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Download file# Molecular Structure of the Aluminum Halides, Al_{2}Cl_{6}, AlCl_{3}, Al_{2}Br_{6}, AlBr_{3}, and AlI_{3},
Obtained by Gas-Phase Electron-Diffraction and ab Initio Molecular Orbital Calculations

journal contribution

posted on 1999-03-04, 00:00 authored by Kirsten Aarset, Quang Shen, Hanne Thomassen, Alan D. Richardson, Kenneth HedbergGas-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 AlX

_{3}(X = Cl, Br, I) and the chloride and bromide dimers Al_{2}Cl_{6}and Al_{2}Br_{6}. No monomeric species were detected in the vapors of Al_{2}Cl_{6}at the experimental temperature of 150 °C, nor in Al_{2}Br_{6}at167 °C, but the vapors of AlCl_{3}at 400 °C and AlBr_{3}at 330 °C contained respectively 29 (3)% and 7 (4)% dimer and the AlI_{3}at 300 °C about 8% I_{2}. The known equilibrium symmetry of the dimers is*D*_{2}*, but the molecules have a very low-frequency, large-amplitude, ring-puckering mode that lowers the thermal average symmetry to*_{h}*C*_{2}*. 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}*V*(Φ) =*V*_{4}^{0}Φ^{4}+*V*_{2}^{0}Φ^{2}, 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 Al_{2}Cl_{6}/Al_{2}Br_{6}with estimated 2σ uncertainties are Al−X_{b}= 2.250(3)/2.433(7), Al−X_{t}= 2.061(2)/2.234(4), X_{b}AlX_{b}= 90.0(8)/91.6(6), X_{t}AlX_{t}= 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 AlCl_{3}, AlBr_{3}, and AlI_{3}(*D*_{3}*symmetry assumed in*_{h}*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.