Matthew Thompson

Solvent-Mediated Electron Hopping: Long-Range Charge Transfer in IBr−(CO2) Photodissociation

CO2 Lends a Hand Solvent plays a complex and multifaceted role in facilitating charge transfer events. One obstacle to understanding its influence is that solvent molecules are in constant motion; just teasing out their arrangement in space at the point in time when an electron hops from one substrate to another is often a great challenge. Sheps et al. (p. 220; published online 4 March) have studied a highly simplified prototype system, in which a single CO2 molecule coordinates, as a solvent might, to an IBr− ion in the gas phase. A combination of ultrafast photoelectron spectroscopy and theoretical simulations was applied that suggests that even this solitary interaction is sufficient to induce electron transfer from iodide to bromine during a dissociation reaction. Energy channeled through CO2-bending vibrations promoted formation of I(CO2) and Br−. The presence of an intervening carbondioxide molecule dramatically changes the electron transfer probability between two halogen atoms. Chemical bond breaking involves coupled electronic and nuclear dynamics that can take place on multiple electronic surfaces. Here we report a time-resolved experimental and theoretical investigation of nonadiabatic dynamics during photodissociation of a complex of iodine monobromide anion with carbon dioxide [IBr–(CO2)] on the second excited (A′) electronic state. Previous experimental work showed that the dissociation of bare IBr– yields only I– + Br products. However, in IBr–(CO2), time-resolved photoelectron spectroscopy reveals that a subset of the dissociating molecules undergoes an electron transfer from iodine to bromine 350 femtoseconds after the initial excitation. Ab initio calculations and molecular dynamics simulations elucidate the mechanism for this charge hop and highlight the crucial role of the carbon dioxide molecule. The charge transfer between two recoiling atoms, assisted by a single solvent-like molecule, provides a notable limiting case of solvent-driven electron transfer over a distance of 7 angstroms.