Theory of Proton Magnetic Shielding

Abstract

If an unsymmetrized product of molecular orbitals is used to represent the ground state of a molecule, the proton magnetic shielding is the sum of contributions from each molecular orbital. In the simplest variation theory of the perturbation of these orbitals by the proton magnetic dipole and an external magnetic field, the perturbation vanishes if the vector potential representing the external field is caused to vanish at the charge centroid of the orbital. Proton magnetic shielding constants are evaluated on this basis with molecular orbitals of the form ψ(1) = [(1—γ)ψa2(1)+γψb2 (1)]½. This form was first examined by an energy variation on H2, the energy being minimized with respect to internuclear distance and a screening constant, and was then applied to proton magnetic shielding in H2. In subsequent calculations γ was evaluated from electric dipole moments when possible. Proton magnetic shielding constants were then evaluated for the C–H bond (methane, ethylene, and acetylene), the Group VI hydrides (H2O, H2S, H2Se), and the hydrogen halides (HF, HCl, HBr, HI).