Electron trapping by polar molecules in alkane liquids: cluster chemistry in dilute solution

Experimental observations are presented on condensed-phase analogues of gas-phase dipole-bound anions and negatively charged clusters of polar molecules. Both monomers and small clusters of such molecules can reversibly trap conduction band electrons in dilute alkane solutions. The dynamics and energetics of this trapping have been studied using pulse radiolysis-transient absorption spectroscopy and time-resolved photoconductivity. Binding energies, thermal detrapping rates, and absorption spectra of excess electrons attached to monomer and multimer solute traps are obtained, and possible structures for these species are discussed. "Dipole coagulation" (stepwise growth of the solute cluster around the cavity electron) predicted by Mozumder in 1972 is observed. The acetonitrile monomer is shown to solvate the electron by its methyl group, just as the alkane solvent does. The electron is dipole-bound to the CN group; the latter points away from the cavity. The resulting negatively charged species has a binding energy of 0.4 eV and absorbs in the infrared. Molecules of straight-chain aliphatic alcohols solvate the excess electron by their OH groups; at equilibrium, the predominant electron trap is a trimer or a tetramer, and the binding energy of this solute trap is ca. 0.8 eV. Trapping by smaller clusters is opposed by the entropy that drives the equilibrium toward the electron in a solvent trap. For alcohol monomers, the trapping does not occur; a slow proton-transfer reaction occurs instead. For the acetonitrile monomer, the trapping is favored energetically, but the thermal detachment is rapid (ca. 1 ns). Our study suggests that a composite cluster anion consisting of a few polar molecules imbedded in an alkane "matrix" might be the closest gas-phase analogue to the core of solvated electron in a neat polar liquid.