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

Infrared spectra are computed for neutral and cationic clusters of Polycyclic Aromatic Hydrocarbon molecules, namely ${\left(C_{16}{H}_{10}\right)}_{n=1,4}^{(0/+)}$, 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.