High coordination number actinide-noble gas complexes; a computational study

Abstract

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 AnHe₁₇^q+ (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 → An^q+ charge transfer. The interatomic exchange–correlation term V_xc 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 V_xc. 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.