Interference Kinetics in a Symmetric Pair of Proton Transfer Reactions

Abstract

Despite the ubiquitous use of quantum mechanics in descriptions of atomic and molecular structure, most chemical reactions are studied as classical local rearrangements of electrons and nuclei. Quantum theory predicts that, under certain circumstances, a system can evolve as a coherent superposition of local basis states. Although quantum effects such as tunneling are known to influence reaction mechanisms, chemists have rarely explored how nonlocal superposition effects might influence reaction kinetics. This dissertation is centered around a proof-of-concept experiment, which aims to use ultrafast time-resolved spectroscopy to observe a superposition signature between two identical proton-transfer reactions launched simultaneously from a solvated molecular exciton. On a homebuilt broadband pump–probe spectrometer, my coworkers and I discovered an unusual, room-temperature kinetic isotope effect (KIE) in the two-site proton-transfer model, Pigment Yellow 101 (PY101). We found that, as the reactive protons were progressively substituted by deuterons, the measured reaction rate first decreased but then increased, with the slowest reaction recorded at ~50% deuteration. We explained the rate enhancement in the fully symmetric isotopologues, which contained both protons or both deuterons, as the effect of a constructive interference stemming from the excitonic coupling between the two sites. Simulations from an illustrative quantum mechanical model, mainly constructed by Luhao Zhang, suggest that inter-site quantum correlations are indeed crucial to the unusual KIE.