Vibrational potential energy surfaces for phthalan and 1,3-benzodioxole intheir S-0 and S-1 (pi,pi*) states

The far-infrared, Raman and ultraviolet spectra of phthalan and 1,3-benzodi oxole 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 quan tum jump transitions between the various ring-puckering energy levels. Many hot bands involving the ring-puckering and ring-flapping vibrations were a lso observed. The ring-puckering energy level spacings possess an irregular pattern and calculations show this to arise from the kinetic energy intera ctions between the puckering and flapping vibrations. A two-dimensional pot ential energy surface, which nicely fits all the observed data, was determi ned. This has a barrier to planarity of 35 cm(-1). Dispersed fluorescence s pectra of jet-cooled phthalan molecules helped to confirm the far-infrared assignments, and the fluorescence excitation spectra were recorded to deter mine the vibrational energy levels from the puckering in the S-1(pi, pi*) e lectronic 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 plan arity of 125 cm(-1). Analysis of the ultraviolet absorption spectra and the fluorescence excitation spectra of the jet-cooled molecules is also in pro gress and shows that the barrier increases in the S-1(pi, pi*) state. (C) 1 999 Elsevier Science Limited. All rights reserved.