Calculation of electron-transfer matrix elements of bridged systems using a molecular fragment approach

A perturbation method for calculating the electronic coupling for electron-transfer reactions between a donor and an acceptor separated by large or small bridges is developed. In this approach the intervening bridge is subdivided into smaller molecular fragments, thereby enabling calculations on larger systems. This method of molecular fragments is tested for a series of polyproline bridged systems. The results obtained for the electron transfer matrix element are compared with those obtained from direct diagonalization of the full bridge and with experimental results. Previously, the result for the direct diagonalization of the bridge had been shown to agree with that obtained from diagonalization of the entire donor bridge-acceptor system. The vertical donor-bridge orbital energy difference is estimated with the aid of a donor bridge charge-transfer spectrum