The infrared spectra of polycyclic aromatic hydrocarbons containing a five-membered ring: symmetry breaking and the B3LYP functional

The infrared spectra of six molecules, each of which contains a five-member ed ring, and their cations are determined using density functional theory; both the B3LYP and BP86 functionals are used. The computed results are comp ared with the experimental spectra. For the neutral molecules, both methods are in good agreement with experiment. Even the Hartree-Fock (HF) approach is qualitatively correct for the neutral species. For the cations, the HF approach fails, as found for other organic ring systems. The B3LYP and BP86 approaches are in good mutual agreement for five of the six cation spectra , and are in good agreement with experiment for four of the five cations wh ere the experimental spectra are available. It is only for the fluoranthene cation where the BP86 and B3LYP functionals yield different results; the B P86 approach yields the expected C-2v symmetry, while the B3LYP approach br eaks symmetry. The experimental spectra support the BP86 spectra over the B 3LYP spectra, but the quality of the experimental spectra does not allow a critical evaluation of the accuracy of the BP86 approach for this difficult system.