Theory of magnetic susceptibilities and NMR chemical shifts in terms of localized quantities. II. Application to some simple molecules

TLDR

The study aims to reformulate a coupled Hartree–Fock IGLO method for computing magnetic susceptibility and NMR shifts of small molecules. The authors apply this reformulated IGLO approach to calculate χ and σ for various small molecules. The method yields excellent agreement with experiment, shows little basis‑size sensitivity, reveals transferable local diamagnetic contributions while local paramagnetic and nonlocal terms are not, and provides pictorial explanations of antishielding in F₂ and H₂CO and geometry‑dependent shifts in CH⁺₃.

Abstract

A coupled Hartree–Fock method with individual gauge for localized orbitals (IGLO) proposed previously is reformulated and applied to the calculation of the magnetic susceptibility χ and the chemical NMR-shifts σ of various small molecules. The agreement with experiment is usually very good. Unlike in traditional methods, the results are not very sensitive to the size of the basis, and the application to large molecules does not pose serious problems. The results are analyzed in terms of orbital contributions, and it is shown that local diamagnetic terms are transferable, while local paramagnetic as well as nonlocal contributions are not. Pictorial explanations of the large antishielding effects in F2 and H2CO are given. In CH+3, a large dependence of χ and σ on changes of the geometry (pyramidalization) is found.