Theoretical and Experimental IR Spectra of Astronomical PAHs and PANHs in the 2000-1650 cm^-1 Range (∼5-6 μm)

Recent James Webb Space Telescope observations reveal unprecedented details in the 2000-1650 cm^-1 (∼5-6 μm) region, including the well-known 5.25 and 5.75 μm bands. This range, dominated by overtone and combination bands mainly involving C-H out-of-plane (CH_oop) bending fundamentals, provides a powerful yet underexplored probe of astronomical polycyclic aromatic hydrocarbons (PAHs) and their nitrogen-containing analogues (PANHs). We present a combined experimental and theoretical investigation of 27 neutral PAHs and PANHs using matrix-isolation spectroscopy and second-order vibrational perturbation theory to disentangle effects of PAH structures and nitrogen substitution in the 2000-1650 cm^-1 region. Coupling among the underlying fundamentals subdivides this region, accommodating the distinct characteristics of the 5.25 and 5.75 μm bands. The higher-frequency subregion (2000-1870 cm^-1; 5.00-5.35 μm) reflects PAH edge topology, where the highest-frequency band shifts from quartet + bay structures (∼5.13 μm) to quartet+solo (∼5.18 μm) and pericondensed PAHs (∼5.20 μm), consistent with the relatively stable astronomical 5.25 μm band. The lower-frequency subregion (1870-1650 cm^-1; 5.35-6.06 μm) shows greater structural variability, consistent with the composite 5.75 μm band. Nitrogen substitution broadens the spectral distribution by shifting bands by ∼10 cm^-1. From our neutral PAHs/PANHs set, we infer that the 5-6 μm region reflects contributions from neutral astronomical PAHs, dominated by pericondensed PAHs, with secondary contributions from catacondensed PAHs with limited quartets and increasing solos. Contributions from irregular edges containing quartets C-H groups and nitrogen-substitution are modest. A corresponding study of cations is needed to fully characterize the 5-6 μm emission, disentangling cation and neutral contributions from the CH_oop region.