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.
Understanding the dynamics of proton transfer along low-barrier hydrogen bonds remains an outstanding challenge of great fundamental and practical interest, reflecting the central role of quantum effects in reactions of chemical and biological importance. Here, we combine ab initio calculations with the semiclassical ring-polymer instanton method to investigate tunneling processes on the ground electronic state of 6-hydroxy-2-formylfulvene (HFF), a prototypical neutral molecule supporting low-barrier hydrogen-bonding. The results emerging from a full-dimensional ab initio instanton analysis reveal that the tunneling path does not pass through the instantaneous transition-state geometry. Instead, the tunneling process involves a multidimensional reaction coordinate with concerted reorganization of the heavy-atom skeletal framework to substantially reduce the donor–acceptor distance and drive the ensuing intramolecular proton-transfer event. The predicted tunneling-induced splittings for HFF isotopologues are in good agreement with experimental findings, leading to percentage deviations of only 20–40%. Our full-dimensional results allow us to characterize vibrational contributions along the tunneling path, highlighting the intrinsically multidimensional nature of the attendant hydron-migration dynamics.
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