Nature of Excess Electrons in Polar Fluids: Anion-Solvated Electron Equilibrium and Polarized Hole-Burning in Liquid Acetonitrile.

Unlike most polar liquids, excess electrons in liquid CH3CN take on two distinct forms − solvated electrons (esolv − ) and solvated molecular anions − that are in equilibrium with each other. We find that excitation of esolv − leads to a short-lived excited state that has no effect on the equilibrium but that excitation of the molecular anion instantaneously leads to the production of new esolv − . We also find that esolv − produced by excitation of dimer anions relocalize to places far enough from their original location to alter their recombination dynamics. Finally, we show using polarized transient holeburning that esolv − in liquid CH3CN have an inhomogeneously broadened spectrum, demonstrating that these electrons almost certainly reside in a cavity. Because there is no polarized hole-burning for esolv − in water or methanol, these results have important implications for the nature of excess electrons in all polar liquids. SECTION: Liquids; Chemical and Dynamical Processes in Solution W happens when an extra electron − one more than is needed to ensure electrical neutrality − is injected into a typical polar liquid? It is easy to imagine two possible fates for such an electron. One possibility is that the excess electron is ‘captured’ by one or more solvent molecules, producing a solvated molecular anion or multimer anion. Even though the solvent molecules in most liquids are closed-shell and will not bind an extra electron in isolation, extra electrons can bind to multiple solvent molecules, and in a liquid environment, the free energy of solvation makes it possible for solvated anions to be stable in solution. The second possibility is that excess electrons are expelled by the closed-shell solvent molecules, creating solvated electrons (esolv − ) that effectively reside between the solvent molecules. Understanding the detailed nature of how excess electrons behave in polar liquids is important because these species play a fundamental role in radiation chemistry and charge-transfer reactions. Liquid acetonitrile (CH3CN), a polar solvent with a dielectric constant and solvation properties similar to those of methanol, provides one of the more interesting environments for excess electrons. Although straight CH3CN molecules will not bind an excess electron, the electron affinity of acetonitrile increases as the molecule bends. The result is that an excess electron can form a covalent bond between the cyano carbons of two bent, antiparallel acetonitrile molecules to make a (CH3CN)2 − molecular dimer anion. Thus, when excess electrons are introduced into liquid CH3CN (or become part of gas-phase (CH3CN)n − clusters,) two species are formed, one which has a weak visible absorption spectrum (high binding energy in clusters) and the other which absorbs more intensely in the near-IR (and has a weak binding energy in clusters); the spectra of these species in liquid CH3CN are shown in Figure 1. We and others have investigated the nature of these two excess electron species in liquid acetonitrile using ultrafast spectroscopy and concluded that the visible absorption results from the solvated (CH3CN)2 − molecular dimer anion and that the near-IR band arises from a solvated electron. These two excess electron species are in equilibrium with each other, with Received: March 20, 2013 Accepted: April 12, 2013 Figure 1. UV−visible absorption spectra of the two excess electron species in liquid CH3CN, as measured in ref 5. The blue dashed curve shows the spectrum of the visible-absorbing species, assigned to the solvated dimer anion, and the red curve shows the spectrum of the IRabsorbing species, assigned to the solvated electron. The inset shows the visible portion of the spectra on an expanded scale with the same units as the main figure. Letter