How perfluorination alters PAH stability: Benchmarking DFT with CCSD(T) isomerization energies of perfluorinated PAHs

Abstract

We construct a comprehensive database of CCSD(T) isomerization energies of 64 polycyclic aromatic fluorinated compounds using the G4(MP2) composite ab initio method. This database includes a diverse range of structural motifs, including planar and non-planar configurations, and exhibits unique stability trends influenced by steric effects and F•••F repulsion. Perfluorination significantly alters the relative stabilities of polycyclic aromatic hydrocarbon isomers, with some isomer sets exhibiting near-complete reversals in energetic ordering. We use the G4(MP2) benchmark isomerization energies to evaluate the performance of DFT functionals. D4 dispersion corrections are critical for attaining mean absolute deviations (MADs) below the threshold of chemical accuracy. The best-performing functionals from each rung of Jacob's Ladder are (MADs given in parentheses): BLYP-D4 (4.5), M06-L-D4 (4.8), B3PW91-D4 (3.4), and PW6B95-D4 (3.3 kJ mol⁻¹). This study underscores the current limitations of many DFT functionals and provides critical guidance for future studies on electron-deficient aromatic systems, including perfluorinated nanomaterials.