Molecular conformation and potentials for out-of-plane ring vibrations in the S and S1 electronic states of 1,4-benzodioxan from supersonic-jet fluorescence and high-resolution absorption measurements

In a study of the 281 nm S1–S0 electronic spectrum of 1,4-benzodioxan, the high-resolution vapor absorption spectrum, the fluorescence excitation spectrum in a supersonic jet, and the dispersed single-vibronic-level fluorescence spectra for excitation into several different S1 levels have been measured. Four out-of-plane ring vibrations have been characterized in each electronic state, and it is shown that the partially saturated six-membered ring is rigidly nonplanar with a twisted C2 conformation. The twisting vibration is nearly harmonic and the barrier to planarity is high in both electronic states; however, there is some lowering of the barrier upon excitation. There is strong Duschinsky mixing in S1 between two of the out-of-plane ring bending modes, one of which is quite anharmonic with a flat-bottomed potential.