Self-Consistent Molecular Orbital Methods. XI. Molecular Orbital Theory of NMR Chemical Shifts

Abstract

Ab initio molecular orbital calculations of first row atom (C, N, O and F) NMR chemical shifts are presented for a variety of polyatomic molecules. Both minimal and extended basis sets are used and it is shown that at the minimal level, basis sets at the Hartree–Fock atomic level give a better description of magnetic shielding than do Slater-type (exponential) orbitals. The extended basis set also gives a good description of magnetic shielding, features associated with anisotropic molecular environment being some-what more adequately treated at this level. Calculated chemical shifts of nuclei which have similar positions relative to the origin of the vector potential describing the magnetic field are in good agreement with experiment. Agreement for the chemical shifts of nuclei which have markedly different positions relative to this origin is not as good.