E. Sánchez Marcos

H2 chemisorption on Ir4 clusters: A HFS-LCAO study

Abstract Hartree-Fock-Slater-LCAO calculations are presented of the interaction of a H 2 molecule with a tetrahedral Ir 4 cluster in the presence or absence of a Mg 2+ ion, with the idea to mimic the catalytic reactivity of metal particles in zeolites. We find only a very weak interaction between the H 2 molecule and Ir 4 in the absence of Mg 2+ , but highly increased interactions in the presence of Mg 2+ . This not only leads to different absorption geometries to be favoured on Ir 4 clusters close to the Mg 2+ ion, but also to a prediction of a lowered barrier for H 2 dissociation.

EXAFS investigation of the second hydration shell of metal cations in dilute aqueous solutions

Abstract EXAFS spectra of aqueous dilute solutions (0.1 M) containing Cr3+, Rh3+, Ga3+ and Zn2+ ions are presented. From the EXAFS fit parameters it is deduced that always the first hydration shell is formed by six water molecules with MO distances between 1.95 and 2.06 A. A second hydration shell is also observed in all cases, although the amplitude of this contribution increases with the stability of aquocomplexes. This can be quantified by the correlation between the Debye-Waller factors for the first and second hydration shells, and the first-shell water release constants of the hydrated cations.

Impact of the number of fitted Debye-Waller factors on EXAFS fitting

EXAFS fit applied to asymmetric systems may imply the fit of parameters for many scattering paths. We illustrate on some examples how fitting too many independent DWs may lead to incorrect distances and mask some structural details. We question the physical meaning of the fitted DW values and we propose some ideas to avoid this problem.

EXAFS Debye-Waller factors issued from Car-Parrinello molecular dynamics: Application to the fit of oxaliplatin and derivatives

One of the main pitfalls in EXAFS fitting is correlation among parameters, which can lead to unreliable fits. The use of theoretical Debye-Waller factors (DWs) is a promising way to reduce the number of fitted parameters. When working with molecular dynamics, it is not only possible to evaluate DWs from the statistical distributions issued from the trajectory but also to estimate the distribution anharmonicity, and to compute simulated average EXAFS spectra that can be fitted as experimental ones, in order to assess the ability of EXAFS fitting to recover information on DWs, as well as other structural and spectroscopical parameters. The case studied is oxaliplatin, a third generation anticancer drug. The structural information and the simulated average spectra were derived from a Car-Parrinello molecular dynamics (CP-MD) trajectory of a compound closely related to oxaliplatin. We present the DWs issued from this simulation and their use, by taking their theoretical absolute values (no DW fitted) or their ratios (one DW fitted). In this second approach, the fit of oxaliplatin experimental spectra leads to DWs values very close to the theoretical ones. This shows that the CP-MD trajectory provides a good representation of the distance distributions for oxaliplatin. Transferability of oxaliplatin DWs, for all relevant single and multiple scattering paths, to closely related compounds is proven for the case of bis(oxalato)platinum(II) and bis(ethylene diamine)platinum(II).

The solvation of bromide anion in acetonitrile: a structural study based on the combination of theoretical calculations and X-ray absorption spectroscopy

This work studies the solvation of bromide in acetonitrile by combining quantum mechanics, computer simulations and X-ray absorption near edge structure (XANES) spectroscopy. Three different sets of interaction potentials were tested, one of them derived from literature and the other two are simple modifications of the previous one to include specificities of the bromide–acetonitrile interactions. Results for microsolvation of bromide were obtained by quantum mechanical optimization and classical minimization of small clusters [Br(ACN) n ]− (n = 9, 20). Analysis of molecular dynamics (MD) simulations has provided structural, dynamic and energetic aspects of the solvation phenomenon. The theoretical computation of Br K-edge XANES spectrum in solution using the structural information obtained from the different simulations has allowed the comparison among the three different potentials, as well as the examination of the main structural and dynamic factors determining the shape of the experimental spectrum.

Molecular Structure of Solvates and Coordination Complexes in Solution as Determined with EXAFS and XANES

x-Ray absorption spectroscopy provides accurate information about the local structure around the majority of the elements in the periodic table, in either solid or liquid states, crystalline or amorphous forms. This contribution describes the application of the technique to the determination of the structure of several species in solution. It includes a brief theoretical description of the photoabsorption phenomena, a description of the experimental setup, a summary of the data analysis procedures, and several examples of applications, mainly to aqua ions and coordination complexes of transition metal and actinide cations.

Molecular solids of actinide hexacyanoferrate: Structure and bonding

The hexacyanometallate family is well known in transition metal chemistry because the remarkable electronic delocalization along the metal-cyano-metal bond can be tuned in order to design systems that undergo a reversible and controlled change of their physical properties. We have been working for few years on the description of the molecular and electronic structure of materials formed with [Fe(CN)6]n- building blocks and actinide ions (An = Th, U, Np, Pu, Am) and have compared these new materials to those obtained with lanthanide cations at oxidation state +III. In order to evaluate the influence of the actinide coordination polyhedron on the three-dimensional molecular structure, both atomic number and formal oxidation state have been varied : oxidation states +III, +IV. EXAFS at both iron K edge and actinide LIII edge is the dedicated structural probe to obtain structural information on these systems. Data at both edges have been combined to obtain a three-dimensional model. In addition, qualitative electronic information has been gathered with two spectroscopic tools : UV-Near IR spectrophotometry and low energy XANES data that can probe each atom of the structural unit : Fe, C, N and An. Coupling these spectroscopic tools to theoretical calculations will lead in the future to a better description of bonding in these molecular solids. Of primary interest is the actinide cation ability to form ionic ? covalent bonding as 5f orbitals are being filled by modification of oxidation state and/or atomic number.

The Aquation of Po(IV): A Quantum Chemical Study.

The aim of this work is to present theoretical results of the hydration of the Po(IV) in solution. Particular attention is paid to the level of calculation needed to properly describe the system under study: Po(IV) coordination number in the first hydration shell and the nature of the polonium‐water bonding. The hydration number of the Po(IV) is found to be in solution between 8 and 9 and the solvation free energy around −1450 kcal/mol. The Po‐H2O bonding is dominated by the strong electrostatic contribution although the peculiar geometry adopted by the different hydrates is due to covalent contributions. This involves the empty 6p orbital of the polonium ion and one lone pair on the oxygen atom of the water molecule. No role of the 6s orbital of the polonium ion is detected.

Coupling MD Simulations and X-ray Absorption Spectroscopy to Study Ions in Solution

The structure of ionic solutions is a key‐point in understanding physicochemical properties of electrolyte solutions. Among the reduced number of experimental techniques which can supply direct information on the ion environment, X‐ray Absorption techniques (XAS) have gained importance during the last decades although they are not free of difficulties associated to the data analysis leading to provide reliable structures. Computer simulations of ions in solution is a theoretical alternative to provide information on the solvation structure. Thus, the use of computational chemistry can increase the understanding of these systems although an accurate description of ionic solvation phenomena represents nowadays a significant challenge to theoretical chemistry. We present: (a) the assignment of features in the XANES spectrum to well defined structural motif in the ion environment, (b) MD‐based evaluation of EXAFS parameters used in the fitting procedure to make easier the structural resolution, and (c) the us...

Combined Study of Vibrational Spectra of α, β, γ and σ Carboline Isomers by IR and AB Initio Calculations

Pyridoindoles, trivially named carbolines, are structural units of numerous naturally occurring alkaloids which possess a wide range of biological and pharmacological properties [1]. In this study the vibrational spectra of α, β;, γ and δ; carbolines have been determined from experimental and theoretical methods.