THEORY OF ODD TORSIONAL TRANSITIONS IN THE $V - N$ RESONANCE RAMAN SPECTRUM OF ETHYLENE
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Date
1996
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Publisher
Ohio State University
Abstract
The $V - N$ resonance Raman spectrum of $ethylene^{a}$ shows a long progression in even quanta of the ground-state torsional mode $\nu_{4}^{0}(a_{v})$. Bands approximately midway between the even quanta have been $assigned^{b}$ to transitions to odd quanta of $\nu_{4}^{0}$. Such transitions are forbidden according to the usual $9\leftrightarrow u$ slection rule of Raman spectroscopy. Here we consider the theory of the intensity of such transitions allowing for the fact that the excited state is twisted by $90^{\circ}$ at equilibrium, using Hougen#### double group $theory^{c}$. From approximate one dimensional torsional potensials of the $V$ and $N$ electronic states, it is shown that good qualitative agreement between observed and calculated intensities is obtained. The electronic transition moment is assumed to be proportional to cos$\theta$, where $\theta$ is the torsional angle, but the calculated relative intensities are not sensitive to the precise torsional dependence. More detailed theory will require consideration of the avoided crossing affecting the $V$ $state^{d}$.
Description
$^{a}$ R. J. Sension and B. S. Hudsor, J. Chem. Phys. 90, 1377 (1989). $^{b}$W. Siebrand and M. Z. Zgierski, J. Raman Spectrosc. 21, 263 (1990). $^{c}$A. J. Merer and J. K. G. Watson, J. Mol. Spectrosc. 47, 499 (1973). $^{d}$C. Petrongolo, R. J. Buenker and S. D. Peyerimhoff, J. Chem. Phys. 76, 3655 (1982).
Author Institution: Steacie Institute for Molecular Science, National Research Council of Canada; Department of Computational Methods in Chemistry, Jagiellonian University
Author Institution: Steacie Institute for Molecular Science, National Research Council of Canada; Department of Computational Methods in Chemistry, Jagiellonian University