Despite their importance in diverse chemical and biochemical processes, low-barrier hydrogen bonds remain elusive targets to classify and interpret spectroscopically. Here the correlated nature of hydrogen bonding and proton transfer in the low-barrier regime has been probed for the ground and excited electronic states of 6-hydroxy-2-formylfulvene by acquiring jet-cooled fluorescence spectra of the parent and monodeuterated isotopologs. While excited-state profiles reveal regular vibronic patterns devoid of obvious dynamical signatures, their ground-state counterparts display a radically altered energy landscape characterized by spectral bifurcations comparable in magnitude to typical vibrational spacings (>100 cm). Quantitative analyses yield unusual deuterium kinetic isotope effects that straddle limiting values attributed to above-barrier vibration and below-barrier tunneling of the proton adjoining donor/acceptor sites. Our findings provide compelling experimental evidence for ultrafast hydron-migration events commensurate with the onset of low-barrier hydrogen bonding and afford a trenchant glimpse of molecular phenomena taking place at the "tipping point" between disparate dynamical regimes.

To elucidate low-barrier hydrogen-bonding (LBHBing) motifs and their ramifications for hydron-migration dynamics, the A ̃1B 2 −X ̃1A 1 (π* ← π) absorption system of 6-hydroxy-2-formylfulvene (HFF) and its monodeuterated isotopolog (HFFd) has been probed under free-jet expansion conditions through synergistic application of fluorescence-based laser spectroscopy and quantum-chemical calculations. Neither the donor−acceptor distance nor the proton-transfer barrier is predicted to change markedly between the X ̃1A 1 and A ̃1B 2 manifolds, yet a radical alteration in the nature of the reaction coordinate, whereby the planar (C 2v ) transition-state configuration of the former is supplanted by a notably aplanar (C 2 ) form in the latter, is suggested to take place following π* ← π electron promotion (owing, in part, to attendant rearrangements of πelectron conjugation about the molecular framework). In contrast to the strongly perturbed vibrational landscape (commensurate with LBHBing) reported for the X ̃1A 1 potential surface, the present measurements have revealed surprisingly regular patterns of A ̃1B 2 vibronic structure which are devoid of obvious band shifts/splittings that would be indicative of efficient proton-transfer processes. Detailed analyses enabled a total of 41 (6) and 28 (5) excited-state vibrational levels (fundamentals) to be assigned for HFF and HFF-d, with extensive activity found for modes involving displacement of the seven-membered chelate ring that harbors the O−H•••O reaction center. Quantitative simulations of partially resolved rotational contours for the HFF origin band showed the transition dipole moment to possess hybrid type-a/b character, thereby allowing the tunneling-induced bifurcation of the vibrationless A ̃1B 2 level to be extracted, Δ 0 A ̃= 0.119(11) cm −1 . This represents an enormous (>1000-fold) decrease over the analogous ground-state metric and implies a pronounced quenching of excited-state hydron migration, in keeping with the kinematic penalties that would be exacted by requisite heavy-atom motion along a putatively aplanar reaction coordinate.

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The vibrational specificity and isotopic dependence of hindered proton-transfer dynamics have been explored in the lowest-lying singlet excited state, Ã1 B 2 (π * π), of 6-hydroxy-2-formylfulvene (HFF) and its monodeuterated isotopolog (HFF-d). Both systems have been probed under bulk-gas conditions by employing polarization-resolved degenerate fourwave mixing (DFWM) spectroscopy, where judicious selection of incident and detected polarization geometries served to alleviate spectral complexity and to allow for the quantitative extraction of rotation-tunneling information. The observed >1000-fold decrease in tunneling rate that accompanies the π * ← π electron promotion (transitioning from ultrafast groundstate dynamics a to near-complete quenching of analogous excited-state behavior) makes HFF a compelling model system for investigating the nuanced nature of low-barrier hydrogen bonding and its ability to regulate attendant hydron-migration events. A thorough analysis of low-energy vibronic bands in the Ã1 B 2 manifold will be presented, with the dependence of unimolecular reactivity on heavy atom motion and isotopic modification being discussed in the context of structural predictions emerging from high-level quantum-chemical calculations.

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