Al27 Quadrupole Coupling in Solid AlCl3

The nuclear quadrupole coupling constant of Al27 in solid anhydrous AlCl3 was found to be 472±24 kc/sec with an electric field gradient asymmetry parameter of zero. Assuming the crystal to be completely ionic, the electric field gradient was calculated for the layered AlCl3 structure by summing the contributions of the near ions, and treating the rest of the crystal as being made up of uniformly charged planes. This calculation gives an Al27 quadrupole coupling constant of —2.78 Mc/sec and an electric field gradient asymmetry parameter of zero. The addition of some covalent bonding tends to reduce the magnitude of the coupling constant and bring it into better agreement with the experimental value. It is estimated that the crystal is two-thirds ionic and one-third covalent.