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 perturbation (FP) method including a previously proposed spin-orbit (SO) interaction. When the SO interaction is included, the calculated chemical shifts agree well with experiment. The SO effects become large in the heavier halogen compounds, AlBr4- and AlI4-. The paramagnetic term and the SO term are important in the chemical shifts of these compounds. The paramagnetic term is governed by the Al valence p electron mechanism and the SO term arises from the Fermi contact interaction in the Al valence s-orbital. The twofold halogen dependences, namely the normal halogen dependence and the inverse halogen dependence, observed for these compounds arise from the SO effect and the p-electron mechanism, respectively.