Hydrogen bond spectroscopy in the near infrared: Out-of-plane torsion and antigeared bend combination bands in (HF)(2)

High-resolution near infrared spectra of the two ''high'' frequency intermolecular modes of (HF), have been characterized in HF-stretch excited states using a slit jet spectrometer. In the spectral region between 4280 and 4480 cm(-1), four vibration-rotation-tunneling (VRT) bands are observed and assigned to tunneling pairs of the out-of-plane torsion (nu(6)) and antigeared bend (nu(3)) intermolecular modes, in combination with the hydrogen bond donor (nu(2)) and acceptor (nu(1)) high-frequency intramolecular HF stretches, respectively. Analysis of the jet-cooled, rotationally resolved spectra provide intermolecular frequencies, rotational constants, tunneling splittings, and predissociation rates for the nu(3)/nu(6) intermolecular excited states. The relatively small changes in the hydrogen bond interconversion tunneling splitting with either nu(3) or nu(6) excitation indicate that neither intermolecular mode is strongly coupled to the tunneling coordinate. The high-resolution VRT linewidths reveal mode specific predissociation broadening sensitive predominantly to intramolecular excitation, but with significant additional effects due to 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 extrapolated from the near-ir data. Agreement between full 6-D quantum calculations and experiment for the out-of-plane torsion (Vg) vibration is remarkably good (0.5%). However, significant discrepancies (> 10%) between theory and experiment are obtained for the antigeared bend (nu(3)), indicating the need for further refinement of the HF dimer 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 for the binding energy, D-e = 1580(35) cm(-1), is slightly higher than semiempirical estimates but now in excellent agreement with recent high level ab initio calculations. (C) 1996 American Institute of Physics.