Spectroscopic Signatures of Floppiness in Molecular Complexes
The challenge of correctly inferring even the qualitative features of the potential energy hypersurface from spectroscopic measurements is heightened dramatically in studies of weakly bound molecular complexes where large amplitude motion is present. This is especially true for data obtained from low temperature, supersonic expansions where Boltzmann distributions limit the range of internally excited states that can be investigated. To stress this point, we present simulated spectra for two model triatomic systems, a “pinwheel” and a “hinge,” with nearly flat potentials that support extremely large amplitude internal rotation and bending, respectively. Even in these highly “floppy” molecular systems, the exact quantum term values can be fitted remarkably well to a standard semirigid, asymmetric top Hamiltonian, but one corresponding to a qualitatively different, vibrationally averaged molecular geometry. These results indicate that simple eigenvalue analysis of jet cooled molecular spectra in the absence of hyperfine resolution may not be sufficiently sensitive to large amplitude angular motion, and that data from a variety of techniques may prove necessary to assess the degree of molecular rigidity.
KeywordsPotential Energy Surface Molecular Complex Rotational Constant Hydrogen Halide Large Amplitude Motion
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