Microsolvation of NO⁺ in Ar_n clusters: A theoretical treatment

At present, we investigate the structure and the stability of NO⁺Ar_n (n ≤ 54) ionic clusters using analytical potential functions. The energy of these systems is described using additive potentials with V_NO⁺Ar and V_Ar–Ar representing the pair potential interactions. To find the geometry of the lowest energy isomers of the NO⁺Ar_n clusters, we use the so-called basin hopping method of Wales et al. which combines a Monte-Carlo exploration and deformation method. The reliability of our model was checked by deriving the structures of the NO⁺Ar_n systems (n = 1, 2, 3 and 4) using ab initio Moller–Plesset perturbation theory up to second order (MP2) in connection with the aug-cc-pVTZ basis set. Magic numbers for sizes n = 8, 12, 18, 22, and 25 are found and they show a high relative stability. Our results reveal that a transition in the NO⁺ ion coordination from 8 (square antiprism) to 12 (icosahedrons) occurs for n = 11. Examination of the stable structures of the ionic clusters demonstrates that the first solvation shell closes at n = 12. Furthermore, we found that the NO⁺Ar_n (n = 12-54) clusters are structurally very similar to the homogenous rare gas clusters with a polyicosahedral packing pattern. The distribution exhibits an additional magic number at n = 54, consistent with the completion of a second solvation sphere around NO⁺. The effects of microsolvation of NO⁺ cation in Ar clusters are also discussed. Generally, our results agree with the available experimental and theoretical findings on NO⁺Ar_n clusters and more generally on diatomics solvated in Ar clusters.