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.