8137
J. Phys. Chem. 1991,95,8137-8142
Effects of Molecular Orlentatlon on Electron-Transfer Colllslons Peter W.Harland,' Howard S,Carman, Jr.,* Leon F. Phillips: and Philip R. Brooks* Department of Chemistry & Rice Quantum Institute, Rice University, Houston, Texas 77251 (Received: February 12. 1991)
K+ ions have been detected from the intersection of a beam of K atoms (5-30 eV) with beams of various simple molecules, such as CH3Br and CF,Br, which had been oriented prior to the collision. Production of ions in the collision is found to be highly dependent on orientation. The effect is most pronounced near threshold (4 eV) and almost disappears at higher (30 eV) energies. Attack at the "reactive' halogen end produces the most ions, regardless of the polarity of that end. For each molecule, the reactive end seems to have the lower threshold energy. These observations may be a result of the electron being transferred to a specific end of the molecule, but the experiments measure only the net result of an electron transfer followed by the separation of the ions. Whether or not electron jump per se depends on orientation is still an open question, but we are able to qualitatively interpret the experimental results as being due to interactions between the ions as they separate in the exit channel. Most of the negative molecular ions dissociate, ejecting a halogen X-in the direction of the (oriented) molecular axis. If the X end is oriented away from the incoming K atom, the ejected X-will travel in the same direction as the K+,making the electron more likely to return to the K+ ion and reducing the K+ signal in this unfavorable orientation. I. Introduction "Chemical intuition", as well as a growing amount of direct experimental observation, tells us that chemical reaction depends on the orientation of the molecules involved. By "orientation" we mean a spatial configuration where one end of a molecule can be distinguished from another. Although reactive species differentiate between ends by not reacting with the "wrong" end, we have little experimental guidance for what constitutes the 'right" end because most experiments include all possible orientations during collision and isolation of orientation effects has not been possible. Several molecular beam techniques have arisen to prepare reagent molecules in known orientations.' The simplest (and most recent) is the "brute force" method of merely applying a strong electric field on extremely cold polar molecules,2 but this method is highly re~trictive.~Most experiments so far have been done with symmetric top molecules, such as CH3Br. The orientation is defined by a weak electric field because these molecules precess in an applied electric field in the same way a child's top precesses in a gravitational field. Each molecule is oriented. Even though all orientations are present in a beam, passing the beam through an inhomogeneous electric field filters out molecules in orientations such that (cos 8) > 0, where 8 is the angle between the dipole moment and electric field. The molecules passed by the inhomogeneous field have (cos8) < 0, and are reacted with an atomic beam in a weak uniform electric field. The weak field is applied parallel or antiparallel to the relative velocity and determines which end of the molecule is presented to the atoms. Orientation effects are large and varied. For example, at thermal energies, CHJ reacts with K or (Rb4) preferentially on the I end, but CF31and CF,Br react at both ends, with different angular distributions. The "harpoon" modelS of electron transfer has been invoked6 to describe the CF31reaction, and this nicely explains the different angular distributions observed in the "heads" and "tails" orientations. In this model, the K is expected to donate an electron (the harpoon) to the CF,I, followed by explosive decomposition of the CFJ-, after which the Coulomb attraction causes the K+ and I- to combine:
K
+ CFJ CF31-
--*
K+ + CFJ CF3 + I-
electron transfer ion dissociation
(1) (2)
I- + K+ KI ionic recombination (3 1 Because the cross section for reaction at the two ends was roughly +
'Permanent address: Chemistry Department, University of Canterbury, Christchurch. New Zealand. 'Permanent address: Chemical Physics Section, Health & Safety Research Division, Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 3783 I .
0022-3654191 12095-8 131302.50/0
equal, step 1 was assumed to be independent of orientation. The dependence on orientation was assumed to be in step 2 because the I- would be ejected in the known direction of the molecular dipole. After we accounted for the momentum of all the partners, this mechanism qualitatively reproduced the experimental angular distribution, not only for heads and tails orientations, but also for sideways orientations.' For the K CF3Br reaction the angular distributions are in qualitative accord with this harpoon model, but the reactive probability is less at the tails (CF,) end, leading us to speculate8 that the electron-transfer process might be orientation dependent. Electron-transfer processes are important to many different kinds of reactions as well as to the harpoon reaction, and we have begun to investigate how orientation might affect electron transfer. We have now studied several reactions, including K CF3X (X = I, Br), a t energies of a few (5-30) eV, so that the ions move fast enough that they can escape from their Coulomb attraction and be detected directly. Collisional ionization has been studied in some detail previously9 (for unoriented molecules) and the products of theselo fast-atom collisions are predominantly K+,CF,, and X-.In the experiments reported here, we orient symmetric top molecules prior to collision, and we detect the K+ ion.
+
+
(1) For reviews see: (a) Brooks, P. R. Science 1976,193, 11. (b) Bernstein, R. B.; Herschbach, D. R.; Levine, R. D. J. Phys. Chem. 1987,91,5365. (c) Parker, D. H.; Bernstein, R. B. Annu. Rev. Phys. Chem. 1989, 40, 561. (d) Stolte, S. In Atomic and Molecular Beam Methods; Scoles, G., Ed.; Oxford: New York, 1988; Vol. 1, p 631. (2) (a) Loescb, H. J.; Remscheid, A. J . Chem. Phys. 1990,93,4779. (b) Friedrich, B.; Herschbach, D. R. 2.Phys., in press. (3) This method can be used if E ,
