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Driving force for proton transfer9/23/2023 In both cases, the vibronic coupling for vibrational states above the barrier was estimated as half the splitting between the third and fourth vibrational states for the fixed barrier. In this model, the red curves correspond to a fixed proton transfer barrier, and the blue curve corresponds to the increase of the barrier as a function of Δ G 0 with a slope of 0.6667, leading to an increase in the barrier height from 7 kcal/mol to 25 kcal/mol for the range of Δ G 0 shown here. The reorganization energy is λ = 8 kcal/mol for the two solid curves and λ = 3 kcal/mol for the dashed curve. In the electronically adiabatic PT model, ω = 3000 cm −1, and the proton transfer barrier frequency and height are 2500 cm −1 and 7 kcal/mol, respectively. Thus, a plausible explanation for experimentally observed inverted region behavior for PT or PCET is that varying the driving force also impacts other properties of the system, such as the proton donor-acceptor distance.ĭriving force dependence of the rate constant for (a) electronically adiabatic PT and (b) PCET models. Moreover, this behavior may be observed for PT or PCET if the proton donor-acceptor distance increases as DeltaG(0) becomes more negative. This behavior may be observed for PT over a limited range of rates and driving forces if the solvent reorganization energy is low enough to cause observable oscillations. As a result, inverted region behavior is predicted to be experimentally inaccessible for PT and PCET if only the driving force is varied. The driving force dependence of the rate constant is qualitatively different for PT and PCET than for ET because of the high proton vibrational frequency and substantial shift between the reactant and product proton vibrational wave functions. The objective of this Letter is to predict the experimental conditions that could lead to observation of inverted region behavior for PT and PCET. This behavior was predicted theoretically for ET but is not well understood for PT and PCET. Inverted region behavior, where the rate constant decreases as the reaction becomes more exoergic (i.e., as DeltaG(0) becomes more negative), has been observed experimentally for ET and PT. The driving force dependence of the rate constants for nonadiabatic electron transfer (ET), proton transfer (PT), and proton-coupled electron transfer (PCET) reactions is examined.
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