A transparent interpretation of the relativistic contribution to the N.M.R. ‘heavy atom chemical shift’

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Abstract
The N.M.R. chemical shift problem is formulated so that a correct nonrelativistic limit is obtained for the paramagnetic term using Extended Hückel level wavefunctions. Then the relativistic contributions are identified from a REX-EHT comparison in second-order perturbation theory. The results obtained for hydrogen halides, HX, agree with the observed experimental trend. The main mechanisms are the spin-orbit induced proton spin density in the π1/2 MO and the spin-orbit induced Zeeman term in the 3σ MO. The results obtained for 13C shifts in haloacetylenes and methyl halides are qualitatively correct, but too small. Possible reasons are discussed.