Absorption and resonance Raman study of the pyromellitic diahydride anion via density functional theory

The electronic structure of the low-energy states of the pyromellitic diahydride (PMDA) anion is investigated in terms of the VWN (Vosco–Wilk–Nusair) the BP (Becke–Perdew) and the B3LYP density functional (DF) methods employed with 6-31G* basis sets. All the methods are shown to reproduce correctly the absorption and resonance Raman spectra in the region corresponding to the low-energy 12Au→12B3g transition. The discrepancies between the theory and experiment are attributed to a (weak) Dushinsky effect predominately due to a mixing of the ν3=1593 cm−1 and ν4=1342 cm−1 vibrations in the 12B3g state of the PMDA radical.