The molecular structure of TbI_3, DyI_3, HoI _3 and ErI_3 as determined by synchronous gas-phase electron diffraction and mass spectrometric experiment assisted by quantum chemical calculations

摘要

A first gas electron diffraction study of TbI_3, DyI_3, HoI_3 and ErI_3 has been carried out in combination with mass spectrometric vapour monitoring at 1117(10), 1117(10), 1132(10) and 1102(10) K, respectively. Up to 4 mol.% of dimeric species was observed in addition to the dominating monomeric molecules. The change of the thermal-averaged r_g-configuration parameters of the molecules in the series TbI_3 → ErI_3 reflect the lanthanide contraction. A low value of the shrinkage 8(I...I) even at such a high temperatures may be considered due to vibration effects in molecule whose equilibrium geometric nuclear structure is planar and which correspond to configurationally-averaged 4f~n electronic state. B3LYP and MP2 calculations performed in this work with large core potential for lanthanide atoms also resulted in equilibrium geometry of D_(3h) symmetry. According to the both quantum chemical methods applied the potential function the non-planar vibration is essentially anharmonic, which is therefore to be taken into account to correctly describe nuclear dynamics in molecules such as LnI_3. Different models applied in this study predict the equilibrium distance r_e(Ln-I) to be shorter than the thermal-averaged r _g(Ln-I) distance by 0.002-0.018(1) ?.