Investigating Intramolecular H Atom Transfer Dynamics in β-Diketones with Ultrafast Infrared Spectroscopies and Theoretical Modeling

Abstract

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.

Interplay between Anion-Receptor and Anion-Solvent Interactions in Halide Receptor Complexes Characterized with Ultrafast Infrared Spectroscopies

“…Although caution must be taken in interpreting the fast component due to pulse overlap effects, the fast decay timescales are consistent with rapid intramolecular relaxation dynamics within the OMCP•Xcomplexes due to the strong NH•••X -H-bonding interactions. 50,55 The initial NH stretch relaxation components are slightly faster for the two DCM solutions, supportive of stronger OMCP•X -H-bonding and weaker solvent interactions. In both solvents, the initial decay component is also faster for the chloride complexes compared to bromide, again consistent with stronger H-bonding between OMCP and chloride that leads to faster intramolecular relaxation.…”

“…The ultrafast infrared spectrometer has been described in detail elsewhere. 50 Briefly, the output of a regenerative amplifier (Coherent Astrella, 800 nm, 30 fs, 1 kHz, 3.6 mJ/pulse) pumped a commercial optical parametric amplifier (Light Conversion, TOPAS Prime) to generate tunable near-IR signal (1.3-1.45 μm) and idler (1.8-2.1 μm) pulses. The signal and idler pulses were combined in a 1 mm thick AgGaS2 crystal (Eksma Optics) in a home-built difference frequency generation setup resulting in IR pulses tuned to ~3300 cm -1 .…”