SPIN-ORBIT EFFECT ON THE MAGNETIC SHIELDING CONSTANT USING THE AB-INITIO UHF METHOD - ELECTRONIC MECHANISM IN THE ALUMINUM COMPOUNDS, ALX(4)(-) (X=H,F,CL,BR AND I)

The Al-27 NMR chemical shifts of the compounds AIX(4)(-) (X = H, F, Cl , Br and I) are studied theoretically by the ab initio UHF/finite pert urbation (FP) method including a previously proposed spin-orbit (SO) i nteraction. When the SO interaction is included, the calculated chemic al shifts agree well with experiment. The SO effects become large in t he heavier halogen compounds, AlBr4- and AlI4-. The paramagnetic term and the SO term are important in the chemical shifts of these compound s. The paramagnetic term is governed by the Al valence p electron mech anism and the SO term arises from the Fermi contact interaction in the Al valence s-orbital. The twofold halogen dependences, namely the nor mal halogen dependence and the inverse halogen dependence, observed fo r these compounds arise from the SO effect and the p-electron mechanis m, respectively.