Vibrational Frequencies and Infrared Spectra of Polycyclic Antiaromatic Hydrocarbons from Biphenylene to Anti-Kekulene: Unexplored Potential Contributors to the Mid-Infrared Region

The [n]phenylene family of molecules,
derivatives
of graphenylene or low density graphene, are shown in this work to
provide intriguing correlation to interstellar infrared (IR) spectra,
showcasing an additional avenue beyond polycyclic aromatic hydrocarbons
(PAHs) for the role that carbonaceous molecules may play in astronomical
iIR spectra. Quantum chemical, anharmonic, vibrational spectra are
generated in this work using pbqff, an open source program
that streamlines the process of calculating quartic force fields (QFFs).
Spectral data for all molecules employ a reparameterization of the
semiempirical method PM6, and smaller molecules also have spectral
data computed via the more traditional B3LYP/N07D level of theory.
Analysis of these theoretical spectra has revealed noteworthy spectral
features for this family of molecules in the region between 1500 and
1300 cm–1, an area typically assigned to PAH cations.
Additionally, the computed phenylene family C–H stretches lie
slightly higher in frequency than those of most PAHs, but this family
still exhibits strong out-of-plane bending motions in the far-IR.
While 20% of the universe’s carbon is hypothesized to be locked
in PAHs, this additional family of polycyclic antiaromatic hydrocarbons
could be used for comparison to future interstellar spectra, possibly
augmenting the forms of interstellar carbon available for astrochemical
exploration.