C-H Stretch Vibrational Modes: Tracers of Interstellar PAH Geometries?

Abstract

Polycyclic aromatic hydrocarbon (PAH) molecules have long been adjudged as carriers of the frequently detected interstellar emission features in the 3-20 μm region. In the present work, PAHs with straight edges having solo-duo (PAHD) and solo-duo-trio (PAHT) C-H modes along with PAHs with irregular edges (PAHI) have been studied theoretically to understand the effect of molecular geometry on the interstellar C-H stretch vibrations at 3.3 μm. The C-H out-of-plane bending vibrations at 11.2 and 12.7 μm are also included for completeness. Using the NASA Ames PAH IR Spectroscopic Database, the mid-infrared spectra have been studied for 125 PAH molecules of varying molecular geometries, sizes, charge states, and symmetries. Results show that the individual solo, duo, and trio C-H stretches follow an order in the peak wavelength (λ3.3 (solo) > λ3.3 (duo) > λ3.3 (trio)) and intensity (I3.3 (solo) < I3.3 (duo) < I3.3 (trio)). If only PAHD’s are considered, the contribution of each charge state is required to account for the observed peak wavelength of the 3.3 μm band, or if only neutrals are contributors, PAHD and PAHT neutrals can explain the 3.3 μm band variations. The observed emission at 11.2 and 12.7 μm is found to match effectively with PAHD with increasing size, and the 11.2 μm band is present at longer wavelengths for PAHT contributing to the red wing. When the solo to duo hydrogen ratio is nearly equal to or greater than 1.0, PAHD neutrals yield better 3.3 μm peak positions. The ratio has a lower limit of 0.8 for the 11.2 μm band and converges at 1.5, indicating a size range of PAHD neutrals with 80 to larger numbers of carbon atoms. The present work examines the presence of solo, duo, and trio modes in the C-H stretching band, which must be taken into consideration when interpreting accurate data from James Webb Space Telescope (JWST) to further explain the observed variations in the interstellar 3.3 μm. © 2023 American Chemical Society.