Theory of Localized Contributions to the Chemical Shift. Application to Fluorobenzenes

An expression for the magnetic shielding tensor is obtained by the use of single-determinant Hartree-Fock molecular wave functions. For nuclei of atoms in which the change in the second-order (paramagnetic) contribution is dominant, LCAO theory is employed to express the shielding in terms of localized bond parameters (ionic character, hybridization, and double bonding) and to compare it with the related treatments of the quadrupole coupling constant. Application of the formulation to the multifluorobenzenes provides an explanation of the available experimental chemical shift data and permits the prediction of shift values for other compounds. Of particular interest is the demonstration that double bonding in the C–F bond plays an important role in the fluorobenzenes. Also, the presence of an ``ortho'' effect in the shift is isolated by a comparison of experimental and theoretical results and tentatively explained in terms of charge repulsions.