Investigation of through-bond coupling dependence on spacer structure

Intramolecular electron transfer rate constants in solution are measured for radical anions and cations of molecules having biphenyl and naphthalene groups as donor and acceptor. The molecules have one or more five-bond hydrocarbon chains between the donor and acceptor groups in trans, gauche, and cis conformations. The rates suggest that one trans chain is as effective as two gauche chains or three cis chains, consistent with Hoffmann's prediction of more effective coupling through trans hydrocarbon chains. Ab initio calculations of the couplings are in reasonable agreement with the experiments. Calculations of electronic coupling pathways provide insight into the reason for the superiority of the trans conformation, which gives constructive interference between the two largest pathways. The calculations also show that ''cross talk'' can enhance couplings in cyclic spacers in contrast to its deleterious effect in norbornyl spacers. Comparisons of couplings through these spacers with simple hydrocarbon chains require that large nonbonded interactions be taken into account.