ORBITAL ANGULAR MOMENTUM IN LINEAR MOLECULES CONTAINING TRANSITION METALS
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Date
2000
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Ohio State University
Abstract
Vibronic coupling arises in the Born-Oppenheimer approximation from the cross-term that appears when the second derivative operator for the nuclear kinetic energy acts on the product of an electronic and a nuclear wave function. This cross-term is usually handled as an effective operator which has matrix elements between vibrational levels of different electronic states. In unsymmetrical linear triatomic molecules these elements follow the selection rules $\Delta \Lambda = \pm 1, \Delta l = \mp 1, \Delta v_{2} = \pm 1,$ where $\nu _{2}$ is the bending vibration. The most important effects occur in $\Pi$; electronic states, because distant $\Sigma^{+}$ and $\Sigma^{-}$ states affect the Born-Oppenheimer components differently. If the $\Sigma$ and $\Pi$ electronic states are well separated the vibrational structure of the $\Pi$ state can be treated as if there is an operator acting within the $\Pi$ state that causes a quadratic splitting between its two components; this is the essence of the Renner- Teller effect. In transition metal-containing molecules the density of electronic states can be high enough for this approach to break down. Every electronic transition then contains intense ``forbidden'' vibrational bands, violating the usual selection rules, and large numbers of seemingly random perturbations occur. Examples will be draw from the spectra of the metal methylidynes and hydroxides.
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Author Institution: Department of Chemistry, University of British Columbia