Spectroscopic determination of potential energy surfaces for the out-of-plane ring vibrations of Indan and related molecules in their s0 and s1 (Π, Π*) states

The laser induced fluorescence excitation spectra of jet-cooled indan and related moelcules along with their ultraviolet absorption spectra have been used to study their S1 (Π ,Π*) excited states. Far-infrared, Raman, and dispersed fluorescence were utilized to obtain the vibrational data for the S ground states. This allowed the potential energy surfaces (PESs) of these molecules to be determined in terms of the ring-puckering and ring-flapping coordinates for both states. These PESs provide barriers to planarity and conformational structures for these bicyclic molecules. Phthalan has a barrier of 35cm-1 in S0 but no barrier for S1. Coumaran has an S0 barrier of 279 cm-1, while 1,3-benzodioxole has barriers of 171 and 264 cm-1 for S0 and S1, respectively, due to the anomeric effect. The barriers for indan are 1077 cm-1 for S0 and 698 cm-1 for S0. Ab initio calculations in general provide good barrier values for the ground state.