Experimental Investigation of the Complete Inner Shell Hydration Energies of Ca2+: Threshold Collision-Induced Dissociation of Ca2+(H2O)x Complexes (x=2-8)

The sequential bond energies of Ca2+(H2O)(x) complexes, where x = 1-8, are measured by threshold collision-induced dissociation (TCID) in a guided ion beam tandem mass spectrometer. From an electrospray ionization source that produces an initial distribution of Ca2+(H2O)(x) complexes where x = 6-8, complexes down to x = 2 are formed using an in-source fragmentation technique. Ca2+(H2O) cannot be formed in this source because charge separation into CaOH+ and H3O+ is a lower energy pathway than simple water loss from Ca2+(H2O)(2). The kinetic energy dependent cross sections for dissociation of Ca2+(H2O)(x) complexes, where x = 2-9, are examined over a wide energy range to monitor all dissociation products and are modeled to obtain 0 and 298 K binding energies. Analysis of both primary and secondary water molecule losses from each sized complex provides thermochemistry for the sequential hydration energies of Ca2+ for x = 1-8 and the first experimental values for x = 1-4. Additionally, the thermodynamic onsets leading to the charge separation products from Ca2+(H2O)(2) and Ca2+(H2O)(3) are determined for the first time. Our experimental results for x = 1-6 agree well with previously calculated binding enthalpies as well as quantum chemical calculations performed here. Agreement for x = 1 is improved when the basis set on calcium includes core correlation.