Relativistic study of nuclear magnetic shielding constants: mercury dihalides

Relativistic ab initio calculations of the 199Hg nuclear magnetic shielding constant and the chemical shift of HgX2 (XCl, Br, I) are presented. The method is a combination of the relativistic spin-free no-pair theory of Sucher and Hess and the spin-orbit unrestricted Hartree-Fock (SO-UHF) method reported previously. For the 199Hg shileding constant, both the spin-free relativistic (SFR) term, like mass-velocity and Darwin terms, and the SO term are important and they strongly couple with each other. Without them the experimental trend in mercury dihalides cannot be explained even qualitatively. Since these relativistic terms stabilize the orbitals closer to the nucleus, a basis set having larger freedom near the nucleus is appropriate.