CORRELATION OF F-19 CHEMICAL SHIELDING AND CHEMICAL-SHIFT NONEQUIVALENCE

The nonequivalence of the chemical shift of nuclei in equivalent bonding environments but inequivalent intermolecular environments can arise in different ways, and electrostatic perturbation is one of the most direct mechanisms. We have carried out large basis set ab initio calculations for a collection of fluorine-substituted hydrocarbons experiencing the electrostatic influence of an external ideal electric dipole. As a function of the separation from the dipole, the chemical shielding at the fluorine nucleus has been evaluated. We find that the change in shielding from the dipole perturbation serves as a secondary signature of the chemical bonding environment since it correlates with the shielding. The biggest effect of an external dipole on the isotopic shielding of a fluorine nucleus comes about when the fluorine is bonded to a doubly-bonded or aromatic carbon. A lesser effect develops from attachment to a triply-bonded carbon, and the least from attachment to a singly-bonded carbon. For the series of molecules, there is a striking, near-linear correlation of the dipole induced shift and the paramagnetic shielding, making the nonequivalence of shifts an indirect measure of the paramagnetic component of the shielding. The size of the nonequivalence effects associated with a nearby dipole can be on the order of 20 ppm for the typical charge fields of small molecules (e.g., water) at van der Waals separations from fluorine.