Abstract
A nearside/farside analysis of differential cross sections has been performed for the complex-forming SN2 reaction Cl(-) + CH3Br → ClCH3 + Br(-). It is shown that for low rotational quantum numbers a direct "nearside" reaction mechanism plays an important role and leads to anisotropic differential cross sections. For high rotational quantum numbers, indirect mechanisms via a long-lived intermediate complex are prevalent (independent of a nearside/farside configuration), leading to isotropic cross sections. Quantum mechanical interference can be significant at specific energies or angles. Averaging over energies and angles reveals that the nearside/farside decomposition in a semiclassical interpretation can reasonably account for the analysis of the reaction mechanism.
Highlights
Ion–molecule reactions, studied experimentally employing crossed-beam imaging,[15] an approach widely used for reactions between neutral species,[3,16] are employed for the investigation of reactions between neutrals and anions, based on the technique of velocity map imaging.[17]
We report on a nearside/farside analysis in the prototypical substitution reaction ClÀ + CH3Br - ClCH3 + BrÀ
The nearside/farside analysis reveals some structure in the differential cross sections with a slight preference of small scattering angles
Summary
State-resolved differential cross sections for chemical reactions, containing information on angular distributions of the scattered products and describing a reactive process at a very detailed level, provide the most stringent test for a quantitatively accurate theoretical model for elementary chemical reactions.[1,2,3,4,5,6] Only a few reactive systems could be studied with quantum-state resolution in scattering experiments,[7,8,9,10,11,12,13,14] where the flux of the reaction products is measured that go into different scattering angles and final ro-vibrational states for different initial levels. On the other hand, a relatively longlived resonance state is produced, corresponding to an intermolecular complex that is bound by long-range ion–dipole forces This complex carries out rotations before dissociating back into the reactants or forming the products. Backward scattering is disfavoured for reaction out of states with small rotational excitation, in particular the rovibrational ground state This is due to a quantum-mechanical effect, the interference of partial waves, that in general can be rationalized by simple classical arguments.
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