Spectroscopic determination of ground and excited state vibrational potential energy surfaces

Far-infrared spectra, mid-infrared combination band spectra, Raman spectra, and dispersed fluorescence spectra of non-rigid molecules can be used to d etermine the energies of many of the quantum states of conformationally imp ortant vibrations such as out-of-plane ring modes, internal rotations, and molecular inversions in their ground electronic states. Similarly, the fluo rescence excitation spectra of jet-cooled molecules, together with electron ic absorption spectra, provide the information for determining the vibronic energy levels of electronic excited states. One- or two-dimensional potent ial energy functions, which govern the conformational changes along the vib rational coordinates, can be determined from these types of data for select ed molecules. From these functions the molecular structures, the relative e nergies between different conformations, the barriers to molecular intercon versions, and the forces responsible for the structures can be ascertained. This review describes the experimental and theoretical methodology for car rying out the potential energy determinations and presents a summary of wor k 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-memb ered rings will be presented, and results for several molecules with unusua l properties will be cited. Potential energy functions for the carbonyl wag ging and ring modes for several cyclic ketones in their S-1(n, pi*) states will also be discussed. Potential energy surfaces for the three internal ro tations, including the one governing the photoisomerization process, will b e examined for trans-stilbene in both its S-0 and S-1(pi, pi*) states. For the bicyclic molecules in the indan family, the two-dimensional potential e nergy surfaces for the highly interacting ring-puckering and ring-flapping motions in both the S-0 and S-1(pi, pi*) states have also been determined u sing all of the spectroscopic methods mentioned above. Here, the effect of the electronic transition on the potential energy surface and hence the mol ecular structure can be ascertained.