Polycyclic (anti)aromatic hydrocarbons: interstellar formation and spectroscopic characterization of biphenylene and benzopentalene†

Formation of biphenylene (C₆H₄)₂ and its isomer benzopentalene, C₁₂H₈, may act as a consumption route for ortho-benzyne (o-C₆H₄) in interstellar clouds such as TMC-1. MRCI-F12 and CCSD(T)-F12 potential energy surfaces show that o-C₆H₄ dimerization is possible through a C_2h-symmetry single-bond association to a (C₆H₄)₂ precursor before isomerization to (C₆H₄)₂ and subsequently C₁₂H₈. Formation of a bimolecular product set from either species is energetically hindered, allowing (C₆H₄)₂ and C₁₂H₈ to stabilize radiatively. To remedy the dearth of spectroscopic data for these species, anharmonic frequencies from explicitly-correlated quartic force fields (QFFs) for o-C₆H₄ and c-C₄H₄ are employed to reparameterize the semiempirical method PM6 for use in lower-cost QFFs for (C₆H₄)₂ and C₁₂H₈. In both cases, at least one reparameterized PM6-QFF spectrum results in the prominent C–H stretch and symmetric C–H out-of-plane-bend features to be accurately predicted with respect to gas-phase experiment or the B3LYP/N07D anharmonic absorption spectrum. B3LYP/N07D accurately recreates the experimental infrared spectrum of (C₆H₄)₂, showing the utility of this method for spectral prediction of small and midsize polycyclic hydrocarbons on the whole. For larger systems, reparameterized PM6-QFF spectra can reproduce the most important infrared features for a species. B3LYP/N07D cascade emission spectra show that the 730 cm⁻¹ C–H symmetric out-of-plane bending feature dominates the emission spectrum of (C₆H₄)₂, while the spectrum of C₁₂H₈ becomes characterized by the collective set of C–H out-of-plane bends. As such, infrared emission spectra of (C₆H₄)₂ will likely be overshadowed by C₂H₂. Derivatives such as cyanobiphenylene are likely better targets for infrared observation.