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

Abstract

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.

Experimental measurements of water molecule binding energies for the second and third solvation shells of [Ca(H 2 O) n ] 2+ complexes

“…Overall, the binding energies reported here have slightly lower values than those recorded by other groups, but for the most part the error bars do overlap with those assigned to the existing data [13,15,18]. In addition to issues discussed above regarding signal levels for the smaller complexes, it is most probable that the ions studied in these experiments have higher internal temperatures than those generated at or close to 298 K [13,15,18]; however, an elevated temperature is more likely to result in a small increase in binding energy [34]. Where temperature could have an effect is in the case of the most obvious mismatch between these results and those recorded previously [13,15,18], namely [Ca(H 2 O) 6 ] 2+ .…”

“…However, because the objective here is to extend the range of measurements out as far as the third solvation shell, it can be seen from figure 2 that this can be achieved with results that have comparatively small experimental errors. In addition, there are already a number of more accurate measurements from other groups on the smaller complexes and some of these are also plotted in figure 2 [13,15,18]. Overall, the binding energies reported here have slightly lower values than those recorded by other groups, but for the most part the error bars do overlap with those assigned to the existing data [13,15,18].…”

“…In addition, there are already a number of more accurate measurements from other groups on the smaller complexes and some of these are also plotted in figure 2 [13,15,18]. Overall, the binding energies reported here have slightly lower values than those recorded by other groups, but for the most part the error bars do overlap with those assigned to the existing data [13,15,18]. In addition to issues discussed above regarding signal levels for the smaller complexes, it is most probable that the ions studied in these experiments have higher internal temperatures than those generated at or close to 298 K [13,15,18]; however, an elevated temperature is more likely to result in a small increase in binding energy [34].…”

“…In addition to issues discussed above regarding signal levels for the smaller complexes, it is most probable that the ions studied in these experiments have higher internal temperatures than those generated at or close to 298 K [13,15,18]; however, an elevated temperature is more likely to result in a small increase in binding energy [34]. Where temperature could have an effect is in the case of the most obvious mismatch between these results and those recorded previously [13,15,18], namely [Ca(H 2 O) 6 ] 2+ . In their measurements of hydration energies for alkaline earth metal ions, Rodriguez-Cruz et al .…”

Thermochemistry of Non-Covalent Ion–Molecule Interactions

“…A typical example can be found in ref. 36. The final result is generally the threshold energy at 0 K for dissociation of a ligand from the reactant metal-ligand complex.…”