A Theory−Experiment Conundrum for Proton Transfer

Abstract

For the past 60 years, the framework for understanding the kinetic behavior of proton transfer has been transition state theory. Found throughout textbooks, this theory, along with the Bell tunneling correction, serves as the standard model for the analysis of proton/hydrogen atom/hydride transfer. In comparison, a different theoretical model has recently emerged, one which proposes that the transition state occurs within the solvent coordinate, not the proton transfer coordinate, and proton transfer proceeds either adiabatically or nonadiabatically toward product formation. This Account discusses the central tenets of the new theoretical model of proton transfer, contrasts these with the standard transition state model, and presents a discrepancy that has arisen between our experimental studies on a nonadiabatic system and the current understanding of proton transfer. Transition state theory posits that in the proton transfer coordinate, the proton must surmount an electronic barrier prior to the formation of the product. This process is thermally activated, and the energy of activation is associated with the degree of bond making and bond breaking in the transition state. In the new model, the reaction path involves the initial fluctuation of the solvent, serving to bring the reactant state and the product state into resonance, at which time the proton is transferred either adiabatically or nonadiabatically to form the product. If this theory is correct, then all of the deductions derived from the standard model regarding the nature of the proton transfer process are called into question. For weakly hydrogen-bonded complexes, two sets of experiments are presented supporting the proposal that proton transfer occurs as a nonadiabatic process. In these studies, the correlation of rate constants to driving force reveals both a normal region and an inverted region for proton transfer. Yet, the experimentally observed kinetic behavior does not align with the recent theoretical formulation for nonadiabatic proton transfer, underscoring the gap in the collective understanding of proton transfer phenomena.

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

“…In particular, the low oxygen sensitivity of the oxidized anisole intermediates rules out the presence of carbon-centered radicals formed via H-abstraction from the methoxy group or via adiabatic proton transfer within a primary radical ion pair, as it has been reported for the system BP/dimethylaniline. [46] The spectral assignment of the BP-based transient as the BP ketyl radical was verified by experiments in the presence of dissolved oxygen that revealed selective quenching of the 540 nm transient with nearly diffusion-controlled rates. The predominance of the BP ketyl radicals in the transient spectra points to an ET reaction from the anisole to the BP triplet [Eq.…”

Accurate Quantum Dynamics Study on the Resonance Decay of Vinylidene

“…[1,2] As H migration processes involve hydrogen, the most "quantum" of atoms, quantum dynamical (QD) effects such as tunneling often play a significant role, and the understanding of the nature of these processes thus requires that accurate full-dimensional QD calculations be performed. However, such calculations are very challenging, even for the benchmark vinylidene-acetylene system.…”

Proton Transfer in Biomolecules Facilitated by Water: Quantum Chemical Investigations

“…14 This finding is in high contrast with the case of smaller Gly -(H 2 O) 3 clusters, in which the zwitterionic Gly is of much higher energy than the canonical forms.…”

“…Proton transfer [1][2][3][4][5] is one of the most fundamental and elementary chemical processess. Acid-base chemistry is essentially the proton transfer process in aqueous solution.…”

“…Energies, Gibbs free energies (at 5 K for sections 1 and 2, and at 298 K for section 3), and the activation barriers were evaluated for typical biomolecules (amino acids, DNA base and nicotine) to analyze the effects of water on their structures and dynamics. We have found that the role of water is profound: the microsolvating water molecules either take part in the process directly (mediating the movement of the proton), or even accelerating it.…”