Reparameterized Semiempirical Anharmonic IR Spectra of Neutral PAHs: Benchmarking and Predictions for PAHs with More than Five Rings

Recently developed, reparameterized PM6 methods can reproduce experimental polycyclic aromatic hydrocarbon (PAH) IR spectra with nearly the same accuracy as state-of-the-art quantum chemical methods but for notably less computational cost. The use of the O(n3) semiempirical method (as opposed to O(n4) for density functional theory or O(n7) for the most accurate coupled cluster theory) allows for full, explicit, quartic force field (QFF), anharmonic computations on PAHs. The anharmonicity also predicts the combination band and overtone frequencies in addition to the fundamentals. As such, anharmonicity is essential for the best predictions of PAH spectra, and semiempirical methods trained to compute anharmonic PAH IR spectra are the best bet at tackling such computations for large numbers of large PAHs. Beyond these benchmarks, this approach is utilized to predict the anharmonic IR spectra for five-, six-, and eight-ring PAHs in benzo[a]pyrene (C20H14), naphtho[2,1,8-qra]naphthacene (C24H14), and naphtho[2,3-b]anthanthrene (C30H16), respectively, for the first time. The larger PAHs give a double-peak in the C–H stretching region around 3000 cm–1 (3.3 μm), whereas benzo[a]pyrene still reports a large single peak, adding support for the notion that larger PAHs are more common than smaller ones in many astronomical regions. Notable IR intensities are also predicted below 1000 cm–1 (10.0 μm) where out-of-plane fundamental frequencies arise. The behavior in these two regions combined with the low (but nonzero) mir-IR features are consistent with the previous hypothesis that PAH cations are responsible for most of the astronomically observed mid-IR features as neutral species like those studied here are not. Hence, this cheaper method is in line with the current best methods of quantum chemical PAH vibrational spectra predictions, is orders of magnitude faster opening the door for explicit anharmonic computations of larger PAH IR spectra, and is already providing insights into PAH behavior where only harmonic computations have previously been possible.