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 ini tio molecular orbital and normal coordinate calculations have been used to determine the structures of the aluminum trihalides AIX(3) (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 1 50 degrees C, nor in Al2Br6 at 167 degrees C, but the vapors of AlCl3 at 40 0 degrees C and AlBr3 at 330 degrees C contained respectively 29 (3)% and 7 (4)% dimer and the AlI3 at 300 degrees C about 8% I-2. The known equilibriu m symmetry of the dimers is D-2h, but the molecules have a very low-frequen cy. large-amplitude, ring-puckering mode that lowers the thermal average sy mmetry to C-2v. The effect of this large-amplitude mode on the interatomic distances was handled by dynamic models of the structures which consisted o f a set of pseudoconformers spaced at even intervals along the ring-puckeri ng angle 2 Phi. The ring-puckering potential was assumed to be V(Phi) = V(4 )(0)Phi(4) + V(2)(0)Phi(2), and the individual pseudoconformers were given Boltzmann weights. The structures were defined in terms of the geometricall y consistent r(alpha) space constraining the differences between correspond ing bond distances and bond angles in the different pseudoconformers to val ues obtained from ab initio calculations at the HF/6-311G(d) level. Results for the principal distances (r(g)/Angstrom), angles (angle alpha,theta/deg ), and potential constants (V-i(0)/kcal mot deg(-1)) from the combined GED/ ab initio study for Al2Cl6/Al2Br6 with estimated 2 sigma uncertainties are Al-X-b = 2.250(3)/2.433(7), Al-X-t=2.061(2)/2.234(4), XbAlXb = 90.0(8)/91.6 (6), X(t)AIX(t) = 122, 1(31)/122. 1(31). [theta]= 180 - 2 Phi, = 165.5(59)/ 158.2(91), V-4(0) = 0.0/75.0 (assumed), V-2(0) = 25.0/0.0 (assumed). The po tential constants could not be refined: although the single-term values lis ted provide good fits, in each case combinations of quadratic and quartic t erms also worked well. For the monomers AlCl3, AlBr3, and AlI3 (D-3h symmet ry assumed in r(alpha) space) the distances (r(g)/Angstrom) with estimated 2 sigma uncertainties are Al-Cl = 2.062(3), AI-Br = 2.221(3), and AI-I = 2. 359(5) Angstrom. Vibrational force fields were evaluated for all molecules. The experimental, theoretical, and vibrational results are discussed.