Spin-orbit effect on the magnetic shielding constant using the ab initio UHF method. Electronic mechanism in the aluminum compounds, A1X4− (X = H, F, Cl, Br and I)

The 27Al NMR chemical shifts of the compounds A1X4− (X = H, F, Cl, Br and I) are studied theoretically by the ab initio UHF/finite perturbation (FP) method including a previously propsed spin-orbit (SO) interaction. When the SO interaction is included, the calculated chemical sshifts 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 those compounds arise from the SO effect and the p-electron mechanism, respectively.