Pressure effects on the rate of electron transfer between tris(1,10-phenanthroline)iron(II) and -(III) in aqueous solution and in acetonitrile

Proton nmr line-broadening experiments at ambient and elevated (to 215 MPa) pressures show that the rate of electron transfer between Fe(phen)32+ and Fe(phen)33+ as bisulfates in D2O/D2SO4 is represented by the activation parameters (at ionic strength I ~ 0.4 mol kg−1) ΔH≠ = 1.6 ± 0.5 kJ mol−1, ΔS≠ = −102.2 ± 1.6 JK−1mol−1, k(276 K) = 1.31 × 107 kg mol−1s−1, and (at I ~ 0.3 mol kg−1 and a mean pressure of 100 MPa) ΔV≠ = −2.2 ± 0.1 cm3mol−1. For the same reaction of the perchlorate salts (total [Fe] 0.046–0.065 mol kg−1) in CD3CN, ΔH≠ = 11.0 ± 1.0 kJ mol−1, ΔS≠ = −72.5 ± 3.6 J K−1 mol−1, k(277 K) = 8.0 × 106 kgmol−1s−1, and ΔV≠ = −5.9 ± 0.5 cm3 mol−1. For water as solvent, ΔV≠ is satisfactorily accounted for by a classical theory of the Stranks–Hush–Marcus type. Volumes of activation for electron self-exchange are shown to provide criteria for non-adiabaticity and for dominance of (non-aqueous) solvent reorganization dynamics; on this basis, it is seen that neither of these factors is important in the title reactions.