How fluorine substituents strengthen aryl C-H bonds

We have investigated the nature and bond dissociation energies (BDEs) of the aromatic C-H bonds in fluorinated benzenes C₆R₅H (each R can be H or F) using quantitative Kohn-Sham molecular orbital theory and a matching energy decomposition analysis (EDA). The C-H bond becomes stronger as the number of fluorine atoms in the benzene ring increases. This increase in the calculated BDE is additive and most pronounced for ortho-substituted C-H bonds. Our analyses of the C-H bond between and H˙ reveal that a fluorine inductive effect is responsible for this. Fluorine polarizes the closed-shell molecular orbitals of away from the carbon radical center and in this way reduces Pauli repulsion between [C˙] and the H˙ radical, leading to a stronger C-H bond. The ortho effect can be accurately modelled by a combination of Pauli repulsion (main contribution) and orbital interactions. We extend our analysis to other substituents, including ones with the opposite effect on C-H bond strength.