PROBING HYDROGEN-BOND POTENTIALS VIA COMBINATION BAND SPECTROSCOPY - A NEAR-INFRARED STUDY OF THE GEARED BEND VAN-DER-WAALS STRETCH INTERMOLECULAR MODES IN (HF)(2)

High resolution near infrared spectra of the two lowest frequency inte rmolecular modes in HF-stretch excited states of (HF), have been chara cterized using a slit-jet infrared spectrometer. In the spectral regio n surveyed, ten vibration-rotation-tunneling (VRT) bands are observed and assigned to the low frequency ''van der Waals stretch'' (nu(4)) an d ''geared bend'' (nu(5)) intermolecular modes, in combination with ei ther the hydrogen bond acceptor (nu(1)) or donor (nu(2)) high-frequenc y intramolecular HF stretches. Analysis of the rotationally resolved s pectra provide intermolecular frequencies, rotational constants, tunne ling splittings, and predissociation rates for the nu(4)/nu(5) intermo lecular excited states. The intermolecular vibrational frequencies in the combination states display a systematic dependence on intramolecul ar redshift that allows far-IR intermolecular frequencies to be reliab ly extrapolated from the near-IR data. Approximately tenfold increases in the hydrogen bond interconversion tunneling splittings with either nu(4) or nu(5) excitation indicate that both intermolecular modes cor relate strongly to the tunneling coordinate. The high resolution VRT l ine shapes reveal mode; specific predissociation broadening sensitive predominantly to intramolecular excitation, with weaker but significan t additional effects due to low frequency intermolecular excitation. A nalysis of the high resolution spectroscopic data for these nu(4) and nu(5) combination bands suggests strong state mixing between what has previously been considered van der Waals stretch and geared bend degre es of freedom. (C) 1996 American Institute of Physics.