The molecular structure of tris(dipivaloylmethanato) thulium: Gas-phase electron diffraction and quantum chemical calculations

Abstract The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione thulium, or Tm(thd) 3 , has been studied by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and quantum chemical (DFT) calculations. Both the DFT(PBE0) calculations and the GED data collected at 400(8) K indicate that the molecules have D 3 symmetry with a distorted antiprismatic TmO 6 coordination geometry. According to GED refinements the twist angle θ , i.e. the angle of rotation of the upper O 3 triangles relative to their position in regular prism is θ  = 16.9(2.0) 0 . This value is close to both the equilibrium value obtained from the DFT calculations and to the thermal average value at the temperature of the GED experiment obtained by integration over the DFT potential energy surface. The bond distances ( r h1 ) in the chelate ring are Tm O = 2.214(5) A, C O = 1.278(4) A, and C C = 1.404(3) A. The DFT calculations yielded structure parameters in close agreement with those found experimentally. As an alternative to conventional Lewis model which was realized in NBO the topological analysis of ρ(r) in the frame of Bader's quantum theory of atoms in molecule (QTAIM) was performed.