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 10^12 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 than the mean distance of Cd2+−Cl- ion pairs in a first coordination sphere of Cd2+ ions (2.6 Å), it is suggested that an overlap between a 4s electron orbital and the localized Cd2+ orbital favors an early inner-sphere electron transfer. For the first time, a nonlinear relationship is defined between the rate of Cd2+ univalent reduction and the energy level of the trapped electron (IR e-p, {Cl··e-··Cd2+}aq, e-aq). We conclude that the short-time water cagings govern the course of PHET events and influence early branchings between elementary oxidoreduction reactions in ionic environments.
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Submitted on : Tuesday, March 15, 2011 - 12:25:26 PM
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Yann Gauduel, A. Hallou, B. Charles. Short-Time Water Caging and Elementary Prehydration Redox Reactions in Ionic Environments. Journal of Physical Chemistry A, American Chemical Society, 2003, 107 (12), pp.2011-2024. ⟨10.1021/jp021745p⟩. ⟨hal-00576732⟩

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