Theoretical Study of [Ni (H2O)n]2+(H2O)m (n ≤ 6, m ≤ 18)

The [Ni-(H2O)(n)](2+)(H2O)(m) (n <= 6, m <= 18) complexes were studied by means of first-principles all-electron calculations performed with the BPW91 gradient corrected functional and the 6-311+G(d,p) basis sets for the H, O, and Ni atoms. Triplet states were found as low-lying states for each (n, m) combination. The estimated Ni2+-(H2O)(n) binding energies (112.8-57.4 kcal/mol for the first layer and 52.0-23.0 kcal/mol for the second one) decreases and the Ni2+-OH2 bond lengths lengthen as n + m increases. With six H2O moieties the Ni2+ ion furnishes its first coordination sphere of octahedral geometry. Further water addition renders the formation of the second layer. The effect of Ni2+ on the (H2O)(n) center dot center dot center dot (H2O)(m) hydrogen bond formation for several "n" and "m" combinations was studied, revealing an enhancement of this kind of bonding, which is of key importance for the stabilization and growth of the clusters. For some n m isomers the second layer appears before the first octahedral layer is fully formed. For example, the square planar Ni2+-(H2O)(4) core originates two-dimensional 4 + 2 and 4 + 4 isomers, where each outer water molecule accepts two H-bonds, lying 2.0 kcal/mol above the 6 and 6 + 2 ground states. The clusters were also studied by IR spectra; the OH stretching vibrational frequencies allowed us to identify the outer solvation shells by the presence of red-shifted hydrogen bond regions.