Joseph Fournier (Washington University in St. Louis)

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Location: 127 Nieuwland Science Hall

Abstract:  H-bonding interactions and proton transfer processes play central roles throughout chemistry and biology. Spectroscopic studies that directly probe strong H-bonds and proton transfer reactions, however, remain a formidable experimental challenge. We aim to characterize vibrational spectral signatures and dynamics of strong intramolecular H-bonds by complementing high-resolution gas phase techniques (cryogenic ion spectroscopy) with ultrafast time-resolved solution phase experiments (transient absorption, 2D IR). Gas-phase studies on a series of proton-coupled electron transfer (PCET) model systems reveal extremely broad shared proton stretch transitions even at cryogenic (30 K) temperatures. Computational modeling reveals that the breadth of the shared proton stretches arises from strong anharmonic coupling of the shared proton to numerous low-frequency soft modes that significantly displace the proton transfer coordinate at the zero‑point level. Solution-phase studies on the prototypical H-bond model system acetylacetone directly reveal the high degree of anharmonic mode mixing between the strongly H-bonded OH stretch, OH bend, fingerprint modes, and low-frequency soft modes that leads to rapid intramolecular population relaxation dynamics. Surprisingly large isotopic differences are observed in polarization anisotropy measurements suggesting significant differences in proton vs. deuteron transfer dynamics in acetylacetone. Finally, exciting progress on a novel experiment that combines ultrafast IR spectroscopies with cryogenic ion techniques for the acquisition of multidimensional and time-resolved spectra of isolated ion ensembles will be discussed.

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