Rotationally resolved electronic spectroscopy of 1,4-benzodioxan: the anomeric effect in the ground and electronically excited state.

We measured the rotationally resolved electronic spectra of the origin and of three vibronic bands of 1,4-benzodioxan. From comparison to various ab-initio-calculated structures of 1,4-benzodioxan, the twisted C(2) symmetric 1,4-benzodioxan was shown to be responsible for all the observed spectral features. We analyzed the inertial defects in both electronic states as sensitive indicators of the non-planarity of the system. The molecule was found to be more planar in the electronic ground state than in the electronically excited singlet state. This effect can be traced back to an increased puckering of the dioxan ring, which also comprises the oxygen atoms, in the excited state. This observation is discussed in terms of natural bond orbitals.