High-Resolution Far- to Near-Infrared Anharmonic Absorption Spectra of Cyano-Substituted Polycyclic Aromatic Hydrocarbons from 300 to 6200 cm−1

Abstract

Cyano-substituted polycyclic aromatic hydrocarbons (CN-PAHs) may contribute to the emission detected in the 7−9 μm (1430−1100 cm⁻¹) and 11−15 μm (900−670 cm⁻¹) regions of astronomical IR spectra. Anharmonic quantum chemical computations of 14 CN-PAH isomers for 4 small PAHs and benzene reveal strong, broad absorption features across the entire 300−6200 cm⁻¹ (33−1.6 μm) frequency range. In particular, when an full width at half maximum (fwhm) of 15 cm⁻¹ is applied, the composite CN-PAH spectrum greatly overlaps with the unsubstituted-PAH spectrum across the entire 300−6200 cm⁻¹ range besides the 2200−2500 cm⁻¹ region that arises from the strong CN stretch fundamental of CN-PAHs and is addressed in a separate publication. At high resolution, however, the infrared absorption spectra reveal unique, identifiable features of CN-PAHs in the 700−950, 1100−1300, 2000−2500, and 3400−3600 cm⁻¹ ranges. The in-plane and out-of-plane CH bending vibrational frequencies of CN-PAHs are shifted when comparing isomers and their unsubstituted counterparts, making their differentiation in mixed laboratory experiments possible. The overall aromatic CH stretch fundamental (2950−3200 cm⁻¹) and first overtone (5950−6200 cm⁻¹) regions are relatively unaffected by the cyano-substitution, with changes only to the frequency range covered by, and the intensity of, the bands. Detailed spectroscopic data on the normal mode components of each state reported herein provide the means to directly assign future laboratory spectra and to guide direct IR observations of astronomical regions with, e.g., James Webb Space Telescope (JWST).