PLUCKING A HYDROGEN-BOND - A NEAR-INFRARED STUDY OF ALL 4 INTERMOLECULAR MODES IN (DF)(2)

The near ir combination band spectra of supersonically cooled (DF)(2) in the 2900 to 3300 cm(-1) region have been recorded with a high resol ution slit jet spectrometer. Twelve vibration-rotation-tunneling (VRT) bands are observed, representing each of the four intermolecular mode s (van der Waals stretch nu(4), geared bend nu(5), out-of-plane torsio n nu(6), and antigeared bend nu(3)) built as combination bands on eith er the nu(1) (free) or nu(2) (bound) DF stretches. Analysis of the rot ationally resolved spectra provide spectroscopic constants, intermolec ular frequencies, tunneling splittings, and predissociation rates as a function of both intra- and intermolecular excitation. The intermolec ular frequencies demonstrate a small but systematic dependence on intr amolecular mode, which is exploited to yield frequency predictions rel evant to far-ir studies, as well as facilitate direct comparison with full 6-D quantum calculations on trial potential surfaces. The tunneli ng splittings demonstrate a much stronger dependence upon intermolecul ar mode, increasing by as much as an order of magnitude for geared ben d excitation. Conversely, high resolution line shape analysis reveals that vibrational predissociation broadening is only modestly affected by intermolecular excitation, and instead exhibits mode specific behav ior controlled predominantly by intramolecular excitation. Detailed H/ D isotopic vibrational shifts are obtained by comparison with previous combination band studies of all four intermolecular modes in (HF)(2). In contrast to the strong state mixing previously observed for (HF)(2 ), the van der Waals stretch and geared bend degrees of freedom are la rgely decoupled in (DF)(2), due to isotopically ''detuning'' of resona nces between bend-stretch intermolecular vibrations. Four-dimensional quantum calculations of the (HF)(2) and (DF)(2) eigenfunctions indicat e that the isotopic dependence of this bend-stretch resonance behavior is incorrectly predicted by current hydrogen bond potential surfaces. (C) 1996 American Institute of Physics.