High Coordination Number Actinide-Noble Gas Complexes; a Computational Study

The geometries, electronic structures and bonding of early actinide-noble gas complexes are studied computationally by density functional and wavefunction theory methods, and by ab initio molecular dynamics. AcHe183+ is confirmed as being an 18-coordinate system, with all of the He atoms accommodated in the primary coordination shell, and this record coordination number is reported for the first time for Th4+ and Th3+. For Pa and U in their group valences of 5 and 6 respectively, the largest number of coordinated He atoms is 17. For AnHe17q+ (An = Ac, q = 3; An = Th, q = 4; An = Pa, q = 5; An = U, q = 6), the average An-He binding energy increases significantly across the series, and correlates linearly with the extent of He  Anq+ charge transfer. The interatomic exchange correlation term Vxc obtained from the Interacting Quantum Atoms approach correlates linearly with the An-He Quantum Theory of Atoms-in-Molecules delocalization index, both indicating that covalency increases from AcHe173+ to UHe176+. The correlation energy in AnHe163+ obtained from MP2 calculations decreases in the order Pa > Th > U > Ac, the same trend found in Vxc. The most stable complexes of Ac3+ with the heavier noble gases Ar–Xe are 12 coordinate, best described as Ng12 cages encapsulating an Ac3+ ion. There is enhanced Ng  Ac3+ charge transfer as the Ng gets heavier, and Ac-Ng covalency increases.