Hydration Energies of Zinc(II): Threshold Collision-Induced Dissociation Experiments and Theoretical Studies

The first experimentally determined sequential bond dissociation energies of Zn2+(H2O) n complexes, where n = 6−10, are measured using threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer coupled with an electrospray ionization source. Kinetic energy dependent cross sections are obtained and analyzed to yield 0 K threshold measurements for the loss of one and two water ligands after accounting for multiple collisions, kinetic shifts, and energy distributions. The threshold measurements are then converted from 0 to 298 K values to give the hydration energies for sequentially losing one water from each parent complex. Theoretical geometry optimizations and single-point energy calculations are performed using several levels of theory for comparison to experiment. Although different levels of theory disagree on the ground-state conformation of most complexes examined here leading to potential ambiguities in the final thermochemical values, calculations at the MP2(full) level provide the best agreement with experiment. On this basis, the present experiments are most consistent with the inner solvent shell of Zn2+ being five waters, except for Zn2+(H2O)6 where all waters bind directly to the metal ion. The charge separation process, Zn2+(H2O) n → ZnOH+(H2O) m + H+(H2O) n−m−1, which is in competition with the loss of water from the parent complex, is also observed for n = 6−8. These processes are analyzed in detail in the following paper.