HOW DO SPIN-ORBIT-INDUCED HEAVY-ATOM EFFECTS ON NMR CHEMICAL-SHIFTS FUNCTION - VALIDATION OF A SIMPLE ANALOGY TO SPIN-SPIN COUPLING BY DENSITY-FUNCTIONAL THEORY (DFT) CALCULATIONS ON SOME IODO COMPOUNDS

A simple concept is delineated for the interpretation of spin-orbit-in duced heavy-atom effects on NMR chemical shifts, The spin polarization induced by heavy-atom spin-orbit coupling is known to interact with t he nuclear magnetic moments of the system mainly by a Fermi-contact me chanism. The rules governing the propagation of these effects through the molecule are thus closely analogous to the well-established mechan isms for indirect Fermi-contact nuclear spin-spin coupling. The scope of this analogy is evaluated by explicit DFT calculations of spin-orbi t shifts and of spin-spin coupling constants in some simple iodo-subst ituted compounds. We find that, for example. the magnitude of the spin -orbit-induced shift on direct neighbor NMR nuclei B of the heavy atom A increases with increasing s-orbital contribution from the NMR atom B to the B-A bond, For the beta-hydrogen atoms in iodoethane, a modifi ed Karplus-type relationship is found to hold between the spin-orbit s hift and the dihedral The patterns of C-13 and H-1 shifts in iodobenze ne are also significantly affected by spin-orbit coupling, and the var iations of the calculated spin-orbit shifts closely follow those of th e calculated reduced spin-spin coupling constants. Tn fact, the observ ed zigzag H-1 shift pattern in iodobenzene is due entirely to spin-orb it effects. Typically, a negative reduced coupling constant correspond s to a shielding spin-orbit shift, and a positive coupling to a deshie lding spin-orbit contribution. Many further consequences of the descri bed analogy are discussed.