Zn²⁺ Has a Primary Hydration Sphere of Five: IR Action Spectroscopy and Theoretical Studies of Hydrated Zn²⁺ Complexes in the Gas Phase

Complexes of Zn²⁺(H₂O)_n, where n = 6−12, are examined using infrared photodissociation (IRPD) spectroscopy, blackbody infrared radiative dissociation (BIRD), and theory. Geometry optimizations and frequency calculations are performed at the B3LYP/6-311+G(d,p) level along with single point energy calculations for relative energetics at the B3LYP, B3P86, and MP2(full) levels with a 6-311+G(2d,2p) basis set. The IRPD spectrum of Zn²⁺(H₂O)₈ is most consistent with the calculated spectrum of the five-coordinate MP2(full) ground-state (GS) species. Results from larger complexes also point toward a coordination number of five, although contributions from six-coordinate species cannot be ruled out. For n = 6 and 7, comparisons of the individual IRPD spectra with calculated spectra are less conclusive. However, in combination with the BIRD and laser photodissociation kinetics as well as a comparison to hydrated Cu²⁺ and Ca²⁺, the presence of five-coordinate species with some contribution from six-coordinate species seems likely. Additionally, the BIRD rate constants show that Zn²⁺(H₂O)₆ and Zn²⁺(H₂O)₇ complexes are less stable than Zn²⁺(H₂O)₈. This trend is consistent with previous work that demonstrates the enthalpic favorability of the charge separation process forming singly charged hydrated metal hydroxide and protonated water complexes versus loss of a water molecule for complexes of n ≤ 7. Overall, these results are most consistent with the lowest-energy structures calculated at the MP2(full) level of theory and disagree with those calculated at B3LYP and B3P86 levels.