Anharmonic vibrational spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs)

TL;DR

This paper introduces a new computational method for anharmonic vibrational spectroscopy of PAHs, enabling accurate IR spectra predictions at manageable costs, improving upon harmonic DFT results, and facilitating studies at non-zero temperatures.

Contribution

A novel code that accounts for all vibrational modes and resonances in PAHs, balancing accuracy and computational efficiency for large molecules.

Findings

Improved band position predictions over harmonic DFT.

Reasonable agreement of calculated intensities with experimental data.

Method demonstrates potential for high-temperature PAH spectra calculations.

Abstract

While powerful techniques exist to accurately account for anharmonicity in vibrational molecular spectroscopy, they are computationally very expensive and cannot be routinely employed for large species and/or at non- zero vibrational temperatures. Motivated by the study of Polycyclic Aromatic Hydrocarbon (PAH) emission in space, we developed a new code, which takes into account all modes and can describe all IR transitions including bands becoming active due to resonances as well as overtones, combination and difference bands. In this article, we describe the methodology that was implemented and discuss how the main difficulties were overcome, so as to keep the problem tractable. Benchmarking with high-level calculations was performed on a small molecule. We carried out specific convergence tests on two prototypical PAHs, pyrene (C$_{16}$H$_{10}$) and coronene (C$_{24}$H$_{12}$), aiming at optimising tunable parameters to achieve both acceptable accuracy and computational costs for this class of molecules. We then report the results obtained at 0 K for pyrene and coronene, comparing the calculated spectra with available experimental data. The theoretical band positions were found to be significantly improved compared to harmonic Density Functional Theory (DFT) calculations. The band intensities are in reasonable agreement with experiments, the main limitation being the accuracy of the underlying calculations of the quartic force field. This is a first step towards calculating moderately high-temperature spectra of PAHs and other similarly rigid molecules using Monte Carlo sampling.