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 diahy dride (PMDA) anion is investigated in terms of the VWN (Vosco-Wilk-Nusair) the BP (Becke-Perdew) and the B3LYP density functional (DF) methods employe d with 6-31G* basis sets. All the methods are shown to reproduce correctly the absorption and resonance Raman spectra in the region corresponding to t he low-energy 1(2)A(u) --> 1(2)B(3g) transition. The discrepancies between the theory and experiment are attributed to a (weak) Dushinsky effect predo minately due to a mixing of the nu(3) = 1593 cm(-1) and nu(4) = 1342 cm(-1) vibrations in the 1(2)B(3g) state of the PMDA radical. (C) 2000 Elsevier S cience B.V. All rights reserved.