Threshold collision-induced dissociation of Sr2+(H2O)x complexes (x=1–6): An experimental and theoretical investigation of the complete inner shell hydration energies of Sr2+

Abstract

The sequential bond energies of Sr(2+)(H(2)O)(x) complexes, where x=1-6, are determined by threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer equipped with an electrospray ionization source. The electrospray source produces an initial distribution of Sr(2+)(H(2)O)(x) complexes, where x=6-9. Smaller Sr(2+)(H(2)O)(x) complexes, where x=1-5, are accessed using a recently developed in-source fragmentation technique that takes place in the high pressure region of a rf-only hexapole ion guide. This work constitutes the first experimental study for the complete inner shell of any multiply charged ion. 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 Sr(2+)(H(2)O) complex to Sr(2+)(H(2)O)(6). Our experimental results agree well with previous literature results obtained by equilibrium and kinetic studies for x=5 and 6. Because there has been limited theory for the hydration of Sr(2+), we also present an in-depth theoretical study on the energetics of the Sr(2+)(H(2)O)(x) systems by employing several levels of theory with multiple effective core potentials for Sr and different basis sets for the water molecules.

Quantum Chemical Studies of the Hydration of Sr2+ in Vacuum and Aqueous Solution

“…Agreement with the calculations of reference [11] is extremely good, and that with the experimental TCID data is excellent when counterpoise corrections are included, with all but the n = 4 and n = 6 SBHs falling within the experimental error bars. Both our computational results and those of reference [11] show a marked improvement over those of reference [24], obtained at the MP2/6-31 + G* level. Our calculated n = 6 SBH is within the error bars of the experimental HPMS value.…”

“…[10] More recently, SBHs, SBSs and SBGs obtained from threshold collision-induced dissociation (TCID) data have been reported for smaller hydrates (1 n 6). [11] Experiments in the aqueous phase are clearly of great relevance to the understanding of microsolvation, since they probe directly the solvation shells of saturated species. It is found that the coordination number (CN) of the Sr dication in solution is higher than its gas-phase value of 6.…”

Threshold Collision‐Induced Dissociation of Hydrated Magnesium: Experimental and Theoretical Investigation of the Binding Energies for Mg2+(H2O)x Complexes (x=2–10)

“…These early reports include the alkaline earth (group 2) elements Ca 2 + and Sr 2 + , [1][2][3] the group 12 elements Zn 2 + and Cd 2 + , [4][5][6][7] as well as the transition metal Fe 2 + . In water, these factors control the acidity of aqueous solutions and constrain the local hydrogenbonding network.…”

“…Complexes smaller than Mg 2 + (H 2 O) 3 cannot be formed in this source because charge separation into MgOH + (H 2 O) and H 3 O + is a lower-energy pathway than simple water loss from Mg 2 + (H 2 O) 3 . From an electrospray ionization source that produces an initial distribution of Mg 2 + (H 2 O) x complexes in which x = 7-10, complexes down to x = 3 are formed by using an in-source fragmentation technique.…”

“…Therefore, sequential modeling of the primary and secondary water losses from the TCID of Mg 2 + (H 2 O) 3 serves as the only means available to us to determine the binding enthalpy for the smallest Mg 2 + (H 2 O) x complexes. [6,7] In addition to the dominant water loss channels, charge separation of the Mg 2 + (H 2 O) 3 For both the dehydration reactions and the charge-separation processes, the thermodynamic onsets measured here are compared to theoretical calculations to provide additional insight into these reactions. [37] For doubly charged solvated metal ions, sequential modeling has been useful in determining binding enthalpies of Ca 2 + (H 2 O) x (x = 1-7), [2] Zn 2 + (H 2 O) x (x = 6-9), [4,5] and Cd 2 + (H 2 O) x (z = 4-11).…”