Far-Infrared, Raman, and Dispersed Fluorescence Spectra, Vibrational Potential Energy Surface, and the Anomeric Effect of 1,3-Benzodioxole

The far-infrared and Raman spectra of 1,3-benzodioxole vapor have been recorded and analyzed. Forty-one infrared and six Raman bands were assigned to transitions between the various ring-puckering energy levels in the ground and excited ring-flapping states. The determination of the energy levels was assisted by analysis of the single vibronic level fluorescence spectra of the jet-cooled molecules. The puckering levels change substantially in the flapping excited state, indicating substantial interaction between the two vibrational modes. From the spectroscopic data, a two-dimensional vibrational potential energy surface was determined. This has a barrier to planarity of 164 cm - 1 and energy minima at puckering and flapping angles of ±24° and ∓3°, respectively. This molecule has a lower barrier to planarity than 1,3-dioxole, reflecting the influence of the benzene ring on the anomeric effect. Nevertheless, the anomeric effect is clearly the origin of the nonplanarity of this bicyclic ring system.