Fluorine chemical shifts in aromatic systems

The fluorine chemical shifts in halogen-substituted benzenes have been calculated and compared with experimental values. It is found that the co-called van der Waals shift makes a substantial contribution to the total chemical shift particularly when the substituent is ortho to the fluorine nucleus. This van der Waals shift is strongly dependent on the molecular geometry so that fluorine chemical shifts could be a possible source of information about overcrowding in molecules.