On a New Path to Computing the Vibrational Spectra of PAHs

Polycyclic aromatic hydrocarbons are big, largely amorphous, and ubiquitous. This makes them both incredibly important (notably for environmental science and astrophysics) and incredibly difficult to precisely describe the infrared spectral features of a single unique PAH specimen type in the laboratory. Quantum chemical computations have recently demonstrated the ability to produce vibrational frequencies for a selection of molecules to as good as within 1.0 cm-¹ of experiment and can do so conclusively for a single molecule since only one chemical system is input into the computations. However, these computations are incredibly costly making them applicable only to small molecules. Recent work in our group has shown that newer quantum chemical theory such as explicitly correlated methods (F12b) can be utilized effectively to produce experimentally-comparable vibrational frequencies at a reduced cost, but this still only extends the sizes of the molecules to be studied potentially up to benzene. However, reparamenterized semi-empirical methods designed solely to treat hydrocarbons are showing promise in predicting the spectra of small molecules with a significant savings in time. Successes include c-C₃H₂, C₃H₅⁺, and HOCO⁺ among others. These methods are currently being extended to PAHs.

NASA Grant NNX17AH15G.