Theory of NMR Chemical Shifts in Conjugated Fluorine Compounds

Abstract

Ramsey's equation for the nuclear magnetic resonance screening constant is analyzed employing a Hartree—Fock ground-state wavefunction. Using the LCAO approximation, a general expression is obtained which relates the screening parameter to localized charge distribution quantities (charge and bond orders). This expression is equivalent to one given by Karplus and Das. Pertinent two-center integrals over atomic orbitals are evaluated, employing, in part, the Fourier integral folding theorem. The relative magnitudes of the integrals over atomic orbitals makes possible a simplification of the general equation for F19 shielding to a form involving quantities associated with the fluorine atom and the adjacent carbon atom. Under the assumption of an invariant sigma framework, the F19 chemical shift between two similar conjugated fluorine compounds is shown to be determined chiefly by the values of the fluorine pi-electron charge densities and the carbon—fluorine pi-electron bond orders. The latter terms are almost as important as the former. The groundwork is laid for applying the derived equations, in a subsequent paper, to several systems of aromatic fluorine compounds.