Steric effects and vector correlations in reactions of K atoms with brute force oriented aromatic molecules

Abstract

In a crossed molecular beam experiment we have measured the angular and time-of-flight (TOF) distributions of the products formed in the reaction K+IR→KI+R, at collision energies of around 3 eV, where IR was systematically varied from iodobenzene to 2-, 3- and 4-iodotoluene. From these data we have deduced the double-differential reaction cross-section in the center-of-mass (CM) frame. All the reactions exhibit preferred sideways and backward scattering of products. Around 70% of the available energy is channeled into the internal degrees of freedom of the products. Orientation of the molecules has been achieved by application of the brute force technique. We have measured differential parallel and perpendicular steric effects over a wide range of scattering angles. The shape and magnitude of both steric effects indicate the existence of a strong vector correlation between the directions of the C–I bond and the recoil velocity. The extent of the correlation depends on the presence and the position of the methyl group on the phenyl ring: it is largest for iodobenzene and weakest for 2-iodotoluene. Also the head (I end) vs. tail (R end) reactive asymmetry depends on the methyl group: it ranges from 5:1 (for 2-iodotoluene) to 12:1 (for 4-iodotoluene). All the reactions proceed via a linear K–I–R transition state. The findings have been rationalized on the basis of simple dynamical models.