Short-Time Water Caging and Elementary Prehydration Redox Reactions in Ionic Environments

Abstract

This article deals with the direct probing of water cages that assist two well-defined prehydration electron transfers (PHETs) with the reactive metal cation Cd2+. Early electron photodetachment processes are triggered by a two-photon UV excitation of aqueous halide ion Cl- (R = [H2O]/[CdCl2] = 110). Concomitant with an ultrafast Cd2+ reduction by IR p-like excited electron (J. Phys. Chem. A 1998, 102, 7795), a subpicosecond oxidoreduction reaction occurs in caged three-body complexes {Cl··e-··Cd2+}aq. Near-IR spectroscopic measurements give a PHET frequency of 1.38 ± 0.02 1012 s-1. This reduction reaction is 70 times faster than a diffusion-controlled bimolecular reaction between aqueous Cd2+ ions and fully hydrated electrons (s-state). Femtosecond spectroscopic data indicate that preexisting bridging water-molecule-bonded Cl-···Cd2+ pairs (SSIP-like configurations) assist efficient prehydration electron transfer. Because the 4s-like orbital radius of nascent {Cl··e-··Cd2+}aq configurations is larger tha...