INTERNUCLEAR DISTANCE DEPENDENCE OF THE SPIN-ORBIT-COUPLING CONTRIBUTIONS TO PROTON NMR CHEMICAL-SHIFTS

The internuclear distance dependence of the electronic spin-orbit coup ling contribution to nuclear magnetic resonance shielding constants - which causes the internal heavy-atom chemical shift of the proton shie lding - is investigated using quadratic response theory. Calculations using a complete active space self-consistent field wavefunction on hy drogen iodide at different internuclear distances show a strong depend ence of the spin-orbit coupling correction to the H-1 shielding consta nt on internuclear separation. These results, combined with comparativ e calculations on HCl and HBr, indicate that the conformational depend ence of the shielding constant is qualitatively changed when relativis tic spin-orbit coupling corrections are important. This change is obse rved for HI and HBr, but not for HCl. (C) 1998 Elsevier Science B.V. A ll rights reserved.