Investigating H Atom Transfer Dynamics in β-Diketones Using Ultrafast Infrared Spectroscopies

Proton and H atom transfer reactions are vital for many chemical and biological systems, but are challenging to directly study due to strong anharmonic effects and ultrafast vibrational relaxation dynamics. In this work, ultrafast transient absorption (TA) and two-dimensional infrared (2D IR) spectroscopies were utilized to characterize the intramolecular H-bonding interactions and H atom transfer dynamics in a series of β-diketones and their deuterated isotopologues. 2D IR spectra in the OH and OD stretch regions display broad, long-lived ground-state bleach signals that derive from the vibrationally hot molecules following ultrafast (<100 fs) relaxation to strongly coupled low-frequency modes. Interestingly, polarization-dependent TA measurements of the OH/OD stretch regions reveal anisotropic signals that persist well beyond the vibrational lifetimes with notable isotopic differences. Orientational relaxation dynamics for the light and deuterated isotopologues occur on timescales of ~600 fs and ~2 ps, respectively. Computational modeling suggests that the orientational dynamics are related to the intramolecular H/D atom transfer events, which are driven by activated and collective internal structural rearrangements.