Imaging Frontside and Backside Attack in Radical Ion-Molecule Reactive Scattering

We report on the reactive scattering of methyl iodide,CH3I, with atomic oxygen anions O - . This radical ion-molecule reaction can produce differentionic products depending on the angle of attack of the nucleophileO( - ) on the target molecule. We presentresults on the backside and frontside attack of O - on CH3I, which can lead to I - and IO - products, respectively.We combine crossed-beam velocity map imaging with quantum chemicalcalculations to unravel the chemical reaction dynamics. Energy-dependentscattering experiments in the range of 0.3-2.0 eV relativecollision energy revealed that three different reaction pathways canlead to I - products, making itthe predominant observed product. Backside attack occurs via a hydrogen-bondedcomplex with observed indirect, forward, and sideways scattered iodideproducts. Halide abstraction via frontside attack produces IO - , which mainly shows isotropic andbackward scattered products at low energies. IO - is observed to dissociate further to I - + O at a certain energy threshold and favors more direct dynamicsat higher collision energies.

Automated summary

We investigate the reactive scattering of methyl iodide (CH3I) with atomic oxygen anions (O-) and identify different ionic products depending on the attack angle. By combining crossed-beam velocity map imaging and quantum chemical calculations, we reveal the reaction dynamics. Our experiments show that three different reaction pathways can lead to the predominant product, I-. Backside attack involves a hydrogen-bonded complex and results in indirectly, forwards, and sideways scattered iodide products, while frontside attack produces IO- through halide abstraction, with mainly isotropic and backward scattered products at low energies. At a certain energy threshold, IO- dissociates further to form I- + O, showing more direct dynamics at higher collision energies.