Rotational spectra of interstellar N- and CN-PAHs: pyrene and coronene

TL;DR

This study reports the rotational spectra of N- and CN-substituted PAHs, specifically pyrene and coronene, using DFT calculations, highlighting their potential detectability in interstellar space, especially in cold molecular clouds.

Contribution

First-time theoretical rotational spectra for N- and CN-PAHs like pyrene and coronene are provided, aiding future astronomical detection efforts.

Findings

CN-PAHs have large dipole moments, making them detectable in space.

CN-pyrene is a promising candidate for detection in cold dark clouds.

Theoretical spectra serve as benchmarks for future experiments and observations.

Abstract

The detection of benzonitrile (C6H5CN), 1- and 2-cyano-naphthalene (C10H7CN) in the cold, dark molecular cloud TMC-1 at centimetre (cm) wavelengths has opened up prospects for the detection of other N- and CN-containing polycyclic aromatic hydrocarbons (PAHs). In this light, the pure rotational spectra of N-pyrene (C15H9N), CN-pyrene (C15H9CN), N-coronene (C23H11N) and CN-coronene (C23H11CN) are reported here for the first time. The B3LYP/6-311+G(d,p) level of theory, in the Density Functional Theory (DFT) calculations, achieves the best performance for calculating the spectroscopic parameters and simulating the rotational spectra. The large permanent dipole moment of CN-PAHs makes them the most suitable PAH species for detection in the interstellar medium. Additionally, pyrene's smaller partition function makes CN-pyrene a prime candidate to be discovered in cold, dark molecular clouds such as the TMC-1. The present work sets a benchmark for theoretical rotational spectra of N- and CN-containing PAHs and may act as a guide for laboratory experiments and observational searches.