Nitrogen nuclear magnetic resonance spectroscopy. Part 8.—Variation of the local diamagnetic term in nuclear magnetic shielding, as derived theoretically and by X-ray photoelectron spectroscopy

Abstract

The dependence of σ^A, the local diamagnetic term in nuclear magnetic shielding, and E_b, the 1s-electron binding energy, on the potential at the nucleus due to the atomic valence electrons, has been used to test the local term approximation in nuclear magnetic shielding. Calculated values of σ^A_d for nitrogen in a group of small molecules were obtained from Pople's equations by a CNDO-CI method, and also by an extended Flygare method (EFM) in which the CNDO net atomic charges are used to interpolate the published Hartree–Fock σ_d values for the free atom and N^± ions. Comparison of the calculated values of Δσ^A_d with experimental values derived from ΔE_b(N1s) measured by X-ray photoelectron spectroscopy shows that the local diamagnetic term spans a range of 10–13 p.p.m., and depends on the degree of oxidation of the nitrogen. There is a variation of 4–5 p.p.m. among the nitrogen reference standards commonly used. In detailed correlations of σ^A_d with ΔE_b, due allowance should be made for differential relaxation in the photo-ionisation process, but the effects are relatively small.