Spectroscopic properties of aromatic heterocyclic systems: XAl3 (XÂ =Â Si, Ge, Sn, Pb) and their anions and cations

The equilibrium geometries, vibrational frequencies, adiabatic energy separations for the ground and several excited states of XAl3 (X = Si, Ge, Sn, Pb) and their positive and negative ions have been investigated by the DFT level, followed by the CCSD(T) level for further energy refining, and the vertical detachment energies (VDE) for XAl3- are computed and compared with the experiment and other theoretical calculations by outer valence Green function (OVGF). On the basis of our computed excited states energy separations, we have successfully suggested assignments for the observed X, A, and B states in the anion photoelectron spectra of XAl3-. The assignments of the excited states are consistent excellently with the experimental excitation energies and the previous OVGF method generally. The canonical MO-NICS analysis suggest that the π and σ MOs of the XAl3- (C2v) clusters both contribute to their aromaticity, and the π MOs contribute mostly.