Spectroscopic determination of the two-dimensional vibrational potential energy surfaces for the ring-puckering and ring-flapping modes of indan in its S and S1(π,π*) electronic states

Abstract

The vapor-phase far-infrared, mid-infrared, ultraviolet, Raman, and laser-induced fluorescence spectra of indan have been recorded and analyzed. The far-infrared spectra, which are very similar to those previously reported, together with the Raman and dispersed fluorescence (SVLF) spectra of the jet-cooled molecules were used to reassign the ring-puckering and ring-flapping energy levels for the S0 ground state. These were then utilized to calculate a two-dimensional vibrational potential energy surface (PES) which nicely fits all of the assigned puckering and flapping levels. The PES has a barrier of 488 cm−1 as compared to a previously reported value of 1979 cm−1, which was based on a one-dimensional analysis and earlier assignments. The dihedral angle of puckering is ±30°. Fluorescence excitation spectra of jet-cooled indan together with ultraviolet absorption spectra were used to assign the flapping and puckering levels in the S1(π,π*) electronic excited state. The PES for this state has a barrier of 441 cm−1 and the energy minima correspond to puckering angles of ±39°. The flapping frequency and the stiffness of the PES along the flapping coordinate both decrease substantially in the excited state. The barriers to planarity for both states are higher than those for analogous molecules due to the two –CH2–CH2– torsional interactions. Ab initio calculations do a fairly good job of predicting the experimental barriers for indan and related molecules in their S0 and S1 states.