Infrared Spectroscopy of Isocyano-Polycyclic Aromatic Hydrocarbons: The NC Stretch

Anharmonic computations reveal an intense, narrow (20 cm^-1, 0.043 μm) absorption feature at approximately 2160 cm^-1 (4.63 μm) in the vibrational spectra of 14 prototypical singly isocyano-substituted polycyclic aromatic hydrocarbons (NC-PAHs) attributed to the NC stretching mode. The intrinsically bright NC stretching mode and strong anharmonic coupling to other states in this region of the spectrum, along with the presence of multiple isomers of each NC-PAH, creates a complex, intense band for each molecule alone, and for the group of molecules as a whole. The NC stretching feature is shifted approximately 130 cm^-1 to lower frequency compared to the CN stretching feature of cyano-substituted PAHs. This shift is due to the weaker NC bond as a result of the zwitterionic character of the NC-PAHs. Advanced resonance polyad matrices are utilized in the second order vibrational perturbation treatment, providing in-depth understanding of the spectroscopic characteristics of each vibrational transition. These detailed spectroscopic data are provided for use in analysis of future laboratory experiments and the search for NC-PAHs in astronomical observations. Such data will be vital to the continued benchmarking of computational methodologies for use in exotic, substituted PAHs that have never been studied before.