Spectroscopic determination of the vibrational potential energy surface and conformation of 1,3-benzodioxole in its S1(π,π*) excited state.: The effect of the electronic excitation on the anomeric effect

The electronic absorption spectra and the laser-induced fluorescence spectra of supersonic-jet-cooled 1,3-benzodioxole molecules have been investigated to map out the vibronic energy levels in the S-1(pi,pi*), electronic excited state. These were used to determine a two-dimensional potential energy surface in terms of the ring-puckering and ring-flapping vibrational coordinates, and the molecule was found to be puckered with a dihedral angle of 22 degrees. The barrier to planarity in the excited state is 264 cm(-1) (3.16 kJ/mol) as compared to 164 cm(-1) (1.96 kJ/mol) in the ground state. This increase is attributed to reduced suppression of the anomeric effect by the benzene ring resulting from decreased pi bonding character in the S-1(pi,pi*) state. As expected, the motion along the flapping coordinate is governed by a more shallow potential energy well. Ab initio calculations carried out for both the ground and excited states support the experimental conclusions.