LEAST SQUARES FITTING OF PERTURBED VIBRATIONAL POLYADS NEAR THE ISOMERIZATION BARRIER IN THE S$_1$ STATE OF C$_2$H$_2$
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Date
2013
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Publisher
Ohio State University
Abstract
The S$_1$ electronic state of acetylene has recently been shown to have two potential minima, corresponding to cis- and trans-bent structures. The trans-bent isomer is the more stable, with the cis-bent isomer lying about 2670 cm$^{-1}$ higher; the barrier to isomerization lies roughly 5000 cm$^{-1}$ above the trans zero-point level. The "isomerization coordinate'' (along which the molecule moves to get from the trans minimum to the barrier) is a combination of the $\nu_3$ (trans bending) and $\nu_6$ (cis bending) vibrational normal coordinates, but the spectrum is very confused because the $\nu_6$ vibration interacts strongly with the $\nu_4$ (torsion) vibration through Coriolis and Darling-Dennison resonances. Since the $\nu_4$ and $\nu_6$ fundamental frequencies are almost equal, nderline{\textbf{98}}, 2742, 1993.} the bending vibrational structure consists of polyads. At low vibrational energies the polyads where these three vibrations are excited can be fitted by least squares almost to experimental accuracy with a simple model of Coriolis and Darling-Dennison interactions, but at higher energies the huge $x_{36}$ cross-anharmonicity, which is a symptom that the levels are approaching the isomerization barrier, progressively destroys the polyad structure; in addition the levels show an increasing even-odd staggering of their $K$-rotational structures, as predicted by group theory. It is not possible to fit the levels near the barrier with a simple model, though some success has been achieved with extended models. Progress with the fitting of the polyads near the barrier will be reviewed.
Description
Author Institution: Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 1Z1; AND Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.; Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA