Hydration Energies of Zinc(II): Threshold Collision-Induced Dissociation Experiments and Theoretical Studies

Abstract

The first experimentally determined sequential bond dissociation energies of Zn(2+)(H(2)O)(n) complexes, where n = 6-10, are measured using threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer coupled with an electrospray ionization source. Kinetic energy dependent cross sections are obtained and analyzed to yield 0 K threshold measurements for the loss of one and two water ligands after accounting for multiple collisions, kinetic shifts, and energy distributions. The threshold measurements are then converted from 0 to 298 K values to give the hydration energies for sequentially losing one water from each parent complex. Theoretical geometry optimizations and single-point energy calculations are performed using several levels of theory for comparison to experiment. Although different levels of theory disagree on the ground-state conformation of most complexes examined here leading to potential ambiguities in the final thermochemical values, calculations at the MP2(full) level provide the best agreement with experiment. On this basis, the present experiments are most consistent with the inner solvent shell of Zn(2+) being five waters, except for Zn(2+)(H(2)O)(6) where all waters bind directly to the metal ion. The charge separation process, Zn(2+)(H(2)O)(n) --> ZnOH(+)(H(2)O)(m) + H(+)(H(2)O)(n-m-1), which is in competition with the loss of water from the parent complex, is also observed for n = 6-8. These processes are analyzed in detail in the following paper.

Extensive Computational Study on Coordination of Transition Metal Cations and Water Molecules to Glutamic Acid

“…By comparing the results for the dihydration and the trihydration, it is not clear how many water molecules are required to transform the global minimum of the Glu−Cu 2+ complex from the canonical form to the zwitterionic structure. However, as the NH 3 Table 10 shows the reaction enthalpies and free energies for the stepwise hydrations of the Glu−TMC complexes with n = 1−3 and may be compared with the future measurements such as blackbody infrared radiative dissociation (BIRD), 18,67−69 collision induced dissociation (CID), 70,71 and high pressure mass spectrometry (HPMS). 72,73 As shown in Table 10, the stepwise reaction enthalpy increases with the cation charge and decreases with the number of water molecules in the complex.…”

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

“…Smaller Mg 2 + (H 2 O) x complexes in which x = 3-6 were accessed by using an in-source fragmentation technique that took place in the high-pressure region of the radio-frequency-only hexapole ion guide. [1][2][3][4][5][6][7][8]43] Details of the experimental methods [38,39,[44][45][46][47] and data analysis procedures [35,[48][49][50][51] can be found in the Supporting Information. Previous work has shown this source arrangement produces thermalized ions at room temperature for ions directly formed, [1,[40][41][42] as well as those created by in-source fragmentation.…”

“…[17] These observations were used to assign the so-called critical size (x c ), defined in these works as Mg 2 + (H 2 O) x complexes that do not dissociate exclusively by water loss, as 3 and 4, respectively. More recent studies by Chen and Stace have identified Mg 2 + (H 2 O) 4 as the smallest stable complex. [18] These dissociative charge-transfer products are not observed in the low-energy collision studies of Kebarle and co-workers or in those of Shvartsburg and Siu (nor in the present study).…”

“…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.…”

“…[34] This procedure does not form complexes smaller than the M 2 + (H 2 O) 3 complex because Mg 2 + (H 2 O) 3 preferentially charge separates into MgOH + (H 2 O) + H 3 O + ions instead of losing a water ligand, as discussed in more detail below. [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). Sequential modeling has been shown to be a viable tool for extracting BDEs for K + (NH 3 ) x (x = 1-4), [35] which have also been determined directly by TCID [36] and HPMS.…”