An automated approach for comparing spectra is also described that minimizes residuals between a given theoretical spectrum and all available experimental spectra. It calculates the MO compositions in terms of the constituent chemical fragments (you can specify them as atoms, groups of atoms, atomic orbitals, fragment molecular orbitals, groups of atomic orbitals, etc.) in the molecule or atom. A comparison of computed spectra in the gas phase indicates that the inclusion of n-octane shifts peaks by +11 nm on average and this change is roughly equivalent for PBE0 and CAM-B3LYP. AOMix is a user-friendly software for the molecular orbital (MO) analysis.
These calculated spectra can be empirically corrected to decrease the uncertainties to 6.5 ± 5.1 and 5.7 ± 5.1 nm for the PBE0 and CAM-B3LYP methods, respectively. Specifically, when solvent is included the PBE0 functional overestimates peaks by 16.1 ± 6.6 nm while CAM-B3LYP underestimates the same transitions by 14.5 ± 7.6 nm. All theoretical spectra visually conform to the profiles of the experimental data but are systematically offset by a small amount. Unusually high-resolution experimental spectra are obtained for comparison by analyzing each PAH at 4.2 K in an n-alkane matrix. Spectra for several of the most toxic PAHs are predicted and compared to experiment, including three isomers of C 24H 14 and a PAH containing heteroatoms. Two methods, PBE0 and CAM-B3LYP, are evaluated both in the gas phase and in solution.
The ability of density functional theory (DFT) methods to predict accurate fluorescence spectra for polycyclic aromatic hydrocarbons (PAHs) is explored.
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