Analysis of the overestimation of the infrared intensity of the C-H stretching band in Polycyclic Aromatic Hydrocarbons

International audienceThe infrared spectra of Polycyclic Aromatic Hydrocarbons (PAH) and related systems, such as fullerenes, have been extensively studied both from an experimental and from a theoretical perspective [1]. In particular in the astrophysical community, the interest in their IR signatures lies in the fact that PAH-like molecules (and/or clusters thereof) are most likely the carriers of the so-called Aromatic Infrared Bands (AIBs) [2]. In this sense, only recently the first unambiguous assignment of an AIB band has been achieved [3]. Typically, the determination of the vibrational structure of large systems (PAH included) relies on vibrational second-order perturbation theory (VPT2) calculations on a potential energy surface (PES) expanded in a Taylor series up to fourth order [4]. The VPT2 method in conjunction with the Density Functional Theory (DFT) has been shown to be very efficient in the computation of anharmonic vibrational energy levels, with the exception of some types of resonances. In the specific case of PAHs, this approach has been shown to be able to predict to a very good agreement the frequencies and intensities of most of the IR bands of PAHs. However, the intensity of the C-H stretching band is constantly overestimated [5]. With this problem in mind, we have carried out a systematic investigation of the IR spectrum, more specifically, of its C-H stretching region, for two related molecules, benzene and the smallest PAH, naphthalene. To this end, we have computed and compared the harmonic and anharmonic IR spectra of these two systems with several DFT functionals, with the Complete Active Space Self Consistent Field (CASSCF) and with its Second-order Perturbation extension (CASPT2).References:[1] G. Rouillé, C. Jäger, M. Steglich, F. Huisken, T. Henning, G. Theumer, I. Bauer, and H. Knölker, Chem. Phys. Chem. 9, 2085 (2008).[2] A. G. G. M. Tielens, Rev. Mod. Phys. 85, 1021 (2013).[3] E. K. Campbell, M. Holz, D. Gerlich, and J. P. Maier, Nature 523, 322 (2015).[4] V. Barone, J. Chem. Phys. 122, 014108 (2005).[5] C. J. Mackie, A. Candian, X. Huang, E. Maltseva, A. Petrignani, J. Oomens, W. J. Buma, T. J. Lee, and A. G. G. M. Tielens, J. Chem. Phys. 143, 224314 (2015)