Ionic and hydrogen bonding dynamics coupled to optical and thermal electron transfer: temperature, pressure and viscosity effects

Abstract

Abstract Electron transfer processes, along with the coupling to the solvent continuum, can also be essentially coupled to, and controlled by the ionic and specific hydrogen bonding dynamics, occuring within the vicinity of the reaction zone. The hexacyanoferrate species, Fe(CN) 6 4−/3− , and related cyanometalate couples lend credence to their consideration as model redox systems suitable for studying the role of such medium dynamics. Recent experimental findings for the temperature, pressure and solvent composition control of three physically related paths for electron transfer (thermal homogeneous, thermal electrochemical, and optically induced homogeneous) detected for this system, allow for the discrimination of microscopic mechanisms responsible for different catalytic effects of electrolyte cations and the solvent control effect of added dextrose. It is argued that for an adequate description of the overall catalytic effect of electrolyte cations, three different factors, viz., the preequilibrium effect of ionic atmosphere, the dynamic effect of stoichiometric ion reorganization, and the superexchange effect of a bridging counter ion should be taken into consideration. The latter two effects may also be “inhibiting”, depending on the composition and geometry of the reactive ionic associate in the precursor and transition states, and the symmetry requirements for the latter. Variation of these three factors in the cases of three different electron exchange paths, leads to the observed interplay of kinetic constants and activation parameters (as well as the CT-band parameters). Participation of the specifically hydrogen bonded water in the first solvation shell of cyanometalate ions, detectable through the differential O-D overtone spectroscopy of heavy water, may be responsible for other peculiarities of the considered redox couple, viz., the solvent isotope effects detected for the homogeneous thermal kinetic constant, the CT-band maximum shift for the optically induced process, and the solvent friction mechanism for the thermal electrochemical process.