Regular Article

Dependence upon charge of the vibrational spectra of small Polycyclic Aromatic Hydrocarbon clusters: the example of pyrene^★,★★

¹ Laboratoire de Chimie et Physique Quantique (LCPQ/IRSAMC), UMR5626, Université de Toulouse (UPS) and CNRS, 118 Route de Narbonne, 31062 Toulouse, France
² Laboratoire Collisions Agrégats Réactivité (LCAR/IRSAMC), UMR5589, Université de Toulouse (UPS) and CNRS, 118 Route de Narbonne, 31062 Toulouse, France

^a e-mail: mathias.rapacioli@irsamc.ups-tlse.fr

Received: 13 February 2020
Received in final form: 18 July 2020
Accepted: 1 September 2020
Published online: 1 November 2020

Abstract

Infrared spectra are computed for neutral and cationic clusters of Polycyclic Aromatic Hydrocarbon molecules, namely ${\left({\mathrm{C}}_{16}{\mathrm{H}}_{10}\right)}_{n=\mathrm{1,4}}^{(0/+)}$, using the Density Functional based Tight Binding scheme combined with a Configuration Interaction (DFTB-CI) in the double harmonic approximation. Cross-comparison is carried out with DFT and simple DFTB. Similarly to the monomer cation, the IR spectra of cluster cations are characterized by a depletion of the intensity of the CH stretch modes around 3000 cm⁻¹, with a weak revival for n = 3 and 4. The in-plane CCC modes in the region 1400–2000 cm⁻¹ are enhanced while the CH bending modes in the range 700–1000 cm⁻¹ are significantly weakened with respect to the monomer cation, in particular for n = 2. Finally, soft modes corresponding to diedral fluctuations of the monomers within the central stack of the ion structure, possibly mixed with monomer folding, are also observed in the region 70–120 cm⁻¹.