Molecular geometry of monomeric and dimeric aluminum tribromide from gas phase electron diffraction

Aluminum tribromide is present in the vapor phase in an equilibrium of monomeric and dimeric forms, with increasing amount of monomers as the temperature of the vapor increases. The molecular geometry of both forms has been determined by high-temperature electron diffraction, the monomer at 830 K and the dimer at 360 K. The electron diffraction investigation was coupled with quadrupole mass spectrometry. The monomer molecule is planar with rg(AlBr) = 2.231 ± 0.005 Å [rα(AlBr) = 2.216 ± 0.005 Å]. The dimer has a halogen-bridged structure with bond lengths of rg(AlBr)t = 2.227 ± 0.005 Å and rg(AlBr)b = 2.421 ± 0.005 Å and bond angles ∠aBrbAlBrb = 93.3 ± 0.2° and ∠aBrtAlBrt = 119.6 ± 0.7°.