Experimental Binding Energies for the Metal Complexes [Mg(CH₃OH)_n]²⁺, [Ca(CH₃OH)_n]²⁺, and [Sr(CH₃OH)_n]²⁺ for n in the Range 4–20

Abstract

A supersonic source of clusters has been used to prepare neutral complexes of methanol in association with an alkaline earth metal atom. From these complexes the following metal-containing dications have been generated through electron ionization: [Mg(CH3OH)n](2+), [Ca(CH3OH)n](2+), and [Sr(CH3OH)n](2+), and for n in the range 4-20, kinetic energy release measurements following the evaporation of a single molecule have been undertaken using a high resolution mass spectrometer. Using finite heat bath theory, these data have been transformed into binding energies for individual methanol molecules attached to each of the three cluster systems. In the larger complexes (n > 6) the results exhibit a consistent trend, whereby the experimental binding energy data for all three metal ions are similar, suggesting that the magnitude of the charge rather than charge density influences the strength of the interaction. From a comparison with data recorded previously for (CH3OH)nH(+) it is found that the 2+ charge on a metal ion has an effect on the binding energy of molecules in complexes containing up to 20 solvent molecules. The results recorded for [Ca(CH3OH)n](2+) show evidence of a very marked transition between n = 6 and 7, which is thought to coincide with the completion of a primary solvation shell and the onset of molecules starting to occupy a second and most probably a third shell.

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

“…Comparison between the experimental data presented here for [Ca(H 2 O) n ] 2+ and results recorded previously for [Ca(NH 3 ) n ] 2+ [26] and [Ca(CH 3 OH) n ] 2+ [27]. …”

“…The error bars (not shown for purposes of clarity) on the results for n  = 4 and 5 are probably too large for any meaningful interpretation; however, for both [Ca(NH 3 ) n ] 2+ and [Ca(CH 3 OH) n ] 2+ it was concluded that the first solvation shell consists of six molecules and so it is interesting to see that all three datasets converge to approximately 80 kJ mol −1 at that point.

Figure 5.Comparison between the experimental data presented here for [Ca(H 2 O) n ] 2+ and results recorded previously for [Ca(NH 3 ) n ] 2+ [26] and [Ca(CH 3 OH) n ] 2+ [27].

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“…Finally, figure 5 makes a comparison between the data presented here for [Ca(H 2 O) n ] 2+ and results presented previously for [Ca(NH 3 ) n ] 2+ and [Ca(CH 3 OH) n ] 2+ that have also been derived from experimental data using finite heat bath theory [26,27]. The error bars (not shown for purposes of clarity) on the results for n  = 4 and 5 are probably too large for any meaningful interpretation; however, for both [Ca(NH 3 ) n ] 2+ and [Ca(CH 3 OH) n ] 2+ it was concluded that the first solvation shell consists of six molecules and so it is interesting to see that all three datasets converge to approximately 80 kJ mol −1 at that point.…”

“…The assumptions made and the parameters specific for this particular series of experiments have been listed elsewhere [26,27]. The binding energies and their associated error limits are recorded in table 1 and plotted in figure 2 as a function of n .…”

“…Each [Ca(H 2 O) n ] 2+ cluster dication was selected using the magnet and the electric sector field voltage was then scanned while the accelerating voltage and magnetic field remained constant. Equations given previously were then used to identify any fragment ions and determine their centre-of-mass kinetic energy [26,27], and for each of the complexes discussed here the principal fragmentation pathway observed was ${[\text{Ca}{false({\text{H}}_2\text{O}false)}_n]}^{2+}false\to {[\text{Ca}{false({\text{H}}_2\text{O}false)}_{n-1}]}^{2+}+{\text{H}}_2\text{O.}$…”