ISOTOPIC SUBSTITUTION OF A HYDROGEN BOND: A NEAR-IR STUDY OF ALL 4 INTERMOLECULAR MODES IN DF DIMER
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Date
1996
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Publisher
Ohio State University
Abstract
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 spectrometer. 12 vibration-rotation-tunneling (VRT) bands are observed, representing each of the 4 intermolecular modes (""van der Waals stretch"" $v_{u}$ ""geared bend"" $v_{y}$ ""out-of-plane torsion"" $v_{u}$ and ""anti-geared bend"" $v_{y}$) built as combination bands on either the $v_{2}$ (""free"") or $v_{2}$ (""bound"") DF stretches. Analysis of the rotationally resolved spectra provide spectroscopic constants, intermolecular frequencies, tunneling splittings and predissociation rates as a function both of intra- and inter-molecular excitation. The intermolecular frequencies demonstrate a small but systematic dependence on intramolecular mode, which is exploited to yield frequency predictions relevant to far-IR studies, as well as facilitate direct comparison with full 6-D quantum calculations on trial potential surfaces. Detailed vibrational energy patterns upon H/D isotopic substitution for the same potential surface are obtained by comparison with previous combination band studies of all four intermolecular modes in $(HF)_{2}$. South analysis reveals, for example, a surprisingly large (25%) and positive shift for the $v_{4}$ ""van der Waals stretch"" upon deuteration. The van der Waals stretch and geared bend degrees of freedom appear to be largely decoupled in $(DF)_{2}$, in contrast to the strong state mixing previously observed for $(HF)_{2}$ and indicating the importance of isotopically tuned resonances between bend-stretch intermolecular vibrations. Quantum mechanical analysis of the 4-D eigenfunctions indicate that the isotopic dependence of such a bend-stretch resonance behavior is incorrectly predicted by current hydrogen bond potential surfaces.
Description
Author Institution: JILA and Department of Chemistry and Biochemistry, National Institute of Standards and Technology and University of Colorado