Approximate Theories of Chemical Shift

Starting from coupled Hartree—Fock perturbation theory, approximate expressions are derived for the magnetic shielding of a nucleus in a magnetic field. Some of these expressions are compared with existent approximate formulas which have been derived by other methods. The concept of localized contributions is explored and illustrated in the case of the diatomic molecules LiH and BH, for which exact coupled Hartree—Fock calculations have been performed.