Experimental and ab initio molecular orbital techniques are developed for study of aluminum species with large quadrupole coupling constants to test structural models for methylaluminoxanes (MAO). The techniques are applied to nitrogen- and oxygen-containing complexes of aluminum and to solid MAO isolated from active commercial MAO preparations. (Aminato)- and (propanolato)aluminum clusters with 3-, 4-, and 6-coordinate aluminum sites are studied with three (27)Al NMR techniques optimized for large (27)Al quadrupole coupling constants: field-swept, frequency-stepped, and high-field MAS NMR. Four-membered (aminato)aluminum complexes with AlN(4) coordination yield slightly smaller C(q) values than similar AlN(2)C(2) sites: 12.2 vs 15.8 MHz. Planar 3-coordinate AlN(2)C sites have the largest C(q) values, 37 MHz. In all cases, molecular orbital calculations of the electric field gradient tensors yields C(q) and eta values that match with experiment, even for a large hexameric (aminato)aluminum cage. A D(3d) symmetry hexaaluminum oxane cluster, postulated as a model for MAO, yields a calculated C(q) of -23.7 MHz, eta = 0.7474, and predicts a spectrum that is too broad to match the field-swept NMR of methylaluminoxane, which shows at least three sites, all with C(q) values greater than 15 MHz but less than 21 MHz. Thus, the proposed hexaaluminum cluster, with its strained four-membered rings, is not a major component of MAO. However, calculations for dimers of the cage complex, either edge-bridged or face-bridged, show a much closer match to experiment. Also, MAO preparations differ, with a gel form of MAO having significantly larger (27)Al C(q) values than a nongel form, a conclusion reached on the basis of (27)Al NMR line widths in field-swept NMR spectra acquired from 13 to 24 T.

Andalusite, Al2SiO5, contains 5- and 6-coordinate aluminum sites, and is a preeminent model for 27Al NMR spectroscopy. We describe a combined NMR, crystallography, and theory project:  single-crystal 27Al NMR spectra at 298 K, the crystal structure of andalusite at 115 K, and electric field gradient calculations. The low-symmetry 5-coordinate site is a stiff test of the computational methods. In addition, the chemical shift tensor is measured for the 5-coordinate site. The small body of 27Al NMR data, especially for rare 5-coordinate sites, inspires the calculation of NMR parameters. We explore the accuracy of two approaches for 27Al EFG calculations:  first, ab initio molecular orbital calculations of small clusters embedded in an array of point charges; second, full-potential linearized augmented plane wave density functional calculations of the crystal. The agreement between the experimental EFG orientation and that from the full-crystal density functional theory is remarkably close, differing by only 0.17° for the 6-coordinate site and 1.56° for the 5-coordinate site. The calculated value of C q is in error by −0.254 MHz for the 5-coordinate site. The embedded cluster molecular orbital results are significantly less accurate, with orientation errors exceeding 45°.