J. Phys. Chem. 1988, 92, 1209-1219
Methane Activation by Ti':
1209
Electronic and Translational Energy Dependence
L. S. Sunderlin and P. B. Armentrout*+ Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: September 14, 1987)
The reaction of Ti+ with methane is studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The effect of electronic excitation is also studied by varying the conditions for forming Ti+. The a2F excited state is found to form the two main products, TiH+ and TiCH2+,substantially more efficiently than the a4F ground and b4F first excited states. The results indicate that reaction occurs primarily through a doublet H-Ti+-CH3 intermediate. The reactivities of the different electronic states of Ti+ can be explained by using simple molecular orbital concepts and spin conservation. The thresholds for these reactions and for related reactions in other systems are interpreted to give Do(Ti+-H) = 54.2 2.5, Do(Ti+-CH3) = 57.5 2.8, Do(Ti+-CH2) = 93.4 3.5, and Do(Ti+-CH) = 121 f 4, all in kcallmol.
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Introduction Extensive progress in understanding the activation of carbonhydrogen and carbon-carbon bonds by transition-metal systems has been made recently. Studies include not only condensed-phase chemistry but also investigations of such processes by atomic transition-metal ions in the gas phase.',2 While the relationship between highly idealized gas-phase systems and systems of true catalytic and synthetic interest is unclear, gas-phase studies can provide quantitative thermochemistry as well as insight into the periodic trends of rea~tivity.'.~ Recent studies in our laboratories have shown that the reactions of atomic transition-metal ions with dihydrogen are sensitive to the electronic state and configuration of the metal ion3 In the case of Ti+,4 the reactivities of the ground state, Ti+(a4F), and first excited state, Ti+(b4F)which is only 0.1 1 eV higher in energy (Table I), cannot be differentiated. This is due to the very close spacing and because the coupling between these states is probably very good. These states react via a statistically behaved intermediate, while the second excited state, Ti+(a*F), and perhaps higher excited states react via direct processes to form TiH+.4 Here we extend these studies to reaction of Ti+ with methane. A question of particular interest is whether the ideas developed to understand the reaction of metals with H2 are also useful in describing the activation of the C-H bonds in methane. In this regard, the present study is a continuation of our recent work on the state-specific reactions of Sc+,I V+,s Cr+,6 and Fe+' with methane. Although the reaction of Ti+ with methane has been studied previously by ion cyclotron resonance (ICR) mass spectrometry,* no reaction was seen and state-specific studies were not carried out. Studies of the reactions of Ti+ with larger molecules indicate high reactivity, predominantly the exothermic loss of one or more H2 or alkane molecules.*-10 Experimental Section General. A complete description of the apparatus and experimental procedures is given elsewhere." Briefly, the apparatus comprises three differentially pumped vacuum chambers. In the first chamber, ions are produced as described below. The resulting ions are extracted, accelerated, and focused into a magnetic sector momentum analyzer for mass analysis. In the second vacuum chamber, the mass-selected ions are decelerated to a desired kinetic energy and focused into an octopole ion guide. Radio-frequency electric fields in the guide create a radial potential well which traps ions over the mass range studied. The velocity of the ions parallel to the axis of the guide is unchanged. The octopole passes through a static gas cell into which methane can be introduced. Pressures are maintained at a sufficiently low level (less than 0.2 mTorr) than multiple ion-molecule collisions are improbable. Product and unreacted beam ions are contained in the guide until they drift out of the gas cell. The ions are then extracted and focused into the third vacuum chamber which contains a quadrupole mass filter for product mass analysis. Ions are detected with a secondary + N S FPresidential Young Investigator 1984-1989; Alfred P. Sloan Fellow.
0022-365418812092-1209%01SO10
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electron scintillation ion detector and processed by pulse-counting techniques. The experiments are automated by use of a computer which collects the ion signals at different masses as it increments the incident ion energy. Use of an octopole ion guide in the interaction region provides two major experimental advantages. The first is that the absolute energy of the ions in the interaction region can be measured easily by using the octopole as a retarding field analyzer. Because the retarding region is physically the same as the interaction region, this energy measurement has minimal uncertainties due to space charge, contact potentials, and focusing aberrations. By scanning through the nominal ion energy zero (where the dc potential on the octopole equals the potential in the ion source), an ion intensity cutoff curve is obtained. The differential of this curve is represented well by a Gaussian peak. The center of this peak is taken to be the true zero of the ion energy, and its width characterizes the kinetic energy distribution of the ion beam. The fwhm of the energy distribution is independent of energy and is generally 0.6 eV in the laboratory frame of reference for these reactions. Uncertainties in the absolute energy scale are f0.05 eV lab. The behavior of the octopole as a retarding analyzer has been verified by timeof-flight measurements" and comparisons with theory.l'J2 Translational energies in the laboratory frame of reference are related to energies in the center of mass (CM) frame by eq 1, where M and m are the masses of the incident ion and neutral reactant, respectively. For these experiments, 48Ti(73.7% natural abundance) was generally used. For some TiH+ data, 5oTi (5.3% natural abundance) was used in order to avoid overlap of @TiH+ with 49Ti+. In all cases, the results are the same in the C M frame. Below -0.3 eV lab (0.08 eV CM), the energies are corrected for truncation of the ion beam energy distribution as described previously.ll The data obtained in this experiment are broadened by two effects: the ion energy spread (determined as discussed (1) Armentrout, P. B. "Gas Phase Inorganic Chemistry"; In Modern Inorganic Chemistry; Russell, D. H., Editor; Plenum: New York, in press, (2) Allison, J. Prog. Inorg. Chem. 1986, 34, 627-676 and references
therein. (3) Elkind, J. L.; Armentrout, P. B. J . Phys. Chem. 1985,89,5626-5636; 1986,90,5736-5745,6576-6586; 1987,91,2037-2045; J. Chem. Phys. 1986, 84, 4862-4871; 1987, 86, 15368-1877. (4) Elkind, J. L.; Armentrout, P. B. Int. J. Mass Spectrom. Ion Processes, in press. (5) Aristov, N.; Armentrout, P. B. J . Phys. Chem. 1987,91,6178-6188. (6) Georgiadis, R.; Armentrout, P. B., work in progress. (7) Schultz, R. H.; Elkind, J. L.; Armentrout, P. B. J . Am. Chem. Soc., in press. (8) Byrd, G. D.; Burnier, R. C.; Freiser, B. S . J . Am. Chem. Soc. 1982, 104, 3565-3569. (9) Tonkyn, R.; Weisshaar, J. C. J . Phys. Chem. 1986, 90, 2305-2308. ( I O ) Tolbert, M. A.; Beauchamp, J. L. J. Am. Chem. Soc. 1986, 108, 7509-75 17. (1 1) Ervin, K. M.; Armentrout, P. B.J. Chem. Phys. 1985,83, 166-189. (12) Ervin, K. M.; Armentrout, P. B. J. Chem. Phys. 1986,84,6738-6749, 6750-6760. Burley, J. D.; Ervin, K. M.; Armentrout, P. B. Inr. J. Mass
Spectrom. Ion Processes, in press.
0 1988 American Chemical Society
1210 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
Sunderlin and Armentrout
TABLE I: Electronic States of TiC state
a4F b4F
a2F a2D a2G
a4P a2P b4P others xstates
>
1 eV
electron confign
energy,a eV
3d24s 3d' 3d24s 3d24s 3d' 3d' 3d' 3d24s
0.028
0.135 0.593
1.082 1.124
1.172
% population
1920 Kb
2260 Kb
2490 Kb
64.83 (94) 33.95 (72) 1.07 (18) 0.04
62.05 (77) 35.82 (47) 1.70 (20) 0.10
60'42 (69) 36.70 (35)f 2.17 (21) 0.16 0.23
0.06
1.232
0.03 0.01
1.236
0.02
21.575
