Reactive scattering from brute force oriented molecules: K+IR -> KI+R (R =i-C3H7 and t-C4H9)

We report results of a crossed molecular beam study on the reactions K + i- C3H7I --> KI + i-C3H7 (R1) and K + t-C4H9I --> KI + t-C4H9 (R2) performed a t an elevated collision energy of 1.55 eV for both nonoriented and oriented reagent molecules. Orientation was achieved by using the brute force techn ique. The most important results are the following: (i) The flux of scatter ed products of R2 consists of a dominant fast and a minor slow component; t he two reaction channels occur with a branching ratio of 100:2. In R1 the f ast component only has been observed. (ii) In the center-of-mass frame the dominant component is preferentially scattered into the backward hemisphere with a propensity for sideways and backward scattering while the minor one is sharply forward scattered and travels on the average with the spectator stripping velocity. (iii) The parallel and perpendicular differential ster ic effects in both R1 and the dominant channel of R2 are very substantial a nd amount to a size close to the theoretical upper boundary. The parallel s teric effect in the minor channel of R2 is rather weak, and the sign is lik ely to be opposite to the one of the dominant channel. (iv) From the differ ential steric effects we have deduced the moments J(10) and J(11) of an exp ansion of the orientation-dependent double-differential cross section in a series of real spherical harmonics. (v) Shape and magnitude of the moments are consistent with a tight vector correlation between the directions of th e main product flux and the molecular principal a-axis. (vi) The steric opa city functions for R1 and the dominant channel of R2 indicate that attacks of the K atoms to the I end of the reagent molecules are favorable for the formation of the fast products. The favored end for the production of the m inor component of R2 is Likely to be the alkyl group. (vii) The emergence o f the minor slow component in R2 has been qualitatively rationalized on the basis of the harpooning mechanism and electronic structure arguments. The model identifies the slow products as KI and electronically excited t-C4H9.