THE POTENTIAL ENERGY SURFACE OF AR-HCN
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Date
1989
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Publisher
Ohio State University
Abstract
The microwave spectrum of AR-HCN is superficially characteristic of a linear molecule with both unusually large centrifugal distortion and unexpectedly large bending $amplitude.^{1}$ Both the centrifugal distortion constants and the vibrationally averaged structure are unusually sensitive to isotopic substitution. Also, the average bending angle derived from the hyperfine interaction constants varies with rotational $state.^{2}$ One explanation for this unusual behavior is the presence of two minima, a global minimum at the hydrogen bonded, Ar-HF like structure with a secondary minimum at the T-shaped, Ar-HCCH like structure. Unusual isotopic dependences arise from tunneling into the secondary minimum, which is not a classically allowed region but is sufficiently low in energy that significant penetration does occur. The first excited bending states are classically allowed in the secondary minimum and are thus predicted to lie at unusually low energies. These low lying vibrational states contribute to the large centrifugal distortion. The secondary, T-shaped, minimum has a shorter center of mass separation than the hydrogen bonded minimum. This means the relative energetics of the two minima change with the overall rotational energy, leading to the observed dependance of the vibrationally averaged angle on rotational state. In solving for the vibrational wavefunctions, a ``reversed Born-Oppenheimer angular radial separation'' technique is used to generate an approximate wavefunction. Numerical relaxation techniques are then used to bring the approximate wavefunctions into agreement with the full Hamiltonian of the ground vibrational state. Coriolis interactions with excited vibrational states are included perturbatively.
Description
$^{1}$ K.R. Leopold, G.T. Fraser, F.J. Lin, D.D. Nelson, and W. Klemperer, J. Chem. Phys. 81 (11), 4922 (1984). $^{2}$ T.D. Klots, C.E. Dykstra, and H.S. Gutowsky, J. Chem. Phys. 90 (1), 30 (1989).
Author Institution: Dept. of Chem., Harvard University
Author Institution: Dept. of Chem., Harvard University