Infrared Spectroscopy of Matrix-Isolated Polycyclic Aromatic Hydrocarbon Ions. 5. PAHs Incorporating a Cyclopentadienyl Ring

The matrix-isolation technique has been employed to measure the mid-infrared spectra of the ions of several polycyclic aromatic hydrocarbons whose structures incorporate a cyclopentadienyl ring. These include the cations of fluoranthene (C16H10), benzo[a]fluoranthene, benzo[b]fluoranthene, benzo[j]fluoranthene, and benzo­ [k]fluoranthene (all C20H12 isomers), as well as the anions of benzo[a]fluoranthene and benzo[j]fluoranthene. With the exception of fluoranthene, which presented significant theoretical difficulties, the experimental data are compared to theoretically calculated values obtained using density functional theory (DFT) at the B3LYP/ 4-31G level. In general, there is good overall agreement between the two data sets, with the positional agreement between the experimentally measured and theoretically predicted bands somewhat better than that associated with their intensities. The results are also consistent with previous experimental studies of polycyclic aromatic hydrocarbon ions. Specifically, in both the cationic and anionic species the strongest ion bands typically cluster in the 1450 to 1300 cm-1 range, reflecting an order-of-magnitude enhancement in the CC stretching and CH in-plane bending modes between 1600 and 1100 cm-1 in these species. The aromatic CH out-of­ plane bending modes, on the other hand, are usually modestly suppressed (e 2x 5x) in the cations relative to those of the neutral species, with the nonadjacent CH modes most strongly affected. The ionization effect on the analogous anion modes is more varied, with both enhancements and suppressions observed. Finally, while no cation features have been observed in the 3100-2950 cm-1 aromatic CH stretching region, bands arising from these modes are observed for each of the anions addressed in these studies. This agrees qualitatively with the theoretical calculations which predict that, in stark contrast to the order of magnitude suppression encountered in the cations, the total intensity in these modes is actually enhanced by about a factor of 3 in the anions relative to the neutral species. This is the first time that the CH stretching features of an isolated PAH ion have been observed experimentally.