화학공학소재연구정보센터
Inorganic Chemistry, Vol.34, No.18, 4572-4580, 1995
Inner-Sphere Self-Exchange Electron-Transfer in Low-Spin Co(III)-Co(II) Couples - Rate Determinations from H-1-NMR Line Broadening and a Simple Vibronic Model of the Reaction Coordinate
Inner-sphere electron transfer self-exchange rate constants have been determined using H-1-NMR line broadening for several low-spin Co(III)-Co(II) couples of the type Co(MCL)(OH2)X(2+)-Co(MCL)(OH2)(2)(2+), where MCL = a tetraazamacrocycle coordinated equatorially and X = Cl, Br, or N-3. This work confirms previous inferences, based mostly on cross-reaction data, that (1) the inner-sphere rate constants are about 10(6) times larger than the equivalent outer-sphere rate constants and (2) variations of some of the inner-sphere rate constants (at least those with X = Cl) parallel differences in reactant and product molecular structure but (3) the inner-sphere rate constants are much less sensitive to structural variations than are the equivalent outer-sphere rate constants. The characteristically smaller inner-sphere nuclear reorganizational barrier can be attributed in part to correlation of the Co(III)-X(-) stretch and the X(-)-Co(II) compression, and this suggests that electron transfer occurs in concert with the motion of the bridging ligand. A simple vibronic model is proposed to accommodate this concerted motion of the bridging ligand and the very strong donor-acceptor coupling in these systems. This model suggests that the nuclear reorganizational parameters for inner-sphere cross-reactions will not be averages of those of the self-exchange reaction components if electron transfer is accompanied by large nuclear displacements and if the bridging ligand is comparable in mass to the donor and acceptor. Further implications of this model for strong vibronic coupling are the lack of Marcus-inverted region behavior and variations of inner-sphere self-exchange rate constants with the mass of the bridging ligand.