The
vibrational signatures and ultrafast dynamics of the intramolecular
H-bond in a series of β-diketones are investigated with 2D IR
spectroscopy and computational modeling. The chosen β-diketones
exhibit a range of H atom donor–acceptor distances and asymmetry
along the H atom transfer coordinate that tunes the intramolecular
H-bond strength. The species with the strongest H-bonds are calculated
to have very soft H atom potentials, resulting in highly red-shifted
OH stretch fundamental frequencies and dislocation of the H atom upon
vibrational excitation. These soft potentials lead to significant
coupling to the other normal mode coordinates and give rise to the
very broad vibrational signatures observed experimentally. The 2D
IR spectra in both the OH and OD stretch regions of the light and
deuterated isotopologues reveal broadened and long-lived ground-state
bleach signatures of the vibrationally hot molecules. Polarization-sensitive
transient absorption measurements in the OH and OD stretch regions
reveal notable isotopic differences in orientational dynamics. Orientational
relaxation was measured to occur on ∼600 fs and ∼2 ps
time scales for the light and deuterated isotopologues, respectively.
The orientational dynamics are interpreted in terms of activated H/D
atom transfer events driven by collective intramolecular structural
rearrangements.