Cd. Clark et Mz. Hoffman, TEMPERATURE AND SPECIFIC ANION EFFECTS IN PHOTOINDUCED INTERMOLECULARELECTRON-TRANSFER REACTIONS, Proceedings of the Indian Academy of Sciences. Chemical sciences, 107(6), 1995, pp. 769-777
The rate constants for the bimolecular oxidative quenching of Ru(bpy)
(3)(2+) by methylviologen (k(q)) and the electron-transfer recombinati
on (k(rec)) reaction of the redox products were determined as a functi
on of the ionic strength and temperature in aqueous solution. At 25 de
grees C for Na+ salts at constant mu, k(q) and k(rec) are in the order
H-2 PO4- < CH3CO2- similar to HPO42- similar to SO42- much less than
ClO4-; activation energies are independent of mu and the anion (simila
r to 16 kJ/mol), except for ClO4- where E(a) is similar to 30% lower.
The trends in k(q) and k(rec) are attributed to the existence of ion-p
aired aggregates of the cationic reactants and anions; correlations wi
th the sizes and hydration enthalpies of the anions can be made. It is
suggested that ClO4-, with the smallest hydration radius and the leas
t negative hydration enthalpy, is more closely ion-paired within the R
u(bpy)(3)(2+) (or Ru(bpy)(3)(3+)) interligand pockets, ''tethering'' t
he MV(2+) (or MV(.+)) into closer proximity, thereby lowering the acti
vation barrier and increasing the rate of electron transfer.