Internuclear distance dependence of the spin–orbit coupling contributions to proton NMR chemical shifts

The internuclear distance dependence of the electronic spin–orbit coupling contribution to nuclear magnetic resonance shielding constants – which causes the internal heavy-atom chemical shift of the proton shielding – is investigated using quadratic response theory. Calculations using a complete active space self-consistent field wavefunction on hydrogen iodide at different internuclear distances show a strong dependence of the spin–orbit coupling correction to the 1H shielding constant on internuclear separation. These results, combined with comparative calculations on HCl and HBr, indicate that the conformational dependence of the shielding constant is qualitatively changed when relativistic spin–orbit coupling corrections are important. This change is observed for HI and HBr, but not for HCl.