Experimental determination of vibrational potential energy surfaces and molecular structures in electronic excited states

For more than three decades far-infrared and Raman spectroscopies, along with appropriate quantum mechanical computations, have been effectively used to determine the potential energy functions which govern the conformationally important large-amplitude vibrations of nonrigid molecules. More recently, we have utilized laser-induced fluorescence (LIF) excitation spectroscopy and ultraviolet absorption spectroscopy to analyze the vibronic energy levels of electronic excited states in order to determine the potential energy surfaces and molecular conformations in these stares. Transitions from the ground vibrational state in an So electronic state can typically be observed only to several excited vibronic levels. Hence, the LIF of the jet-cooled molecules generally provides data on only a few excited state levels. Ultraviolet absorption spectra recorded at ambient temperatures, however, often provide data on many additional excited vibronic levels. However, these can only be correctly interpreted if the electronic ground state levels have been accurately determined from the far-infrared, Raman, and dispersed fluorescence studies. In this article, we will first present our results for bicyclic molecules in the indan family in their S-0 and S-1(pi,pi*) electronic states. Two-dimensional potential energy surfaces in terms of the ring-puckering and ring-flapping vibrations were utilized for the analyses. Next, we review our work on trans-stilbene in its S-0 and S-1(pi,pi*) states and examine the data from which two-dimensional. potential energy surfaces were determined for the phenyl torsions and one-dimensional functions were calculated for the torsion about the C=C bond, which governs the photoisomerization. Finally, we consider seven cyclic ketones in their S-0 and S-1(pi,pi*) states. The carbonyl wagging vibration of each was studied in its electronic excited state in order to determine the barrier to inversion and the wagging angle. The barrier to inversion was found to increase with angle strain. Conformational changes between the ground and excited electronic states were also examined in terms of the out-of-plane ring motions.