An intricate molecule: Aluminum triiodide. Molecular structure of AlI 3 and Al2i6 from electron diffraction and computation

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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, Al 2I6, 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, rg, 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 D3h, 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 D2h symmetry structure.

Original languageEnglish
Pages (from-to)3770-3777
Number of pages8
JournalJournal of Physical Chemistry A
Issue number10
Publication statusPublished - Mar 16 2006

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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