Anharmonic vibrational spectroscopy of polycyclic aromatic hydrocarbons (PAHs)

Giacomo Mulas (1, 2) , Cyril Falvo (3) , Patrick Cassam-Chenaï (4) , Christine Joblin (5)

1 OAC - INAF - Osservatorio Astronomico di Cagliari
2 IRAP - Institut de recherche en astrophysique et planétologie
3 ISMO - Institut des Sciences Moléculaires d'Orsay
4 JAD - Laboratoire Jean Alexandre Dieudonné
5 CESR - Centre d'étude spatiale des rayonnements

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 dierence bands. In this article, we describe the methodology that was implemented and discuss how the main diculties were overcome, so as to keep the problem tractable. Benchmarking with high-level calculations was performed on a small molecule. We carried out specic 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 signicantly 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 eld. This is a rst step towards calculating moderately high-temperature spectra of PAHs and other similarly rigid molecules using Monte Carlo sampling.

Domaines: Chimie théorique et/ou physique

Giacomo Mulas, Cyril Falvo, Patrick Cassam-Chenaï, Christine Joblin. Anharmonic vibrational spectroscopy of polycyclic aromatic hydrocarbons (PAHs). Journal of Chemical Physics, 2018, 149 (14), ⟨10.1063/1.5050087⟩. ⟨hal-01890989⟩