Infrared Spectroscopy of Jet-cooled "grandPAHs" in the 3-100 μm Region

Although large polycyclic aromatic hydrocarbons (PAHs) are likely to be responsible for IR emission of gaseous and dusty regions, their neutral experimental high-resolution gas-phase IR spectra - needed to construct accurate astronomical models - have so far remained out of reach because of their nonvolatility. Applying laser desorption to overcome this problem, we report here the first IR spectra of the jet-cooled large PAHs coronene (C24H12), peropyrene (C26H14), ovalene (C32H14), and hexa(peri)benzocoronene (C42H18) in the 3-100 μm region. Apart from providing experimental spectra that can be compared directly to astronomical data, such IR spectra are crucial for assessing the accuracy of theoretically predicted spectra used to interpret interstellar IR emission. Here we use the experimental spectra to evaluate the performance of conventional calculations using the harmonic approximation, as well as calculations with an anharmonic (GVPT2) treatment. The harmonic prediction agrees well with the experiment between 100 and 1000 cm-1 (100 and 10 μm) but shows significant shortcomings in the combination band (1600-2000 cm-1, 6.25-5 μm) and CH-stretch (2950-3150 cm-1, 3.4-3.17 μm) regions. Especially the CH-stretch region is known to be dominated by the effects of anharmonicity, and we find that large PAHs are no exception. However, for the CH out-of-plane region (667-1000 cm-1, 15-10 μm) the anharmonic treatment that significantly improves the predicted spectra for small PAHs leads to large and unrealistic frequency shifts, and intensity changes for large PAHs, thereby rendering the default results unreliable. A detailed analysis of the results of the anharmonic treatment suggests a possible route for improvement, although the underlying cause for the large deviations remains a challenge for theory.