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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.

The interactions of alkali metal cations (M(+) = Li(+), Na(+), K(+), Rb(+)) with the amino acid cysteine (Cys) are examined in detail. Experimentally, bond energies are determined using threshold collision-induced dissociation of the M(+)(Cys) complexes with xenon in a guided ion beam mass spectrometer. Analyses of the energy dependent cross sections provide 0 K bond energies of 2.65 +/- 0.12, 1.83 +/- 0.05, 1.25 +/- 0.03, and 1.06 +/- 0.03 eV for complexes of Cys with Li(+), Na(+), K(+), and Rb(+), respectively. All bond energy determinations include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations at the MP2(full)/6-311+G(2d,2p), B3LYP/6-311+G(2d,2p), and B3P86/6-311+G(2d,2p) levels with geometries and zero-point energies calculated at the B3LYP/6-311G(d,p) level for the lighter metals show good agreement with the experimental bond energies. For Rb(+)(Cys), similar calculations using the HW* basis set and ECP underestimate the experimental bond energies, whereas the Def2TZVP basis set yields results in good agreement. Ground state conformers are tridentate for Li(+) and Na(+), and subtle changes in the Cys side-chain orientation are found to cause noticeable changes in the alkali metal binding energy. For K(+) and Rb(+), tridentate and carboxylic acid bound (both charge-solvated and zwitterionic) structures are nearly isoenergetic, with different levels of theory predicting different ground conformers. The combination of this series of experiments and calculations allows the influence of the sulfur functional group of Cys on the overall binding strength to be explored. Comparison to previous results for serine elucidates the influence of sulfur for oxygen substitution.

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.

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.