Pressure effects and solvent dynamics in the electrochemical kinetics of the tris(hexafluoroacetylacetonato)ruthenium(III)/(II) couple in nonaqueous solvents

Rate constants and reactant diffusion coefficients for the Ru(hfac)(3)(0/-) electrode reaction have been measured at 25 degreesC as functions of press ure (0-200 MPa) in acetone, acetonitrile, methanol, and propylene carbonate . In sharp contrast to the negative volumes of activation DeltaV(ex)(double dagger) found for the corresponding bimolecular self-exchange reaction in organic solvents, the volumes of activation DeltaV(el)(double dagger) for t he electrode reaction are markedly positive, ranging from 8 to 12 cm(3) mol (-1). The volumes of activation DeltaV(diff)(double dagger) for reactant di ffusion (which can be equated to the volume of activation DeltaV(visc)(doub le dagger) for viscous flow) range from 12 to 19 cm(3) mol(-1). For the Deb ye solvents acetonitrile and acetone at least, DeltaV(el)(double dagger) is given within the experimental uncertainty by DeltaV(diff)(double dagger) (deltaV(ex)(double dagger)/2). In this relation, the numerical value of De ltaV(diff)(double dagger) represents indirectly the dominant contribution o f solvent dynamics (solvent friction) to DeltaV(el)(double dagger), and Del taV(ex)(double dagger)/2 represents the pressure dependence of the free-ene rgy barrier height for the electrode reaction. It is proposed that solvent friction is important in nonaqueous electrode processes but not in the corr esponding bimolecular self-exchange reactions because the free-energy activ ation barrier is twice as high in the latter.