The molecular structure of TbI3, DyI3, HoI3 and ErI3 as determined by synchronous gas-phase electron diffraction and mass spectrometric experiment assisted by quantum chemical calculations

A first gas electron diffraction study of TbI3, DyI3, HoI3 and ErI3 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 rg-configuration parameters of the molecules in the series TbI3 → ErI3 reflect the lanthanide contraction. A low value of the shrinkage δ(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 4fn electronic state. B3LYP and МР2 calculations performed in this work with large core potential for lanthanide atoms also resulted in equilibrium geometry of D3h 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 LnI3. Different models applied in this study predict the equilibrium distance re(Ln-I) to be shorter than the thermal-averaged rg(Ln-I) distance by 0.002-0.018(1) Å.