Probing computational methodologies in predicting Mid-Infrared spectra for Large Polycyclic Aromatic Hydrocarbons (PAHs)

Abstract We extend the prediction of vibrational spectra to large sized Polycyclic Aromatic Hydrocarbon (PAH) molecules comprising up to ∼1500 carbon atoms by evaluating the efficiency of several computational chemistry methodologies. We employ classical mechanics methods (Amber and Gaff) with improved atomic point charges, semi-empirical (PM3, and DFTB) and density functional theory (B3LYP) and conduct global optimizations and frequency calculations in order to investigate the impact of PAH size on the vibrational band positions. We primarily focus on the following mid-infrared (MIR) emission bands 3.3, 6.2, 7.7, 8.6, 11.3, 12.7 and 17.0 $\mu$m. We developed a general Frequency Scaling Function ($\mathcal {FSF}$) to shift the bands and to provide a systematic comparison versus the three methods for each PAH. We first validate this procedure on IR scaled spectra from the NASA Ames PAH Database, and extend it to new large PAHs. We show that when the $\mathcal {FSF}$ is applied to the Amber and Gaff IR spectra, an agreement between the normal mode peak positions with those inferred from the B3LYP/4-31G model chemistry is achieved. As calculations become time intensive for large sized molecules Nc > 450, this proposed methodology has advantages. The $\mathcal {FSF}$ has enabled extending the investigations to large PAHs where we clearly see the emergence of the 17.0 $\mu$m feature, and the weakening of the 3.3 $\mu$m one. We finally investigate the trends in the 3.3 $\mu$m/17.0 $\mu$m PAH band ratio as a function of PAH size and its response following the exposure to fields of varying radiation intensities.