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-infrare d spectra of the ions of several polycyclic aromatic hydrocarbons whose str uctures incorporate a cyclopentadienyl ring. These include the cations of f luoranthene (C16H10), benzo[b]fluoranthene, benzo[b]fluoranthene, benzo-[k] fluoranthene, and benzo[k]fluoranthene (all C20H12 isomers), as well as the anions of benzo[a]fluoranthene and benzo[j]fluoranthene. With the exceptio n of fluoranthene, which presented significant theoretical difficulties, th e experimental data art: compared to theoretically calculated values obtain ed using density functional theory (DFT) at the B3LYP/4-31G level. In gener al, there is good overall agreement between the two data sets, with the pos itional agreement between the experimentally measured and theoretically pre dicted bands somewhat better than that associated with their intensities. T he results are also consistent with previous experimental studies of polycy clic aromatic hydrocarbon ions. Specifically, in both the cationic and anio nic species the strongest ion bands typically cluster in the 1450 to 1300 c m(-1) range, reflecting an order-of-magnitude enhancement in the CC stretch ing and CH in-plane bending modes between 1600 and 1100 cm(-1) in these spe cies. The aromatic CH out-of-plane bending modes, on the other hand, are us ually modestly suppressed (less than or equal to 2x - 5x) in the cations re lative to those of the neutral species, with the nonadjacent CH modes most strongly affected. The ionization effect on the analogous anion modes is mo re 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 t heoretical 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 rela tive to the neutral species. This is the first time that the CH stretching features of an isolated PAH ion have been observed experimentally.