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Plucking a Hydrogen Bond: A Near Infrared Study of all Four Intermolecular Modes in (DF)2



S Davis, D T. Anderson, David Nesbitt


The near IR combination band spectra of supersonically cooled (DF)2 in the 2900 cm-1 to 3300 cm-1 region have been recorded with a high resolution slit jet apectrometer. Twelve vibration-tunneling (VRT) bands are observed, representing each of the four intermolecular modes (van der Waals stretch Ņ4, geared bend nu)5, out-of-plane torsion Ņ6, and anti-geared bend Ņ3) built as combination bands on either the Ņi (free) or Ņ2 (bound) DF stretches. Analysis of the rotationally resolved spectra provide spectroscopic constants, intermolecular frequencies, tunneling splitting and predissociation rates as a function both of intra- and intermolecular excitation. The intermolecular frequencies demonstrate a small but systematic dependence on intramolecular mode, which is explited by as much as an order of magnitude. Conversely, high resolution line shape analysis reveals that vibrational predissociation broadening is only modestly affected by intermolecular excitation, and instead exhibits a strong mode specificity controlled predominatly by intramolecular excitation. Detailed H/D isotopic vibrational shifts for the same potential surface are obtained by comparison with previous combination band studies of all four intermolecular modes in (HF)2. Such analysis reveals, for example, a surprisingly large (23 %) and positive shift for the Ņ4 van der Waals stretch upon deuteration. In contrast to the strong state mixing previously observed for (HF)2, the van der Waals stretch and geared bend degrees of freedom are largely decoupled in (DF)2, due to isotopically detuning of resonances between bend-stretch intermolecular vibrations. 4-D quantum calculations of the (HF)2 and (DF)2 eifenfunctions indicate that the isotopic dependence of this bend-stretch resonance behavior is invcorrectly predicted by current hydrogen bond potential surfaces.
Journal of Chemical Physics
No. 16


high resolution, hydrogen bonding, hydrogen fluoride, infrared, potential surfaces, predissociation


Davis, S. , Anderson, D. and Nesbitt, D. (1996), Plucking a Hydrogen Bond: A Near Infrared Study of all Four Intermolecular Modes in (DF)<sub>2</sub>, Journal of Chemical Physics (Accessed April 25, 2024)
Created September 30, 1996, Updated October 12, 2021