Hydrated metal ions in the gas phase

Ab initio investigation of the hydration of deprotonated amino acids

The complexation of five deprotonated anionic amino acids (glycine, L-alanine, L-valine, L-Aspartic acid, and L-glutamine) with one water molecule, has been investigated using a MP2/63-11++G(d,p) approach fully accounting for the basis set superposition errors. For each amino acid, several energetic minima have been identified, and we provide spectroscopic information allowing to discriminate them. Our results strongly suggest that two complexes should coexist under the experimental conditions for [Ala - H](-), [Val - H](-), and [Asp - H](-). Comparisons with the experimental enthalpies, entropies, and Gibbs free energies recently obtained by Wincel [J. Am. Soc. Mass Spectrom. 2008, 19, 1091-1097] show that our simulation reproduces the most significant structure/energy experimental trends, though the entropic changes induced by hydration are slightly overestimated.

Investigation of energy deposited by femtosecond electron transfer in collisions using hydrated ion nanocalorimetry

Ion nanocalorimetry is used to investigate the internal energy deposited into M (2+)(H 2O) n , M = Mg ( n = 3-11) and Ca ( n = 3-33), upon 100 keV collisions with a Cs or Ne atom target gas. Dissociation occurs by loss of water molecules from the precursor (charge retention) or by capture of an electron to form a reduced precursor (charge reduction) that can dissociate either by loss of a H atom accompanied by water molecule loss or by exclusively loss of water molecules. Formation of bare CaOH (+) and Ca (+) by these two respective dissociation pathways occurs for clusters with n up to 33 and 17, respectively. From the threshold dissociation energies for the loss of water molecules from the reduced clusters, obtained from binding energies calculated using a discrete implementation of the Thomson liquid drop model and from quantum chemistry, estimates of the internal energy deposition can be obtained. These values can be used to establish a lower limit to the maximum and average energy deposition. Not taking into account effects of a kinetic shift, over 16 eV can be deposited into Ca (2+)(H 2O) 33, the minimum energy necessary to form bare CaOH (+) from the reduced precursor. The electron capture efficiency is at least a factor of 40 greater for collisions of Ca (2+)(H 2O) 9 with Cs than with Ne, reflecting the lower ionization energy of Cs (3.9 eV) compared to Ne (21.6 eV). The branching ratio of the two electron capture dissociation pathways differs significantly for these two target gases, but the distributions of water molecules lost from the reduced precursors are similar. These results suggest that the ionization energy of the target gas has a large effect on the electron capture efficiency, but relatively little effect on the internal energy deposited into the ion. However, the different branching ratios suggest that different electronic excited states may be accessed in the reduced precursor upon collisions with these two different target gases.

Biasing the center of charge in molecular dynamics simulations with empirical valence bond models: free energetics of an excess proton in a water droplet

Multistate empirical valence bond (EVB) models provide an accurate description of the energetics of proton transfer and solvation in complex molecular systems and can be efficiently used in molecular dynamics computer simulations. Within such models, the location of the moving protonic charge can be specified by the so-called center of charge, defined as a weighted average over the diabatic states of the EVB model. In this paper, we use first-order perturbation theory to calculate the molecular forces that arise if a bias potential is applied to the center of charge. Such bias potentials are often necessary when molecular dynamics simulations are used to determine free energies related to proton transfer and not all relevant proton positions are sampled with sufficient frequency during the available computing time. The force expressions we derive are easy to evaluate and do not create any significant computational cost compared with unbiased EVB simulations. As an illustration of the method, we study proton transfer in a small liquid water droplet consisting of 128 water molecules plus an excess proton. Contrary to predictions of continuum electrostatics, but in agreement with previous computer simulations of similar systems, we observe that the excess proton is predominantly located at the surface of the droplet. Using the formalism developed in this paper, we calculate the reversible work required to carry the protonic charge from the droplet surface to its core, finding a value of roughly 4 k(B)T.

Study of doubly charged alkaline earth metal and 3-azidopropionitrile complexes by electrospray ionization mass spectrometry

The present work describes a study of the complexation of calcium and magnesium by 3-azidopropionitrile by means of electrospray ionization mass spectrometry (ESI-MS). Complexes were obtained from solutions of calcium and magnesium salts of the type CaX2 and MgX2 (where X = Cl or NO3) in water and methanol/water. The complexes detected were mainly double positively charged, with various stoichiometries not depending on the solvent, since water and 3-azidopropionitrile were always the main ligands. Solvation with methanol was not observed unlike in a previous study of complexation of nickel and cobalt by 3-azidopropionitrile. The complex ions [M(II)Az4(H2O)](2+), [M(II)Az5](2+) (where M = Ca and Mg) are the most abundant for both metals, and both counter ions. Tandem mass spectrometric (MS/MS) analysis showed that, under collision-induced dissociation (CID) conditions, the most important processes occurring were loss of neutral ligands and the replacement of 3-azidopropionitrile by water. A complex species containing reduced alkaline earth metal was due to radical loss, resulting from homolytic cleavage in the azide ligand. Some terminal ions, in the fragmentation sequences, point to the nitrile group as the coordination site in the 3-azidopropionitrile. Density functional theory (DFT) calculations confirmed this coordination site and proved that 3-azidopropionitrile behaves as a monodentate ligand in the systems under study. Moreover, the theoretical study proved that the presence of water ligand introduces stability through a hydrogen bond established between the water molecule and one nitrogen atom of the azido group. In addition, the strong dipole moment of 3-azidopropionitrile (4.76 D), which is mainly related to presence of the nitrile group, favors the stabilization of the metal-ligand complexes through charge-dipole interactions and the coordination of the metal to the nitrile group.

Micro-Hydration of the MgNO3+ Cation in the Gas Phase

Coordination complexes of the magnesium nitrate cation with water [MgNO(3)(H(2)O)(n)](+) up to n=7 are investigated by experiment and theory. The fragmentation patterns of [MgNO(3)(H(2)O)(n)](+) clusters generated via electrospray ionization indicate a considerable change in stability between n=3 and 4. Further, ion-molecule reactions of mass-selected [MgNO(3)(H(2)O)(n)](+) cations with D(2)O reveal the occurrence of consecutive replacement of water ligands by heavy water, and in this respect the complexes with n=4 and 5 are somewhat more reactive than their smaller homologs with n=1-3 as well as the larger clusters with n=6 and 7. For the latter two ions, the theory suggests the existence of isomers, such as complexes with monodentate nitrato ligands as well as solvent-separated ion pairs with a common solvation shell. The reactions observed and the ion thermochemistry are discussed in the context of ab initio calculations, which also reveal the structures of the various hydrated cation complexes.