Isolating the Effects of the Proton Tunneling Distance on Proton-Coupled Electron Transfer in a Series of Homologous Tyrosine-Base Model Compounds

Abstract

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, giving rise to values ranging from 2.567 to 2.8487 Å, and resulting in calculated proton tunneling distances, r, 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 as a variable compared to earlier reports. Rates of PCET were determined by time-resolved optical spectroscopy with flash-quench generated [Ru(bpy)] and [Ru(dce)], where bpy = 2,2'-bipyridyl and dce = 4,4'-dicarboxyethylester-2,2'-bipyridyl. The rates increased as d decreased, as can be expected from a static proton tunneling model. An exponential attenuation of the PCET rate constant was found: k(d) = kexp[-β(d - d)], with β ∼ 10 Å. The observed kinetic isotope effect (KIE = k/k) ranged from 1.2 to 1.4, where the KIE was observed to decrease slightly with increasing d. 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.

On the Structure of [K(crypt‐222)]+

“…Computationally moving the proton within the F 3 C···H‐C( β ‐crypt) moiety toward the gave rise to F 3 C‐H···C( β ‐crypt), which may be viewed as one of the three possible isomers of 2 . The C···C separations and the calculated proton transfer distances r 0 = 2.28 and 2.38 Å seem too long for the proton to tunnel efficiently . Unlike electron tunneling that can span tens of Å, tunneling of much heavier protons is very sensitive to distance and is typically observed for r 0 ≤ 1 Å .…”

“…[60 -63] Unlike electron tunneling that can span tens of A, tunneling of much heavier protons is very sensitive to distance and is typically observed for r 0 ≤ 1 A. [63] Although proton tunneling within 1 to give 2 may not be completely ruled out even for the F 3 CÁÁÁH-C(crypt) separations exceeding 3.5 A and r 0 > 2.2 A, this process is highly endergonic. The computed energetics 18 suggest that like in THF (Scheme 4), [15] [16] the proposed equilibrium between 1 and 2 in the solid is shifted entirely to 1.…”

Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions

“…Many laboratories, including ours, have studied two-component systems in which intermolecular electron transfer is coupled to an intramolecular proton transfer across a hydrogen bond to a covalently tethered base. 15-23 The covalent tether promotes formation of the hydrogen bond and can probe the effects of different hydrogen bond distances. 18,19,23 …”

“…15-23 The covalent tether promotes formation of the hydrogen bond and can probe the effects of different hydrogen bond distances. 18,19,23 …”

Catalytic‐Type Excited‐State N−H Proton‐Transfer Reaction in 7‐Aminoquinoline and Its Derivatives

“…In this study, for generalization, we lift the boundary of the unimolecular‐type ESIPT and extend the research to systems in which the proton donor (‐NHR) and acceptor are greatly separated, precluding the formation of an intramolecular H‐bond. Therefore, the occurrence of excited‐state proton transfer (ESPT), if available, requires the assistance of surrounding solvent molecules .…”