Theory of Nuclear Magnetic Shielding in Diatomic Molecules

Abstract

A variation‐perturbation method is employed for the calculation of the magnetic shieldingtensor for the nuclei of a series of diatomic molecules (H2, Li2, N2, F2, LiH, HF, LiF). The SCF—LCAO—MO functions of Ransil are used as unperturbed solutions and polynomials are introduced to represent the orbital perturbations. For certain nuclei (e.g., F in HF, N in N2), special perturbation functions are required to account for the pi‐orbital nodes. Although no total shielding measurements are available, the paramagnetic contributions can be compared with those calculated from spin—rotational constants. Good agreement is found for H2, H in LiH and HF, Li and F in LiF, but larger errors appear for the other nuclei. Estimates for the shielding anisotropy Δσ N and average shielding 〈σ N 〉 of some of the nuclei are given that should be accurate to ±10%. Since relative chemical shifts for H, Li, and F in several molecules can be obtained from these results, some experimental tests of the theory by standard NMR measurements are possible.