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) 3 2+ and Fe(phen) 3 3+ as bisulfates in D 2 O/D 2 SO 4 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 −1 mol −1 , k(276 K) = 1.31 × 10 7 kg mol −1 s −1 , and (at I ~ 0.3 mol kg −1 and a mean pressure of 100 MPa) ΔV ≠ = −2.2 ± 0.1 cm 3 mol −1 . For the same reaction of the perchlorate salts (total [Fe] 0.046-0.065 mol kg −1 ) in CD 3 CN, ΔH ≠ = 11.0 ± 1.0 kJ mol −1 , ΔS ≠ = −72.5 ± 3.6 J K −1 mol −1 , k(277 K) = 8.0 × 10 6 kgmol −1 s −1 , and ΔV ≠ = −5.9 ± 0.5 cm 3 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.