Spectroscopic determination of the vibrational potential energy surface and conformation of 1,3-benzodioxole in its S-1(pi,pi*) excited state. The effect of the electronic excitation on the anomeric effect
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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 S1(,+*) 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. 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 bonding character in the S1(,:*) state. As expected, the motion along the flipping 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.