THEORETICAL-STUDY ON METAL NMR CHEMICAL-SHIFTS - ARSENIC AND ANTIMONY COMPOUNDS

The group XV As and Sb chemical shifts of the compounds AsClxF6-x- (x = 0-6), AsO43-, AsMe(4)(+), SbClxF6-x- (x = 0-6), SbS43-, SbMe(4)(+), SbCl5, and Sb(OH)(6)(-) are theoretically investigated by the ab initio Hartree-Fock/finite perturbation method. The calculated values are in good agreement with the experimental ones: large basis sets including FOBFs (first order basis functions) proposed previously are necessary to accomplish this agreement. Theoretical analysis reveals that the As and Sb chemical shifts are mainly dominated by the paramagnetic term and are due to the p-electron mechanism, that is, the shifts are parallel with the number of the valence p-electrons at the As and Sb atoms. As the coordination number increases, the valence p-electron population decreases and the chemical shift shows an upfield shift. In the four-coordinate compounds the major factor of the p-mechanism is the excitation energy. The U-shaped relationship observed for the MCl(x)F(6-x)(-) compounds (M = As and Sb; x 0-6) reflects the change in the valence p-electron density of the metal.