Spectroscopic determination of ground and excited state vibrational potential energy surfaces
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Far-infrared spectra, mid-infrared combination band spectra, Raman spectra, and dispersed fluorescence spectra of non-rigid molecules can be used to determine the energies of many of the quantum states of conformationally important vibrations such as out-of-plane ring modes, internal rotations, and molecular inversions in their ground electronic states. Similarly, the fluorescence excitation spectra of jet-cooled molecules, together with electronic absorption spectra, provide the information for determining the vibronic energy levels of electronic excited states. One- or two-dimensional potential energy functions, which govern the conformational changes along the vibrational coordinates, can be determined from these types of data for selected molecules. From these functions the molecular structures, the relative energies between different conformations, the barriers to molecular interconversions, and the forces responsible for the structures can be ascertained. This review describes the experimental and theoretical methodology for carrying out the potential energy determinations and presents a summary of work that has been carried out for both electronic ground and excited states. The results for the out-of-plane ring motions of four-, five-, and six-membered rings will be presented, and results for several molecules with unusual properties will be cited. Potential energy functions for the carbonyl wagging and ring modes for several cyclic ketones in their Si(n, 7t*) states will also be discussed Potential energy surfaces for the three internal rotations, including the one governing the photoisomerization process, will be examined for trans-stilbene in both its S0 and Si(7t, 7t*) states. For the bicyclic molecules in the indan family, the twodimensional potential energy surfaces for the highly interacting ring-puckering and ring-flapping motions in both the S0 and Si(7t, 7t*) states have also been determined using all of the spectroscopic methods mentioned above. Here, the effect of the electronic transition on the potential energy surface and hence the molecular structure can be ascertained. © 1999, Taylor & Francis Group, LLC. All rights reserved.
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