Quantum Chemical Computation of Vibrational Spectra for CN-PAHs

The long theorized polycyclic aromatic hydrocarbons (PAHs) crashed onto the astrochemical scene with the radioastronomical discovery of cyanobenzene (benzonitrile) in 2018, but the astronomical observation of the CN stretch for this or any other known CN-PAHs in the IR has yet to be reported. With the wealth of James Webb Space Telescope (JWST) data being actively returned, the need for reference data for IR features of known interstellar molecules like CN-PAHs has never been greater. Due to complexities in working with PAHs in the laboratory, quantum chemistry provides the best means of high-throughput vibrational reference data generation. This work gives an overview for the state-of-the-art in quantum chemical approaches and results for computing IR spectroscopic data for CN-PAHs, specifically for the CN stretch. The use of the hybrid rDSD/junTZ + B3LYP/N07D method through quartic force fields (QFFs) and second-order vibrational perturbation theory (VPT2) has computed the CN stretch of C2v 9-cyanoanthracene to be 2207 cm–1 (4.531 μm), exactly the same as free electron laser experiments report. Additionally, due to the structural regularity of the attached PAHs, the CN stretch does not vary greatly (less than 20 cm–1) from CN-PAH molecule to different CN-PAH molecule. However, this spectroscopic region is home to Mg IV and Ar VI lines that may hinder observations of the CN stretch in astronomical environments where vibrational emission is possible.