ARe hydrogen bonds unique among weak interactions in their ability to mediate electronic coupling?

Superexchange effects on the electronic coupling element for electron transfer are investigated using water dimers and atomic donors and acceptors. We compare the electronic coupling elements obtained with H-bonded dimers to those obtained for other water dimer geometries at given donor-acceptor and oxygen-oxygen distances. The H-bonded orientation does not yield significantly different coupling elements from non-H-bonded orientations at a given oxygen-oxygen distance. In addition, the distance dependence of the coupling mediated by H-bonds is not significantly different from that for other dimer geometries. It is found that protonation of the intervening waters has a significant effect on coupling elements for donor/acceptor pairs, with low ionization potentials. The implications of these results are discussed for condensed-phase ground-and excited-state electron transfers.