Binding energies for the inner hydration shells of Ca2+: An experimental and theoretical investigation of Ca2+(H2O)x complexes (x = 5-9)

The sequential bond energies of Ca2+(H2O)x complexes, where x = 5-9, are determined by collision-induced dissociation (CID) using a guided ion beam tandem mass spectrometer with a recently developed electrospray ionization source. To our knowledge, this represents the first quantitative threshold CID study of multiply charged ions. The kinetic energy dependent cross sections are determined over a wide energy range to monitor all possible dissociation products and are modeled to obtain 0 and 298 K binding energies for loss of a single water molecule. These binding energies decrease monotonically for the Ca2+(H2O)5 complex to Ca2+(H2O)7 and plateau for Ca2+(H2O)7, Ca2+(H2O)8, and Ca2+(H2O)9. This suggests that six water molecules bind directly to the calcium ion and that three outer shell water molecules bind to inner shell water molecules through similar binding motifs. Our experimental results agree well with previous literature results obtained by equilibrium and BIRD studies. We also present an in-depth theoretical study of the structures and energetics of the Ca2+(H2O)x systems, employing several levels of theory. The present theoretical results focus on the larger hydrates (x = 8 and 9) where multiple low lying conformations are possible and there is little previous theory.