An Intricate Molecule:  Aluminum Triiodide. Molecular Structure of AlI₃ and Al₂I₆ from Electron Diffraction and Computation

The molecular structure of aluminum triiodide was investigated in the gas phase by high-temperature gas-phase electron diffraction and high-level computations. The geometries of monomeric, AlI₃, and dimeric, Al₂I₆, molecules were determined from two separate experiments carried out under carefully controlled conditions to prevent decomposition. This is the first experimental determination of the dimer structure by modern techniques. The computed geometrical parameters strongly depend on the applied methods and basis sets as well as on core-valence correlation effects. The electron diffraction thermal average bond length, r_g, of AlI₃ at 700 K is 2.448(6) Å; while those of Al₂I₆ at 430 K are 2.456(6) Å (terminal) and 2.670(8) Å (bridging). The equilibrium geometry of the monomer molecule is planar with D_3h symmetry. The dimer molecule is extremely floppy, and it is difficult to determine the symmetry of its equilibrium geometry by computation, as it is sensitive to the applied methods. MP2 and CCSD calculations find the Al₂I₆ molecule puckered with C_2v symmetry (although with a very small barrier at planarity), while density functional methods give a structure with a planar central ring of D_2h symmetry. Comparison of the computed vibrational frequencies with the gas-phase experimental ones favors the D_2h symmetry structure.