Molecular Structure of the Aluminum Halides, Al₂Cl₆, AlCl₃, Al₂Br₆, AlBr₃, and AlI₃, Obtained by Gas-Phase Electron-Diffraction and ab Initio Molecular Orbital Calculations

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_2h, but the molecules have a very low-frequency, large-amplitude, ring-puckering mode that lowers the thermal average symmetry to C_2v. 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) = V₄⁰Φ⁴ + V₂⁰Φ², 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_i⁰/kcal mol deg^-1) from the combined GED/ab initio study for Al₂Cl₆/Al₂Br₆ with estimated 2σ uncertainties are Al−X_b = 2.250(3)/2.433(7), Al−X_t = 2.061(2)/2.234(4), X_bAlX_b = 90.0(8)/91.6(6), X_tAlX_t = 122.1(31)/122.1(31), 〈θ〉 = 180 − 2Φ = 165.5(59)/158.2(91), V₄⁰ = 0.0/75.0 (assumed), V₂⁰ = 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_3h 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.