Isolating the effects of the proton tunneling distance on protoncoupled electron transfer in a series of homologous tyrosine-base model compounds

The distance dependence of concerted proton-coupled electron transfer (PCET) reactions was probed in a series of three new compounds, where a phenol is covalently bridged by a 5, 6, or 7 membered carbocycle to the quinoline. The carbocycle bridge enforces the change in distance between the phenol oxygen (proton donor) and quinoline nitrogen (proton acceptor), d_O•••N, giving rise to values ranging from 2.567 to 2.8487 Å, and resulting in calculated proton tunneling distances, r0, that span 0.719 to 1.244 Å. Not only does this series significantly extend the range of distances that has been previously accessible for experimental distance dependent PCET studies of synthetic model compounds, but it also greatly improves the isolation of d_O•••N as a variable compared to earlier reports. Rates of PCET were determined by time-resolved optical spectroscopy with flash-quench generated [Ru(bpy)₃]³⁺ and [Ru(dce)₃]³⁺, where bpy = 2,2′-bipyridyl and dce = 4,4′-dicarboxyethylester-2,2′-bipyridyl. The rates increased as d_O•••N decreased, as can be expected from a static proton tunneling model. An exponential attenuation of the PCET rate constant was found: kPCET(d) = k_PCETexp[-β(d-d₀)], with β ∼ 10 Å-1. The observed kinetic isotope effect (KIE = k_H/k_D) ranged from 1.2 to 1.4, where the KIE was observed to decrease slightly with increasing d_O•••N. Both β and KIE values are significantly smaller than what is predicted by a static proton tunneling model. We conclude that vibrational compression of the tunneling distances, as well as higher vibronic transitions, that contribute to concerted proton coupled electron transfer must also be considered.