J. Phys. Chem. 1986, 90, 2305-2308 increases. Unlike the case for monomer collisions, a constant enhancement cannot correct for overall uncertainties in these rates. In both monomer and multimer cases, these results are consistent with the fit done by Soler et aL4 to their data on C02. Those authors found that, in monomer-dominated growth, their liquid drop model collisional growth rates24were capable of being used to compute the envelope of their measured size distributions. In contrast, those same growth rates did not accurately account for the cluster size distribution when the monomer was depleted and growth occurred via multimer-multimer collisions. For these larger clusters, the measured size distribution was narrower and less symmetric than the computed one. Beyond the question of multimer coagulational growth, these results imply that the multimer long-range interaction energy must be included in formulating a theory of homogeneous nucleation that is valid at supersaturations which are sufficiently high that multimermultimer collisional growth processes are important. A final observation based on these results arises from the nonuniform separation (or bunching) of the collision rates, Figure 2. This is due to a combination of cluster morphology and interaction potential and may have observable consequences. For example, small-cluster size distributions generally are not smooth (e.& Figure 1 of ref 25) indicating the possibility that bunching
2305
of the collision rates could play an important role in the formation of such size distributions. We may conjecture that these collision rate effects could also play a role, in addition to purely self-energy considerations, in cluster size distributions at all sizes. In this picture, the formation and evolution of the cluster size distribution is subject to two factors. The morphology and stability of a cluster of N monomers is determined by the customary shape and electronic structure factors, i.e., by “short-range ‘forces’ ”. Accommodation or sticking factors enter cluster formation questions at this point. The probability density that a cluster of N monomers will be “formed” in gas-phase collisions is dependent upon the long-range intercluster potential energies and the morphologies of all the possible collision pairs J , K such that J + K = N . Thus, questions of cluster mass and composition26 distributions may require examination from the viewpoint that the collision rate densities themselves can impose a preferred cluster size distribution prior to the clusters’ stabilization and assumption of their minimal energy configurations.
Acknowledgment. This research was performed under the auspices of the United States Department of Energy under Contract No. DE-ACO2-76CH00016. (25) Geusic, M. E.; Morse, M. D.; Smalley, R. E. J . Chem. Phys. 1985, 82, 590.
(24) In that study, all rates were normalized to the monomer-monomer rate and a unit sticking coefficient was assumed.
(26) Jonkman, H. T.; Even, U.; Kommandeur, J. J . Phys. Chem. 1985,89, 4240.
Transition-Metal Cation Chemistry in 1 Torr of He: M+ -t C,H, Reaction Rates Russ Tonkyn and James C. Weisshaar* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received: February 24, 1986)
We have used laser vaporization of solid metal targets to create first transition series gas-phase metal cations M+ in a fast flow reactor with 1 Torr of He buffer gas. In contrast with single-collision results, all ten first transition series M+ ions, Sc+ through Zn’, react with C2H6 in our multicollision experiment. The major primary product is usually the adduct ion MC2H6+arising from third-body collisional stabilization of long-lived intermediates. The primary reaction rates vary a factor of 250 across the series. M+ ions having ground-state or low-energy 3d” electron configurations react the fastest with alkanes in 1 Torr of He.
Introduction With the advent of novel ion sourcesl~zand the steady development of sensitive mass spectrometric3q4 and ion beam5v6techniques, gas-phase organometallic ion chemistry has become a lively area of research. Fundamental interest in such work stems from the possibility of studying intrinsic molecular interactions unperturbed by solvent effects. As compared to the solution phase, gas-phase ion studies may permit clean isolation and mass identification of mrdinatively unsaturated (radical) species, of primary photochemical products, and of intriguing reaction intermediates. A wide range of single-collision chemistry of first and second linear alkanes,&lo~zz alkenes,” transition series metal ions with H2,’ ~~~
Jones, R. W.; Staley, R. H. J . Am. Chem. SOC.1982, 104, 1235. Allison, J.; Ridge, D. P. J. Am. Chem. SOC.1979, 101, 4998. Cody, R. B.; Freiser, B. S. Anal. Chem. 1982, 54, 1431. Gross, M. L.; Chess, E. K.;Lyon, P. A.; Crow, F. W.; Evans, S.; Tudge, H.In?. J . Mass Soectrom. Ion Phvs. 1982. 42. 243. (5) ArmentroG, P. B.; Beauchimp, J. L: J . Chem. Phys. 1981,74, 2819. (6) Ervin, K.; Loh, S. K.; Aristov, N.; Armentrout, P.B. J . Phys. Chem. 1983.87. 3593. ( 7 j (a) Elkind, J. L.; Armentrout, P. B. J . Phys. Chem. 1985, 89, 5626. (b) Armentrout, P. B.; Beauchamp, J. L. Chem. Phys. 1980, 50, 37. (1) (2) (3) (4)
0022-3654/86/2090-2305$01 .50/0
cyclic alkanes,12 and other organic species has been studied by ion cyclotron resonance and ion beam techniques. The reactions of M+ with alkanes are remarkable in that selective cleavage of C-H and C-C bonds is frequently 0b~erved.l~The first transition series single-collision M+ reactions with linear alkanes at thermal energies can be summarized as follow^.^-'^^'^^^^ Sc’, Ti+, and (8) (a) Allison, J.; Freas, R. B.; Ridge, D. P. J . Am. Chem. SOC.1979,101, 1332. (b) Freas, R. B.; Ridge, D. P. J . Am. Chem. Sor. 1980,102, 7129. (c) Larsen, B. S.;Ridge, D. P. J. Am. Chem. SOC.1984,106, 1912. (d) Peake, D. A.; Gross, M. L.; Ridge, D. P. J. Am. Chem. Sor. 1984, 106, 4307. (9) (a) Armentrout, P. B.; Beauchamp, J. L. J. Am. Chem. Sor. 1981,103, 784. (b) Houriet, R.; Halle, L. F.; Beauchamp, J. L. Organometallics 1983, 2, 1818. (c) Mandich, M. L.; Halle, L. F.; Beauchamp, J. L. J . Am. Chem. SOC.1984, 106, 4403. (10) (a) Byrd, G. D.; Burnier, R. C.; Freiser, B. S. J. Am. Chem. Sor. 1982, 104, 3565. Byrd, G. D.; Freiser, B. S. J. Am. Chem. SOC.1982, 104, 5944. (b) Jacobson, D. B.; Freiser, B. S.J . Am. Chem. SOC.1983,105, 5197. ( c ) Uppal, J. S.; Staley, R. H. J . Am. Chem. SOC.1982, 104, 1235. (1 1) (a) Armentrout, P. B.; Beauchamp, J. L. J. Chem. Phys. 1981, 74, 2819. (b) Jacobson, D. B.; Freiser, B. S. J . Am. Chem. SOC.1983, 105, 7484. (12) (a) Jacobson, D. B.; Freiser, B. S.J. Am. Chem. SOC.1983, 105,7492. (b) Armentrout, P. B.; Beauchamp, J. L. J . Am. Chem. Sor. 1981, 103,6628. (13) Halle, L. F.; Houriet, R.; Kappes, M. M.; Staley, R. H.; Beauchamp, J. L. J. Am. Chem. Sor. 1982, 104, 6293. (14) Tolbert, M. A.; Beauchamp, J. L. J . Am. Chem. Sor. 1984,106,8117.
0 1986 American Chemical Society
2306 The Journal of Physical Chemistry, Vol. 90, No. 11, I 986
Letters
M+ SOURCE AND FAST FLOW R E A C T O R
?OOOl 500
LIF
w
rr
100
O' Z
50
w
20
5
10
2
DP,
Y
DP2
>
Figure 1. Schematic of fast flow reactor with excimer laser vaporization source of M'. Neutral reactants (XU)are added midstream. Ions are sampled downstream, mass analyzed in a quadrupole mass spectrometer (QMS), and detected by a Channeltron particle multiplier. A Roots blower (RB, 850 CFM) and 6-in. (DP,) and 4-in. (DP,)diffusion pumps maintain pressures of 0.5-1 Torr (flow tube), lo4 Torr (front chamber), Torr (QMS chamber). Ions or neutrals can be probed by and laser-induced fluorescence (LIF, detected by a photomultiplier tube, PMT) or by laser photodissociation (PD). Mean He flow speed is 7000 cm s-'. For the rate constant measurements of Figure 3, the target-CzH6 inlet and inlet-sampling orifice distances are 136 and 62 cm, respectively.
V+ are observed to dehydrogenate linear alkanes, while Fe', Co+, and Nit preferentially "demethanate" linear alkanes. Alkanes are inert to Cr', Mn', Cu', and Zn'. Examples of the observed single-collision reactions are
Ti+ Fe'
+ C2H,
+C H
-
TiC2H4++ H,
, FeC,H,' + H2 "FeC2H4'
+ CH,
-1
$
5 2 1
0
30
60
90
120 150
180
210
C2H, FLOW UNITS Figure 2. Semilog plot of mass-selected ion currents vs. ethane flow for the Ti' C,H6 reaction sequence. Reaction length is 39 cm. The majority species (stoichiometries)at m/e 48, 76, 7 8 , 106, 108, and 136
+
are Ti+, TiC2H4+,TiC2H6+,TiC4Hlo+,TiC4H12+,and TiC6HI6+,respectively. Solid lines are calculated fits from the reaction sequence of eq I , the rate constants of Table I, and a TiCzH6*/TiC2H4'branching were ratio of 1.20. Small contributions from minority isotopes &Tior included where appropriate.
(30%) (70%)
The observed products have been rationalized by an "oxidative addition" step (insertion of M+ into a C-H or C-C bond) followed by &hydrogen migration to the metal and "reductive elimination" of neutral H, or CH,. Initial C-H or C-C insertion would thus lead to elimination of H 2 or CH,, respectively. It is inferred that Ti+ inserts in C-H bonds, while Fe+ preferentially inserts in C-C bonds, an interesting proposition. An alternative possible path to CH, elimination involves initial C-H insertion followed by P-methyl migration to the metal.' We report our initial kinetics results using a new combination of techniques, namely pulsed laser vaporization of a metal target1*I0 to produce metal cations M' in a fast flow reactor15 at 1 Torr of He buffer gas. Such "high pressure" conditions illuminate important complementary aspects of the reactions with alkanes. First, in the multiple-collision environment of the flow tube, we observe sequential reaction steps leading from bare M+ to large organometallic "terminal ions" whose stoichiometries are intriguing. We can obtain reaction rate constants for not only M', but also certain ligated metal ions. At each reaction step, we observe a competition between H, or CH4 elimination (the single-collision channels) and collisional stabilization of long-lived reaction intermediates. Thus metal ions that are apparently inert under single-collision conditions can be quite reactive in our experiment. Finally, quantitative branching ratios between elimination products and adducts can yield estimates of the lifetime of the activated intermediate. Experimental Section Figure 1 shows a schematic of the fast flow reactor. Details of the technique will be published elsewhere.I6 M+ ions are created upstream by focusing 1.5-11s fwhm, 308-nm laser pulses (15) (a) Ferguson, E. E.; Fehsenfeld, F. C.; Schmeltekopf, A. L. Ado. At. Mol. Phys. 1969, 5, 1 , (b) Albritton, D.L. At. Data Nucl. Data Tables 1978, 22, 1. (c) Smith, D.; Adams, N.G. In Gas Phase Ion Chemistry, Vol. I, Bowers, M. T., Ed.; Academic Press: New York, 1979. (16) Tonkyn, R.; Weisshaar. J. C., manuscript in preparation.
(XeCI excimer laser, 10-30 mJ/pulse, 10-25 pulse/s) with a 30-cm lens onto a rotating metal disk (I-cm diameter, 0.7 rpm). Ions created upstream are sampled downstream through a 1-mm hole in a 0.25-mm-thick Mo disk, mass analyzed by a quadrupole filter, and detected by a Channeltron particle multiplier and boxcar integrator. Small flows of neutral reagents are added midstream through "showerhead" inlets. Ionic reaction products are followed by changes in the mass spectrum; neutral products are inferred. Reaction rate constants are measured by monitoring the change in mass-selected ion signal vs. neutral partial In contrast to results in high vacuum,18 the laser-produced plasma in our experiment is entrained in 1 Torr of He. The He helps to thermalize ion, neutral, and electron kinetic energies; greatly increases the fraction of ionized gas phase metal species; and tends to relax the M' electronic state distribution toward Boltzmann at 300 K. Preliminary laser-induced fluorescence measurements16 on Ti and Ti' indicate that, 30-cm downstream of the target, the Ti number density is roughly 10 times that of Ti+, the Ti' ions are almost entirely in the ground (a4F) electronic state, and the neutral Ti atoms are more highly excited. Thus at least in the case of Ti', we are certain that we are studying ground-state chemistry, a definite strength of the technique. The Channeltron detects 2-3-111s fwhm ion "packets" whose width is governed by a "space charge explosion" at the target and subsequent axial diffusion. Each laser pulse creates 10" metal ions, leading to peak ion densities > 2 X lo8 at the orifice (peak mass-selected currents > 0.5 nA). For Sc and Ti, we often observe M2' signals comparable to the M' signals at the detector. The semilog plots of M' decay vs. ethane flow are still linear, indicating that M2' + C,H6 M' C2H6+does not occur significantly, a surprising r e s ~ l t . ' ~Shot-to-shot peak current fluctuations are about &lo%. Electrons are the negatively charged species in the flow tube.
-
-
+
(1 7) For the Ar+ + O2reaction, we obtain a rate constant of (5.4 f 1.1) cm3 sd. Literaturevalues include (5.2 1.0) X lo-" cm3 s-l and (4.6 i 1.2) X lo-" cm3 s-'. See ref 1%. (18) (a) Ready, J. F. Effects of High Power Laser Radiation; Academic: New York, 1971. Kang, H.; Beauchamp, J. L. J . Phys. Chem. 1985,89,3364. (19) Tonkyn, R.; Weisshaar, J. C., manuscript in preparation. X 10"
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2301
Letters TABLE I: Effective Bimolecular Rate Constants for the Ti+ + C2H, Reaction Sequenceo at 1 Torr of He, 300 K
10-9
1
1
1 ,
1
1
1
1
1
1
1
0
1--1--1
I I I
rate constant, reactionb
I O-’O cm3 molecule-I s-’
kLlP
0.75 & 0.17
kl
15 3.9
2.7 f 0.5 6.3 f 1.3 130 7.8 >65
