Coordination numbers in hydrated Cu(II) ions

The potential energy surface of [Cu(H₂O)_n]²⁺ clusters with n?=?12, 16, and 18 was explored by using a modified version of the simulated annealing method. Such exploration was carried out by using the PM7 semiempirical method to obtain around 100,000 isomers, which provide candidates to be optimized with PBE0-D3, M06-2X, and BHLYP exchange-correlation functionals coupled with the 6–311++G** basis set. These methods based on the Kohn-Sham approach delivered isomers with coordination numbers of 4, 5, and 6. The analysis used to obtain coordination numbers was based on geometrical parameters and the quantum theory of atoms in molecules (QTAIM) approach. Our methodology found only one isomer with fourfold coordination and its probabilities to appear in these clusters are quite small for high temperatures. The procedure used in this article predicts important populations of fivefold and sixfold coordination clusters, in fact, the fivefold coordination dominates for PBE0-D3 and BHLYP methods, although the sixfold coordination starts to be important when the number of water molecules is increased. The nature of axial and equatorial contacts is discussed in the context of the QTAIM and the noncovalent interaction index (NCI), which gives a clear classification of such orientations. Also, these methods suggest a partial covalent interaction between the Cu²⁺ and water molecules in both positions; equatorial and axial.