A Density Functional Theory Study of the Structure and Energetics of Zincate Complexes

We have conducted a density functional theory (DFT) investigation of zincate species. The accuracy of the DFT/B3LYP method and the adequacy of the atomic basis sets employed were established through investigation of the ionization potentials of Zn, the geometry and bond energy of ZnO, and the geometries and energies of selected Zn−OH and Zn−H2O complexes. Our investigation revealed that the Zn(OH)+, Zn(OH)2, and Zn(OH)3- zincate complexes are stable in the gas phase. However, we found that dissociated Zn(OH)3- + OH- is more stable than Zn(OH)42- in the gas phase and that the gas-phase geometry of Zn(OH)42- differs significantly from that gleaned from experimental studies of aqueous KOH/zincate solutions. We also investigated zincate complexes involving molecular water and K+ cations in order to better understand the influence of condensed phase effects in aqueous KOH solutions on the stability and geometry of the zincate complexes. We found that water does not significantly influence complex binding energies or the geometries of the underlying Zn(OH)n2-n complexes for n = 1, 2, and 3. In contrast, for Zn(OH)42- the introduction of water strongly stabilizes the complex relative to the gas phase and results in a structure close to that observed experimentally. We were unable to find a stable Zn(OH)4(H2O)22- complex with a planar Zn(OH)4 arrangement and close Zn−H2O coordination, corresponding to a Zn−O coordination of number of six, as has been suggested in some interpretations of experiments. We found through investigation of the K2Zn(OH)4 complex that K+ cations are also effective in engendering a structure that is very close to experiment and that K+ ions are even more strongly bound to the Zn(OH)42- complex than water. Finally, we determined the structure and stability of ZnO(OH)22-(oxodihydroxozincate), a species that has been hypothesized to be important in water-poor zincates solutions.