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.