A new method for distinguishing between Al2X6 (X=Cl, Br) conformers based on ab initio calculated nuclear quadrupole coupling constants

Nuclear quadrupole coupling constants (χ) of 27Al, 35Cl and 81Br in AlX3 monomers as well as Al2X6 (X=Cl, Br) dimers are calculated at the RHF/6-311G* and B3LYP/6-311G* levels, using Gaussian 98 package. Correlations are made between χ and dihedral angles θ, of Al2X6. These θs are produced through the ring puckering motions about the hinge line which joins the two bridge halogens (Xb). Nuclear quadrupole coupling constants of 35Cl, 81Br and 27Al are used as probes for monitoring the departure of the symmetry of Al2X6 from a high symmetry point group D2h to a lower one. The χs of chlorine nuclei of AlCl3 differ significantly from those of Al2Cl6. These differences appear negligible for AlBr3 in comparison to Al2Br6. This work demonstrates the considerable sensitivity of nuclear quadrupole resonance in distinguishing between Al2X6 conformers. This is in comparison to the usage of energy differences which is customarily employed.