Thermochemical Investigations of Hydrated Nickel Dication Complexes by Threshold Collision-Induced Dissociation and...

The experimental bond energies of Ni²⁺(H₂O)_x complexes, where x = 4-11, are determined by threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer with an electrospray ionization source. The electrospray ionization source produces a distribution of Ni²⁺(H₂O)_x complexes, where an in-source fragmentation technique is employed to access the x = 4-6 complexes and control the population of excited isomers. The kinetic energy-dependent cross sections are modeled to yield 0 K bond energies for sequential loss of neutral water molecules, which are converted to 298 K binding energies. Analysis of the primary and secondary water losses from the Ni²⁺(H₂O)_x reactant ion complexes, x = 4-11, provide accurate thermochemistry for the hydration energies of Ni²⁺ and yield the first experimental values for x = 4 and 5 binding energies. Speculative thermochemistry for excited isomers of the x = 4-6 complexes is also obtained. Quantum chemical calculations explore the relative energies of possible geometries. Theoretical bond energies for ground structures are used for direct comparison with experimental values. Our experimental results agree well with previously calculated and experimentally obtained binding enthalpies as well as with the more extensive quantum chemical calculations performed here.