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Gas-phase electron-diffraction (GED) data together with results from ab initio molecular orbital and normal coordinate calculations have been used to determine the structures of the aluminum trihalides AlX3 (X = Cl, Br, I) and the chloride and bromide dimers Al2Cl6 and Al2Br6. No monomeric species were detected in the vapors of Al2Cl6 at the experimental temperature of 150 °C, nor in Al2Br6 at167 °C, but the vapors of AlCl3 at 400 °C and AlBr3 at 330 °C contained respectively 29 (3)% and 7 (4)% dimer and the AlI3 at 300 °C about 8% I2. The known equilibrium symmetry of the dimers is D 2 h , but the molecules have a very low-frequency, large-amplitude, ring-puckering mode that lowers the thermal average symmetry to C 2 v . The effect of this large-amplitude mode on the interatomic distances was handled by dynamic models of the structures which consisted of a set of pseudoconformers spaced at even intervals along the ring-puckering angle 2Φ. The ring-puckering potential was assumed to be V(Φ) = V 4 0Φ4 + V 2 0Φ2, and the individual pseudoconformers were given Boltzmann weights. The structures were defined in terms of the geometrically consistent r α space constraining the differences between corresponding bond distances and bond angles in the different pseudoconformers to values obtained from ab initio calculations at the HF/6-311G(d) level. Results for the principal distances (r g/Å), angles (∠α,θ/deg), and potential constants (V i 0/kcal mol deg-1) from the combined GED/ab initio study for Al2Cl6/Al2Br6 with estimated 2σ uncertainties are Al−Xb = 2.250(3)/2.433(7), Al−Xt = 2.061(2)/2.234(4), XbAlXb = 90.0(8)/91.6(6), XtAlXt = 122.1(31)/122.1(31), 〈θ〉 = 180 − 2Φ = 165.5(59)/158.2(91), V 4 0 = 0.0/75.0 (assumed), V 2 0 = 25.0/0.0 (assumed). The potential constants could not be refined; although the single-term values listed provide good fits, in each case combinations of quadratic and quartic terms also worked well. For the monomers AlCl3, AlBr3, and AlI3 (D 3 h symmetry assumed in r α space) the distances (r g/Å) with estimated 2σ uncertainties are Al−Cl = 2.062(3), Al−Br = 2.221(3), and Al−I = 2.459(5) Å. Vibrational force fields were evaluated for all molecules. The experimental, theoretical, and vibrational results are discussed.

The feasibility of multibeam diffraction for determining phases of structure factors is assessed on the basis of recent contributions and experiments in progress. It is shown that the method works well in situations in which the global interaction between X-ray photons and the crystal is weak, in which case diffraction takes place by single scattering events, and crystal perfection does not play a role in interpreting the experimental results. Two successful examples of phase determinations using the notion of virtual Bragg scattering are presented. One case is of particular interest, because the crystal (V3Si) is mosaic, and the phases were a priori unknown. Some problems and limitations of the method are encountered when trying to extend this technique to organic crystals of relatively large cell size, with spherical or irregular shape. Some data and calculations are presented for benzil, C14H1002, a crystal isomorphous 0108-7673/86/060533-06501.50 with quartz. It is concluded that the resolution presently available from standard laboratory set-ups is not adequate for application of this method to a crystal like benzil, but that use of synchrotron radiation beams will probably remove these obstacles.

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