Application of the sequential formula: the electronic coupling and the distance dependence in the electron transfer of ferrocene-terminated alkanethiol monolayers

A sequential formula developed by Hsu and Marcus [J. Chem. Phys., 106 (1997) 584] is applied to calculate the electronic coupling between a gold electrode and each of the molecules (eta(5)-C5H5)Fe(eta(5)-C5H4)CO2(CH2)(n)SH and (eta(5)-C5H5)Fe(eta(5)-C5H4)(CH2)(n)SH (n = 3 to 50). Most of the calculated coupling strengths, if converted to rate constants according to a high-temperature non-adiabatic expression, agree well with the experimental rate constants. The exponential coefficients are obtained by a linear fit of the logarithm of the coupling strengths to the chain length. The calculated exponential coefficient is 1.05 per methylene unit for both molecules and for large n. For shorter chains the exponential coefficient is slightly larger with an irregularity caused by an oscillation of coupling strengths between even and odd numbers of units. The physical origin of such oscillation among short chains is discussed. (C) 1997 Elsevier Science S.A.