Hydrogen bond spectroscopy in the near IR via combination bands: Out-of-plane torsion and anti-geared bending modes in (HF)2
D T. Anderson, S Davis, David Nesbitt
High-resolution near-infrared spectra of the two high frequency intermolecular modes of (HF)2 have been characterized in HF-stretch excited states using a slit-jet spectrometer. In the 200 cm-1 spectral region between 4280 and 4480 cm-1, 4 vibration-rotation-tunneling (VRT) bands are observed and assigned to tunneling pairs of the out-of-plane torsion (υ6) and anti-geared bend (υ3) intermolecular modes, in combination with the hydrogen bond donor (υ2) and acceptor (υ1) high-resolved spectra provide intermolecular frequencies, rotational constants, tunneling splittings, and predissociation rates for the υ3/υ6 intermolecular excited states. The relatively small changes in the hydrogen bond interconversion tunneling splitting VRT line widths reveal mode specific predissociation broadening sensitive predominately to intramolecular excitation, but with significant additional effects due to initial low-frequency intermolecular excitation as well. The intermolecular vibrational frequencies in the combination states display a systematic dependence on intramolecular redshift that allows all four intermolecular fundamental frequencies to be reliably extrapolated from the near-IR data. Agreement between full 6-D quantum calculations and experiment for the out-of-plane torsion vibration is remarkably good (0.5%). However, significant (>10%) discrepancies between theory and experiment are obtained for the anti-geared bend, indicating the need for further refinement of the HF dimmer potential surface. Finally, the observation of all four intermolecular modes allows zero-point contributions to the binding energy to be reliably estimated. The revised value, De=1580(30)cm-1, is appreciably higher than semi-empirical estimates, and now in excellent agreement with recent high level ab initio calculations.
, Davis, S.
and Nesbitt, D.
Hydrogen bond spectroscopy in the near IR via combination bands: Out-of-plane torsion and anti-geared bending modes in (HF)<sub>2</sub>, Journal of Chemical Physics
(Accessed June 6, 2023)