Reminiscence of benzene in the spectroscopy of 1,3-benzodioxole: A computational study

We present a theoretical investigation on the structure and spectroscopy of 1,3-benzodioxole. Ground state structure and vibrational frequencies are computed at several levels of theory ranging from density functional theory (DFT) to complete active space self consistent field (CASSCF) and are employed to provide a coherent assignment of IR and Raman spectra. In addition, excitation energies and transition dipole moments are estimated with the time dependent (TD) DFT approach and with configuration interaction calculations limited to single excitations (CIS). It is shown that the lowest excited singlet states correlate with those of benzene. Excited state equilibrium structures are obtained at CIS and at CASCCF levels. Optimized geometries, vibrational normal modes and frequencies computed for the ground and lowest excited states are employed to derive the parameters that govern the S0-S1 Franck-Condon (FC) activities in absorption and fluorescence spectra. The intensity of vibronically induced bands are estimated with the Herzberg-Teller approach. These parameters are used to identify correctly the most active modes in the single vibronic level (SVL) fluorescence spectra. The analysis of these spectra complements and completes the IR and Raman identification of ground state vibrational frequencies and provides evidence for substantial frequency changes upon S0-S1 excitation.