studies of this type, which provide product energy and angular distributions, will provide detailed insights into the mechanisms of organometallic reactions. Several investigators, including Depuy (20), McDonald (21), Squires (22), and Weissharr (23), have utilized flowing afterglow techniques to study organometallic reactions. These experimental methods all complement each other in terms of the type of information they provide and the accessible range of experimental parameters such as pressure and temperature. Experimental Studies of the Reactions of Atomic Metal Ions with Hydrocarbons Endothermic Reactions and the Determination of Bond Dissociation Energies for Organometallic Fragments. The reaction of atomic nickel ion with molecular hydrogen to yield N i H is substantially endothermic. Reaction cross sections for this process, measured using the ion beam apparatus shown in Figure 1, are displayed in Figure 3 for reactions 1 and 2 with HD as the neutral. +


14

H I G H - E N E R G Y P R O C E S S E S IN O R G A N O M E T A L L I C C H E M I S T R Y

Quadrupole. ,

J^>\

n i


A v > ^Eiitroiict l / Ion Source

Horizontol 8 Vtrtical Biam Deflectors

60* « V

M o

i n 2 t l

Menzlnger Deceleration

.

Focusing Lens

Accelerating A Electrodes

JS^t

Unit .Collision Chamber Modulation

Gas Inlet to Capacitance Manometer Retarding Grid

Multiplier

Quadrupole Mass Filter

Ion

Sources electron beam ,^-oven ionizing / filoment

repeller

filament

Figure 1. Schematic view of ion beam apparatus and surface ionization source. Magnetic

Electrostatic

Pulte Counting Equipment

Figure 2. Schematic view of reversed geometry double focusing mass spectrometer used for kinetic energy release measurements.


2.

BEAUCHAMP

+

Ni


+ HD

H U

15

NiH

+

+ D

(1)

NiD

+

4- H

(2)


+

An analysis of the threshold region for reaction 1 yields a N i - H bond dissociation energy (Table I). Determination of a threshold involves assuming a functional form for the variation of the cross section with energy in excess of the threshold value and folding in the velocity distribution of the neutral target molecules (Doppler broadening) to fit the experimental data (25). Examination of endothermic reactions in more complex systems has yielded a wide range of bond dissociation energies for neutral and ionic organometallic fragments. For example, the threshold identified for the formation of C0CH2 " in reaction 3 yields a cobalt ion carbene bond dissociation energy (26). 4

Co

+

+ C2H4 ^ C o C H

+

+ CH

2

2

(3)

Representative bond dissociation energy data for organometallic fragments are summarized in Table I. Several trends in these data warrant comment. Not surprisingly, C r forms unusually weak bonds to ligands such as H and CH3. This results from the disruption of the unusually stable half filled 3d shell in the atomic ion to form a sigma bond. Periodic trends of the metal hydride ion bond dissociation energies are shown in Figures 4 and 5 for the first and second row metals, respectively, where they are compared to the results of ab initio generalized valence bond dissociation consistent configuration interaction (GVB-DC-CI) calculations (27). The agreement is generally very good, with the experimental results tending to be slightly higher than the theoretical values. A careful analysis of the wave functions indicates that first row metals use a mixture of the 4s and 3d orbitals to form strong sigma bonds. The second row metals form strong sigma bonds with only minor participation of the 5s orbitals. It has been noted that the bond strengths can be correlated with the promotion energy from the ground state to the lowest state derived from a 4s!3dn configuration (which is thus assumed to form a strong bond to hydrogen) (28). The correlation is somewhat better with a more appropriately defined promotion energy in which exchange terms are accounted for (29) and suggests an intrinsic metal hydrogen bond dissociation energy of around 60 kcal/mol. It is of particular interest to note that the metal ion methyl bond dissociation energies in Table I are typically greater than the metal hydrogen bond energies. In part this can be attributed to the greater polarizability of the methyl group compared to hydrogen and the effect which this has on stabilizing the positive charge. Since D°[H-H] and D°[CH3-H] are approximately equal, the relative nickel ion hydrogen and methyl bond strengths are confirmed by the observation that reaction 4 of nickel hydride ion with methane is a fast process (30). +

NiH

+

+ C H -> N i C H 4

+ 3

+ H

2

(4)

According to the data in Table I this process is exothermic by 9 kcal/mol. Reaction 4 is one of the few processes in which methane is known to undergo a facile reaction at a transition metal center. Interestingly, the analogous reaction with F e H is not observed, even though it is exothermic by 10 kcal/mol (31). +


16


Table I . Bond Dissociation Energies (eV) at 298 K for Organometallic Fragments


M Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Ru Rh Pd Ag

+

D°(M -H) 1.95 2.44 2.43 2.09 1.40 2.10 2.12 2.02 1.72 0.96

± ± ± ± ± ± ± ± ± ±

+

D°(M -CH ) 2

+

D°(M -CH ) 3

± ± ± ± ± ± ± ±

D°(M-CH ) 3

a

0.2 0.10 0.10 0.06 0.11 0.15 0.06* 0.06 0.08 0.13 b

c

d

e

f

h

h

b

>4.05 ± 0.2 3.69 ± 0.2 3.30 ± 0.2J 2.21 ± 0.2 4.16 ± 0.2 4.2 ± 0.2 3.7 ± 0.3 3.7 ± 0.2 a

a

k

1

1

1

b

2.56 ± 0.2 2.45 ± 0.2 2 . 1 7 ± 0.2J 1.28 ± 0.2 2.2 ± 0.2 2.56 ± 0.2 2.6 ± 0.2 2.13 ± 0.2

a

a

1.8 ± 0.2°

m

a

1

a

h

3.03 ± 0.2 2.52 2.34 2.31 1.80 1.78 1.56 2.06 0.65

D°(M-H)

1.9 2.3 2.8 2.9

± 0.2P ± 0.4P ± 0.2P ± 0.3 > E i , E 2). To reduce the high level of internal excitation which results from association of the metal ion with the olefin, adducts are prepared using the ligand exchange reaction 14 and the product is allowed to further +


a

a

CoCO+ + C5H10 -

Co(C Hi ) 5

0

+

+ CO

(14)

relax by infrared emission over the long time scale of trapped ion ICR experiments. Starting at the bottom of the well in Figure 13 it should in principle be possible to use collisional activation to identify the lowest energy pathway. Such studies have been used by Freiser and co-workers to identify isomeric ion structures, but the control of the relative kinetic energy and the number of collisions in such experiments is only semiquantitative at best (51). Absorption of infrared photons provides a relatively easy method for depositing small increments (2.5 kcal mol-l photon ) of energy into a molecule. Using low-power continuous wave infrared laser radiation, gasphase ions have been shown to undergo decomposition at energies near threshold (52). This technique has been utilized to identify the lowest energy pathway, which may be different for different isomers, in complex systems. With reference to Figure 13, if case I best represents the reaction coordinate, only the thermodynamically most stable product would be observed. In case II the higher energy products which have the lower activation energy would be observed. Finally, if a common transition state were involved as in case HI, both sets of produts would be formed. In addition, isomeric ions may exhibit dissociation yields which vary with wavelength in a manner which allows them to be distinguished (16). Thus, infrared mutltiphoton decomposition can yield details relating to activation parameters as well as structural information. Figure 15 shows the results of infrared laser excitation of the Co(2pentene) adduct (6). The decrease in the abundance of the adduct ion is matched by an increase in the abundance of only a single product in which methane is eliminated. Compared to the bimolecular collision process, a high degree of selectivity is achieved in the IR activation experiment. This information can thus be used in constructing the reaction coordinate diagram shown in Figure 16, where only the elimination of hydrogen and methane are considered. Elimination of methane is the most exothermic process and in addition has the lowest activation energy. The lowest energy process available to the adduct involves cleavage of the allylic C-C bond in preference to the allylic C-H bond and subsequent rearrangement and elimination of methane. The analogous experiment with 1-pentene also yields a single product as indicated in Table IV. In addition, the product, in which ethylene is eliminated, is different from that observed in the case of 2-pentene! In this case, the process with the lowest activation energy does not correspond to the most exothermic process (Table IV). Again the process of lowest energy involves cleavage of the allylic C-C bond in preference to the allylic C-H 1

+



36


Scheme I I I

A.

a

u

"VI / -

—1

LOMr On

- V \ |

—i

Vf"

i—

LOMr On h-

a

c

Co (C H )*

-

i

—1

s

A

r—

LoMr On Co(C H )* 5 8

-

•

E.

W\

—1

-

-

r—

-

—1

Law On

Lastr On

Co(C Hj*

Co(C M )

a

2

Figure 15. Changes in reactant and product ion yields resulting from cw infrared laser multiphoton activation of the Co(2-pentene) adduct. Data from reference 6. +


4

4


2.

BEAUCHAMP


37

Figure 16. Reaction coordinate diagram for the elimination of hydrogen and methane in the reaction of C o with 2-pentene. +

Table IV. Product Distribution for Exothermic Reactions of C o with C5H10 Isomers +

Product Distributions C5H10 Isomer

a

Neutral AH Lost (kcal/mol)

. l o n

beam

metastable

-46 -55

0.11 0.13

0.02 0.01

0 0

C2H4

-29

0.58

0.95

1.00

C H

6

-25

0.18

0.01

0

4

-43 -52 -27

0.28 0.33 0.29

0.47 0.43 0.09

0 1.00 0

-23

0.10

0.01

0

u

1- pentene

H CH

2 4

3

2- pentene

H CH C H

2

2

4

C3H6

. . . . activation

Data from reference 6.



38

HIGH-ENERGY


CHEMISTRY

bond. The suggested mechanism for the elimination of ethylene starting from the 1-pentene adduct is included in Scheme E L Also included in Table IV are the metastable product yields for comparison to the ion beam and IR activation results. From these data it appears that the processes involving elimination of hydrogen and methane involve a competitive dissociation from a common intermediate as shown in Figure 16. However, a common intermediate may not be involved in the elimination of ethylene and propylene (the latter product appears to be formed in a faster process), and Scheme i n is overly simplistic. A prerequisite for the success of the multiphoton IR laser activation experiment is that the adduct must absorb in the 10 micron region. Unfortunately, none of the adducts of atomic metal ions with saturated hydrocarbons which we have examined to date absorb in this region. As the tunability and intensity of infrared sources improve, the general applicability of this experimental methodology for probing the potential energy surfaces for organometallic reactions will be greatly increased. Future Studies Considering the extensive efforts which have been directed at understanding the reactions of atomic transition metal ions with hydrocarbons, it is somewhat surprising that our understanding of these processes is rudimentary at best. It is imperative that continued studies be coupled with high quality ab initio theory to provide a more fundamental understanding of not only the molecular and electronic structure of organometallic fragments but complete potential energy surfaces for some of the simpler processes. Studies utilizing state selected reactants clearly represent an important frontier. Benchmark quantitative cross section data obtained by Armentrout and co-workers (53) demonstrate that with an energy difference of 0.25 eV, excited state F e (4F derived from the 3d? configuration) is much more reactive with H than is the ground state (6D derived from the 4s3d6 configuration). These studies utilized a surface ionization source with a calculated reactant state distribution that could be modified by quenching higher excited states in a drift cell. The use of single photon (54) and resonance enhanced multiphoton ionization processes (55) to prepare state selected reactants will allow for extensive studies of this type in the future. Collisional excitation can also be used to prepare state selected reactants. Beyond an examination of the effects of electronic excitation, a study of the effects of vibrational excitation in the hydrocarbon on reactivity would be of particular interest. Enhanced reactivity of a species such as ethane would verify the idea that intrinsic barriers for insertion of metal ions into C-H bonds are responsible for the low reactivity of this species. Complementing studies of the effect of reagent excitation are studies which determine energy disposal to products. Measurements of kinetic energy release distributions are only a first step in this direction. The examples of kinetic energy release distributions presented above are selected from measurements for several hundred reactions (56). These data are still being digested to extract not only thermochemical data but mechanistic information as well. In numerous cases bimodal distributions have been obtained. The dehydrogenation of propane by F e and the elimination of propylene in the reaction of C o with 1-pentene are examples of processes which yield bimodal distributions. These could +

2

+

+



2.

BEAUCHAMP


39

result from distinct reactivity of different states of the metal ion, from the formation of two different internal energy states of the products, or from the formation of dissimilar transition states (which may then yield either the same or distinct products). The application of newer methods to studies of gas phase organometallic reactions will lead to the development of routine techniques for determination of the thermochemistry of organometallic species. The examples discussed above demonstrate that an analysis of kinetic energy release distributions for exothermic reactions yields accurate metal ligand bond dissociation energies. This can be extended to include neutrals as well as ions. For example, reaction 15 has been used to determine accurate bond dissociation energies for Co-H and C0-CH3 (57). Co

+

+ RSi(CH ) -> CoR + S i ( C H ) 3

3

3

+

(15)

3

Related reactions can be used to determine metal hydrogen and metal carbon bond dissociation energies in coordinately saturated neutral species. Experiments in progress in our laboratory using magnetic bottle techniques (58) will extend investigations of kinetic energy release distributions to include measurements for bimolecular in addition to unimolecular reactions. This will be particularly useful in the examination of direct processes which do not involve long lived intermediates. From the examples given above it is clear that the development of a novel approach for real time measurements of the dissociation kinetics of reaction intermediates would greatly assist the unravelling of complex multistep processes associated with the transformation of even simple molecules at transition metal centers. While techniques are available for such measurements over a limited range of times, none of the methods are sufficiently general to be useful for extensive measurements. An examination of the reactions of small molecules with organometallic fragments and species which possess only a single coordination vacancy will permit more direct comparisons with condensed phase studies. Studies of specific organometallic functional groups such as metal carbenes (59), metal alky Is (60) and metal oxides (61) have already revealed some exciting results. The addition of one or more ligands to a metal center greatly modifies the reactivity toward hydrocarbons. A species such as C p N i will readily dehydrogenate hydrocarbons but does not exhibit any reactions in which C-C bonds are cleaved (62). Extensions of studies of the reactivity of hydrocarbons at transition metal centers to include a variety of functional groups have been reported and remain to be explored in greater detail. Of particular interest are processes which lead to single and multiple bonding of elements other than carbon to the transition metal center. Little is known about the energetics of such bonds and their functional group chemistry. We were recently surprised to find that metal silylenes (M = SiH2 ) are exceptionally stable, with bond dissociation energies as high as 70 kcal/mol (63). The observation and characterization of metal silylenes in the gas phase has now inspired attempts by several groups to prepare these species in more conventional condensed phase studies. Anionic and cationic species are particularly easy to study in the gas phase using mass spectrometric techniques. Studies of organometallic reactions involving neutral species are becoming more prevalent. The elegant study of the catalytic hydrogenation of ethylene by photochemical +

+



40

HIGH-ENERGY


CHEMISTRY

activation of Fe(CO)5 by Grant and co-workers is an example of the type of investigation that can be carried out in the gas phase (64). Crossed beam studies involving both neutrals and ions will provided detailed information relating to the potential energy surfaces and reaction mechanisms for organometallic reactions (19). These studies are especially revealing when the reactions are direct and involve intermediates whose lifetime does not exceed a rotational period. Finally, the general field of metal cluster chemistry deserves mention. Techniques for the preparation of metal clusters of virtually any size and composition are virtually routine (65). An ultimate objective in studying these species is to characterize the structure, properties, and reactions of clusters of a particular size. In addition to physisorption and chemisorption processes, more complex reactions such as the extensive dehydrogenation of alkanes by metal clusters have been reported for cluster of varying size (66). The use of ion beam (67) and ICR techniques (68) facilitates such studies since particular clusters can be mass selected and examined. Considering the complexity of processes occurring on a single metal center, it will be a real challenge to extract mechanistic details from cluster studies. To the extent that this can be accomplished the results may well revolutionize our understanding of reactions on catalyst surfaces. In the meantime it seems worthwhile to answer simpler questions. Can atomic metal ions directly insert into C-C bonds? What determines the ability of a distal hydrogen or alkyl group to transfer to a metal center? How important are multicenter reactions in the elimination of small molecules from organometallic reaction intermediates? Acknowledgments This work was supported in part by the National Science Foundation under Grant CHE-8020464 and by the Petroleum Research Foundation. Citations in Literature 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Byrd, G.D.; Burnier, R.C.; Freiser, B.S. J . Am. Chem. Soc. 1982, 104, 5944. For a recent review, see Crabtree, R.H., Chem. Rev. 1985, 85, 245. Muller, J.; Goll, W. Chem. Ber. 1973, 106, 1129. Houriet, R.; Halle, L.F.; Beauchamp, J.L. Organometallics 1983, 2, 1818 and references contained therein. Halle, L.F.; Houriet, R.; Kappes, M.M.; Staley, R.H.; Beauchamp, J.L. J . Am. Chem. Soc. 1982, 104, 6293. Hanratty, M. A.; Paulson, C.M.; Beauchamp, J.L. J . Am. Chem. Soc. 1985, 107, 5074. Hanratty, M.A.;Illies,A.J.; Bowers, M.T.; Beauchamp, J.L. J. Am. Chem. Soc. 1985, 107, 1788. Armentrout, P.B.; Beauchamp, J.L. J . Amer. Chem. Soc. 1981, 103, 784. Ervin, K . M . ; Armentrout, P.B. J . Chem. Phys. 1985, 83, 166. Halle, L.F.; Armentrout, P.B.; Beauchamp, J.L. J. Amer. Chem. Soc. 1981, 103, 962. Kang, H.; Beauchamp, J.L. J . Phys. Chem. 1985, 89, 3364 and references contained therein. For a recent example, see Mandich, M.L.; Halle, L.F.; Beauchamp, J.L. J . Am. Chem. Soc. 1984, 106, 4403.



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13. Beauchamp, J.L. Ann. Rev. Phys. Chem. 1971, 22, 527. 14. Wise, M.B.; Jacobson, D.B.; Huang, Y.; Freiser, B.S. Organometallics, in press. 15. Cody, R.B.; Burnier, R.C.; Freiser, B.S. Anal. Chem. 1982, 54, 96. 16. Thorne, L.R. Beauchamp, J.L. in"Ionsand Light" (Gas Phase Ion Chemistry, Vol. 3), Bowers, M.T., ed., Academic Press, New York, 1983. 17. Hettich, R.L.; Freiser, B.S. J. Amer. Chem. Soc., in press. 18. Jarrold, M.F.;Illies,A.J.; Kerchner, N.J.; Wagner-Redeker, W.; Bowers, M.T.; Mandich, M.L.; Beauchamp, J.L. J. Phys. Chem. 1983, 87, 2313, and references therein. 19. Sonnenfroh, D.M.; Farrar, J.M. J. Amer. Chem. Soc. 1986, 108, 3521. 20. Walba, D.M.; Depuy, C.H.; Grabowski, J.J.; Bierbaum, V.M. Organometallics 1984, 3, 498. 21. McDonald, R. N.; Schell, P.L.; McGhee, W.D. Organometallics 1984, 3, 182 22. Lane, K.R.; Sallans, L.; Squires, R.R. J. Am. Chem. Soc. 1986, 108, 4368. 23. Tonkyn, R.; Weisshaar, J.C. J. Phys. Chem. 1986, 90, 2305. 24. Armentrout, P.B.; Beauchamp, J.L. J. Chem. Phys. 1980, 50, 37. 25. Elkind, J.L.; Armentrout, P.B. J. Phys. Chem. 1984, 88, 5454. 26. Armentrout, P.B.; Beauchamp, J.L. J. Chem. Phys. 1981, 74, 2819. 27. Schilling, J.B.; Goddard, W.A.,III;Beauchamp, J.L. J. Am. Chem. Soc. 1986, 108, 582. 28. Armentrout, P.B.; Halle, L.F.; Beauchamp, J.L. J. Am. Chem. Soc. 1981, 103, 6501. 29. Schilling, J.B.; Goddard, W.A.,III;Beauchamp, J.L., to be published. 30. Carlin, T.J.; Sallans, L.; Cassady, C.J.; Jacobson, D.B.; Freiser, B.S., J. Am. Chem. Soc. 1983, 105, 6320. 31. Halle, L.F.; Klein, F.S.; Beauchamp, J.L. J. Am. Chem. Soc. 1984, 106, 2543. 32. Beauchamp, J.L.; Stevens, A.E.; Corderman, R.R. Pure and Appl. Chem. 1979, 51, 967. 33. Halle, L.F.; Armentrout, P.B.; Beauchamp, J.L. Organometallics 1982, 1, 963. 34. Schilling, J.B.; Beauchamp, J.L., to be published. 35. Tolbert, M.A., Mandich, M.L., Halle, L.F.; Beauchamp, J.L. J. Am. Chem. Soc. 1986, 108, 5675. 36. Tolbert, M.A.; Beauchamp, J.L. J. Amer. Chem. Soc., in press. 37. Tolbert, M.A.; Beauchamp, J.L. J. Amer. Chem. Soc. 1984, 106, 8117. 38. Hanratty, M.A.; Beauchamp, J.L.;Illies,A.J.; van Koppen, P.; Bowers, M.T. J. Amer. Chem. Soc. submitted for publication. 39. Watson, P.L.: Roe, D.C. J. Am. Chem. Soc. 1982, 104, 6471. 40. Chesnovich, W.J.; Bass, L.; Su, T.; Bowers, M.T. J. Chem. Phys. 1981, 74, 2228 41. Huisken, F.; Kranovich, D.; Zhang, Z.; Shen, Y.R.; Lee, Y.T. J. Chem. Phys. 1983, 78, 3806. 42. Jacobson, D.B.; Freiser, B.S. J. Am. Chem. Soc. 1983, 105, 5197. 43. Low. J.J.; Goddard, W.A. J. Am. Chem. Soc. 1984, 106, 8321. 44. Low, J.J.; Goddard, W.A. J. Am. Chem. Soc. 1984, 106, 6928. 45. Obara, S.; Kitaura, K.; Morokuma, K. J. Am. Chem. Soc. 1984, 106, 7482. 46. Brookhart, M.; Green, M.L.H. J. Organomet. Chem. 1983, 250, 395.


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47. 48. 49.


50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68.


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