Hydration Enthalpies of Ba(2+)(H2O)x, x = 1-8: A Threshold Collision-Induced Dissociation and Computational Investigation.

The sequential bond dissociation energies (BDEs) of Ba(2+)(H2O)x complexes, where x = 1-8, are determined using threshold collision-induced dissociation (TCID) in a guided ion beam tandem mass spectrometer. The electrospray ionization source generates complexes ranging in size from x = 6 to x = 8 with smaller complexes, x = 1-5, formed by an in-source fragmentation technique. The only products observed result from sequential loss of water ligands. Charge separation, a process in which both hydrated singly charged barium hydroxide and hydronium ion are formed, was not observed except for Ba(2+)(H2O)3 yielding BaOH(+) + H5O2(+). Modeling of the kinetic energy-dependent cross sections, taking into account the number of collisions, energy distributions, and lifetime effects for both primary and secondary water loss, provides 0 K BDEs. Experimental thermochemistry for the x = 1-3 complexes is obtained here for the first time. Hydration enthalpies and reaction coordinate pathways for charge separation are also examined computationally at several levels of theory. Our experimental and computational work are in excellent agreement in the x = 1-6 range. The present experimental values and theoretical calculations are also in reasonable agreement with the available literature values for experiment, x = 4-8, and theory, x = 1-6. Of the numerous calculations performed in the current study, B3LYP/DHF/def2-TZVPP calculations including counterpoise corrections reproduce our experimental values the best, although MP2(full)/DHF/def2-TZVPP//B3LYP/DHF/def2-TZVPP results are comparable.

Threshold collision-induced dissociation and theoretical studies of hydrated Fe(II): binding energies and Coulombic barrier heights.

The first experimentally determined bond dissociation energies for losing water from Fe(2+)(H(2)O)(n) complexes, n = 4-11, are measured using threshold collision-induced dissociation (TCID) in a guided ion beam tandem mass spectrometer coupled to an electrospray ionization source that forms thermalized complexes. In this technique, absolute cross-sections for dissociation induced by collisions with Xe at systematically varied kinetic energies are obtained. After accounting for multiple collisions, kinetic shifts, and energy distributions, these cross-sections are analyzed to yield the energy thresholds for losing one, two, or three water ligands at 0 K. The 0 K threshold measurements are converted to 298 K values to give the hydration enthalpies and free energies for sequentially losing water ligands from each complex. Comparisons to previous results for hydration of Zn(2+) indicate that the bond energies are dominated by electrostatic interactions, with no obvious variations associated with the open shell of Fe(2+). Theoretical geometry optimizations and single-point energy calculations are performed using several levels of theory for comparison to experiment, with generally good agreement. In addition to water loss channels, the charge separation process generating hydrated FeOH(+) and protons is observed for multiple reactant complexes. Energies of the rate-limiting transition states are calculated at several levels of theory with density functional approaches (B3LYP and B3P86) disagreeing with MP2(full) results. Comparisons to our kinetic energy dependent cross-sections suggest that the energetics of the MP2(full) level are most accurate.

Structural elucidation of biological and toxicological complexes: investigation of monomeric and dimeric complexes of histidine with multiply charged transition metal (Zn and Cd) cations using IR action spectroscopy.

The gas-phase structures of singly and doubly charged complexes involving transition metal cations, Zn and Cd, bound to the amino acid histidine (His) as well as deprotonated His (His-H) are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser. IRPMD spectra are measured for CdCl(+)(His), [Zn(His-H)](+), [Cd(His-H)](+), Zn(2+)(His)(2), and Cd(2+)(His)(2) in the 550-1800 cm(-1) range. These studies are complemented by quantum mechanical calculations of the predicted linear absorption spectra at the B3LYP/6-311+G(d,p) and B3LYP/Def2TZVP levels. The monomeric spectra are similar to one another and indicate that histidine coordinates to the metal in a charge-solvated (CS) tridentate form in the CdCl(+)(His) complex and has a similar tridentate configuration with a deprotonated carboxylic acid terminus in the [M(His-H)](+) complexes. The preference for these particular complexes is also found in the relative energetics calculated at the B3LYP, B3P86, and MP2(full) levels. The spectra of the dimer complexes have obvious CS characteristics, suggesting that at least one of the His ligands is charge solvated; however, there are also signatures for a salt-bridge (SB) formation in the second His ligand. The definitive assignment of a SB ligand is complicated by the presence of the CS ligand and conflicting relative energetics from the different levels of theory.