Subscriber access provided by RICE UNIV Effects of molecular orientation on electron-transfer collisions Peter W. Harland, Howard S. Carman Jr., Leon F. Phillips, and Philip R. Brooks J. Phys. Chem., 1991, 95 (21), 8137-8142 • DOI: 10.1021/j100174a023 Downloaded from http://pubs.acs.org on December 9, 2008 More About This Article The permalink http://dx.doi.org/10.1021/j100174a023 provides access to: • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article The Journal of Physical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 J. Phys. Chem. 1991,95,8137-8142 8137 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 (4eV) 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 equal, step 1 was assumed to be independent of orientation. The "Chemical intuition", as well as a growing amount of direct dependence on orientation was assumed to be in step 2 because experimental observation, tells us that chemical reaction depends the I- would be ejected in the known direction of the molecular on the orientation of the molecules involved. By "orientation" we dipole. After we accounted for the momentum of all the partners, mean a spatial configuration where one end of a molecule can this mechanism qualitatively reproduced the experimental angular be distinguished from another. Although reactive species dif- distribution, not only for heads and tails orientations, but also for ferentiate between ends by not reacting with the "wrong" end, sideways orientations.' we have little experimental guidance for what constitutes the For the K + CF3Br reaction the angular distributions are in 'right" end because most experiments include all possible orien- qualitative accord with this harpoon model, but the reactive tations during collision and isolation of orientation effects has not probability is less at the tails (CF,) end, leading us to speculate8 been possible. that the electron-transfer process might be orientation dependent. Several molecular beam techniques have arisen to prepare Electron-transfer processes are important to many different reagent molecules in known orientations.' The simplest (and most kinds of reactions as well as to the harpoon reaction, and we have recent) is the "brute force" method of merely applying a strong begun to investigate how orientation might affect electron transfer. electric field on extremely cold polar molecules,2 but this method We have now studied several reactions, including K + CF3X (X is highly re~trictive.~Most experiments so far have been done = I, Br), at energies of a few (5-30) eV, so that the ions move with symmetric top molecules, such as CH3Br. The orientation fast enough that they can escape from their Coulomb attraction is defined by a weak electric field because these molecules precess and be detected directly. Collisional ionization has been studied in an applied electric field in the same way a child's top precesses in some detail previously9 (for unoriented molecules) and the in a gravitational field. Each molecule is oriented. Even though products of theselo fast-atom collisions are predominantly K+,CF,, all orientations are present in a beam, passing the beam through and X-. In the experiments reported here, we orient symmetric an inhomogeneous electric field filters out molecules in orientations top molecules prior to collision, and we detect the K+ ion. such that (cos 8) > 0, where 8 is the angle between the dipole moment and electric field. The molecules passed by the inho- (1) For reviews see: (a) Brooks, P. R. Science 1976,193, 11. (b) Bern- stein, R. B.; Herschbach, D. R.; Levine, R. D. J. Phys. Chem. 1987,91,5365. mogeneous field have (cos8) < 0, and are reacted with an atomic (c) Parker, D. H.; Bernstein, R. B. Annu. Rev. Phys. Chem. 1989, 40, 561. beam in a weak uniform electric field. The weak field is applied (d) Stolte, S. In Atomic and Molecular Beam Methods; Scoles, G., Ed.; parallel or antiparallel to the relative velocity and determines which Oxford: New York, 1988; Vol. 1, p 631. end of the molecule is presented to the atoms. (2) (a) Loescb, H. J.; Remscheid, A. J. Chem. Phys. 1990,93,4779. (b) Friedrich, B.; Herschbach, D. R. 2. Phys., in press. Orientation effects are large and varied. For example, at (3) This method can be used if E, << MEand requires either very cold thermal energies, CHJ reacts with K or (Rb4) preferentially on molecules or very high fields, but it is the only method available for orienting the I end, but CF31and CF,Br react at both ends, with different 2-state molecules. For these 2 molecules, various optical methods have bctn angular distributions. The "harpoon" modelS of electron transfer used to produce polarization, where the plane of rotation is selected but one end is not distinguished from the other (described in ref 1 b). has been invoked6 to describe the CF31reaction, and this nicely (4) Chakravorty, K. K.; Parker, D. H.; Bernstein, R. B. Chem. Phys. 1982, explains the different angular distributions observed in the "heads" 68, 1. These latter experiments have been reinterpreted in terms of a steric and "tails" orientations. In this model, the K is expected to donate cone of nonreaction of 43O: Choi, S. E.; Bernstein, R. B. J. Chem. Phys. an electron (the harpoon) to the CF,I, followed by explosive 1985, 83, 4463. (5) Herschbach, D. R. Adv. Chem. Phys. 1966, IO, 319. decomposition of the CFJ-, after which the Coulomb attraction (6) Brooks, P. R. Faraday Discuss. Chem. Soc. 1973,55, 299. causes the K+ and I- to combine: (7) Brooks, P. R.; McKillop, J.; Pippen, H. G. Chem. Phys. Lett. 1979,615, 144. K + CFJ --* K+ + CFJ electron transfer (1) (8) Carman, H. S.;Harland, P. W.; Brooks, P. R. J. Phys. Chem. 1986, 90,944. CF31- - CF3 + I- ion dissociation (2) (9) For reviews of collisional ionization, see: (a) Kleyn, A. W.; Los, J.; Gislason, E. A. Phys. Rep. 1982.90, 1. (b) Lacmann, K. I. Potential Energy Surfaces; Lawley, K. P.; Ed.; Wiley: New York, 1980, p 513. (c) Bade, A. I- K+ + ionic recombination (3 1 + KI P. M. Adv. Chem. Phys. 1975, 30, 463. (d) Los, J.; Kleyn, A. W. Alkali Because the cross section for reaction at the two ends was roughly Halide Vapors; P. Davidovits, P., McFadden, D. L., Eds.; Academic Press: New York, 1979; p 279. (10) For collision with K: (a) McNamee, P. E.; Lacman, K.; Herschbach, 'Permanent address: Chemistry Department, University of Canterbury, D. R. Faraday Discuss. Chem. Soc. 1973,55,318. (b) Kleyn, A. W.; Hubers, Christchurch. New Zealand. M. N.; La, J. 10th ICPEAC Abstracts of Papers, Paris, 1977; p 1162. For 'Permanent address: Chemical Physics Section, Health & Safety Research collision with other alkali metals: (c) Compton, R. N.; Reinhardt, P. W.; Division, Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN Cooper, C. D. J. Chem. Phys. 1978,68,4360. (d) Tang, S. Y.; Mathur, B. 3783 I. P.; Rothe, E. W.; Reck, G. P. J. Chem. Phys. 1976, 64, 1270. 0022-3654191 12095-8 131302.50/0 0 1991 American Chemical Society 8138 The Journal of Physical Chemistry, Vol. 95, No. 21, 1991 Harland et al. Preliminary results have been obtained for CH$ and CF31.11*'2 io00 L I I I I Here we report on a survey of related molecules, CH3X (X = F, CI, Br, and I); CF,Y (Y = C1, Br, and I); t-butyl Z (Z = C1, Br); and CX3H (X = C1, F). We find in all cam a strong dependence on orientation at energies near threshold, and this effect decreases as the energy is increased. Regardless of the polarity of the dipole, the end producing the most ions is the "heads" end (the end with the most labile halogen). These results may stll be understood in terms of the orientation of the dissociating molecular ion. 11. Experimental Section The experimental apparatus and procedure for data collection 5 10 5 10 5 10 have been previously discussed in paper I (ref 12), and we provide E (eV) here only a brief summary of the experiment.