The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione aluminium: gas-phase electron diffraction, quantum chemical calculations and X-ray crystallography

Abstract

The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione aluminium, or Al(thd)3, has been determined by quantum chemical (DFT) calculations, X-ray crystallography and gas-phase electron diffraction monitored by mass spectrometry (GED/MS). The DFT calculations yield an equilibrium structure of D 3 symmetry. The space group imposes no on the molecule in the crystalline phase. Nevertheless, the molecule structure obtained by X-ray crystallography has close to D 3 symmetry as indicated by the calculations. Refinement of a model of D 3 symmetry to the gas electron diffraction data yields the bond distances (r h1) Al–O = 1.891(4) A, C–O = 1.270(3) A and C–C = 1.406(3) A in the Al-chelate ring. The twist angle, defined as the angle of rotation of the triangle defined by three upper O atoms starting from the D 3h (triangular prism) reference configuration, is θ = 28.9(13)°. The coordination polyhedron of the Al atom is best described as slightly distorted octahedron.