Thursday, November 5, 2009: 1:40 PM
Santa Fe (Camino Real Hotel)
The vibrational spectra of a molecule should contain ample information on its electronic structure, the strength of its bonds, its geometry, and its conformational flexibility. This information however is hidden in the properties of the normal vibrational modes because the latter are always delocalized involving the collective movement of groups of atoms rather than the isolated movement of just a couple of atoms. The question is how to extract from delocalized modes localized vibrational modes associated with a given structural unit as, e.g., a bond. This can be accomplished with the help of the adiabatic mode concept of Cremer, Kraka, and Konkoli. Adiabatic modes are localized modes obtained by perturbing a leading parameter, such as a bond length, bond angle, etc., and then relaxing all other parameters of the molecule. The resulting vibrations are those that initiate bond dissociation, molecular rearrangements or a conformational change of a molecule. It can be shown that McKeanxs isolated CH stretching modes are directly related to the adiabatic modes. The adiabatic mode concept is applied to different bonding situations involving multiple bond character of formal single bonds, breaking or forming of bonds in transition states, or very weak van der Waals bonds. It is demonstrated how the adiabatic vibrational modes help to determine single or multi-bond character of a given bond, characterize changes in bonding in dependence on the environment, and provide a close account on the electronic situation during bond breaking. An application of the adiabatic mode concept to characterizing materials is discussed.
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