Molecular structure of monomeric scandium trichloride by gas electron diffraction and density functional theory calculations on ScCl₃ and Sc₂Cl₆ †

Abstract

The molecular structures of monomeric and dimeric scandium trichloride were optimised by DFT calculations with basis sets of valence shell TZ + P quality, and the molecular force fields and normal vibrational modes calculated. Optimisation of ScCl₃ yielded an equilibrium geometry of D_3h symmetry and bond distance Sc–Cl 228.5 pm. Optimisation of a model of the dimer with double Cl bridges indicated an equilibrium geometry of D_2h symmetry, the terminal and bridging bond distances Sc–Cl_t 226.0 and Sc–Cl_b 247.5 pm, and the valence angles Cl_t–Sc–Cl_t 114.9 and Cl_b–Sc–Cl_b 86.6°. Synchronous gas electron diffraction (GED) and mass spectrometric (MS) data were recorded with the effusion cell kept at 900 ± 10 K. The gas was found to consist of 92 ± 2% monomer and 8 ± 2% dimer. Least-squares refinement of a trigonal pyramidal (C_3v) model of the monomer yielded the bond distance r_g(Sc–Cl) = 229.1(3) pm and a valence angle α Cl–Sc–Cl 119.8(5)°. The concentration of the dimer was too low for the GED data to give accurate structure parameters for this species. Bond energies for both monomer and dimer were calculated from thermochemical data in the literature and compared to corresponding energies in MCl₃ and M₂Cl₆, M = Al, Ga or In.

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