Temperature effects on anharmonic infrared spectra of large compact polycyclic aromatic hydrocarbons

¹ Leiden University, Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: chen@strw.leidenuniv.nl
² School of Engineering Sciences in Chemistry, Biotechnology and Health, Department of Theoretical Chemistry & Biology, Royal Institute of Technology, 10691 Stockholm, Sweden
e-mail: taochen@kth.se

Received: 26 October 2018
Accepted: 14 December 2018

Abstract

Aims. Large compact polycyclic aromatic hydrocarbon molecules (PAHs) present special interest in the astrochemical community. A key issue in analyses of large PAHs is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the infrared (IR) spectra for molecules under various conditions.

Methods. Because of the huge amount of interactions/resonances in large PAHs, no anharmonic IR spectrum can be produced with static/time-independent ab initio method, especially for the molecules with D6h symmetry, e.g., coronene and circumcoronene. In this work, we performed molecular dynamics simulations to generate anharmonic IR spectra of coronene and circumcoronene.

Results. The method is validated for small PAHs, i.e., naphthalene and pyrene. We find that the semiempirical method PM3 produces accurate band positions with an error <5 cm⁻¹. Furthermore, we calculate the spectra at multiple temperatures and find a clear trend toward band shifting and broadening.