The molecular structure of PrI3 and GdI3 as determined by synchronous gas-phase electron diffraction and mass spectrometric experiment assisted by quantum chemical calculations

A gas electron diffraction study of PrI₃ and GdI₃ has been carried out in combination with mass spectrometric vapour monitoring at 1110(10) K and 1100(10) K, respectively. Up to 3 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 LaI₃ → LuI₃ reflects 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 4f ⁿ electronic state. B3LYP calculations performed in this study with large core potential for lanthanide atoms also resulted in equilibrium geometry of D _3h symmetry. According to the quantum chemical calculations, 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.