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

Abstract

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.