Vibrational potential energy surfaces for phthalan and 1,3-benzodioxole in their S0 and S1( π, π*) states

Abstract

The far-infrared, Raman and ultraviolet spectra of phthalan and 1,3-benzodioxole were recorded and analyzed. In addition, the fluorescence excitation spectra and the dispersed fluorescence spectra of the jet-cooled molecules were also investigated. The far-infrared spectrum of phthalan shows single, double, and triple quantum jump transitions between the various ring-puckering energy levels. Many hot bands involving the ring-puckering and ring-flapping vibrations were also observed. The ring-puckering energy level spacings possess an irregular pattern and calculations show this to arise from the kinetic energy interactions between the puckering and flapping vibrations. A two-dimensional potential energy surface, which nicely fits all the observed data, was determined. This has a barrier to planarity of 35 cm −1. Dispersed fluorescence spectra of jet-cooled phthalan molecules helped to confirm the far-infrared assignments, and the fluorescence excitation spectra were recorded to determine the vibrational energy levels from the puckering in the S 1( π, π*) electronic excited state. Ultraviolet absorption spectra were used to better understand the excited state energy levels. The far-infrared spectra of 1,3-benzodioxole were also reanalyzed. Because of the anomeric effect, 1,3-benzodioxole is puckered with a barrier to planarity of 125 cm −1. Analysis of the ultraviolet absorption spectra and the fluorescence excitation spectra of the jet-cooled molecules is also in progress and shows that the barrier increases in the S 1( π, π*) state.