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 in termolecular modes of (HF), have been characterized in HF-stretch exci ted states using a slit jet spectrometer. In the spectral region betwe en 4280 and 4480 cm(-1), four vibration-rotation-tunneling (VRT) bands are observed and assigned to tunneling pairs of the out-of-plane tors ion (nu(6)) and antigeared bend (nu(3)) intermolecular modes, in combi nation with the hydrogen bond donor (nu(2)) and acceptor (nu(1)) high- frequency intramolecular HF stretches, respectively. Analysis of the j et-cooled, rotationally resolved spectra provide intermolecular freque ncies, rotational constants, tunneling splittings, and predissociation rates for the nu(3)/nu(6) intermolecular excited states. The relative ly small changes in the hydrogen bond interconversion tunneling splitt ing with either nu(3) or nu(6) excitation indicate that neither interm olecular mode is strongly coupled to the tunneling coordinate. The hig h-resolution VRT linewidths reveal mode specific predissociation broad ening sensitive predominantly to intramolecular excitation, but with s ignificant additional effects due to low-frequency intermolecular exci tation as well. The intermolecular vibrational frequencies in the comb ination states display a systematic dependence on intramolecular redsh ift 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) vibrati on 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 po tential surface. Finally, the observation of all four intermolecular m odes allows zero-point contributions to the binding energy to be relia bly estimated. The revised value for the binding energy, D-e = 1580(35 ) cm(-1), is slightly higher than semiempirical estimates but now in e xcellent agreement with recent high level ab initio calculations. (C) 1996 American Institute of Physics.