Riccardo Spezia

Revised Ionic Radii of Lanthanoid(III) Ions in Aqueous Solution

A new set of ionic radii in aqueous solution has been derived for lanthanoid(III) cations starting from a very accurate experimental determination of the ion-water distances obtained from extended X-ray absorption fine structure (EXAFS) data. At variance with previous results, a very regular trend has been obtained, as expected for this series of elements. A general procedure to compute ionic radii in solution by combining the EXAFS technique and molecular dynamics (MD) structural data has been developed. This method can be applied to other ions allowing one to determine ionic radii in solution with an accuracy comparable to that of the Shannon crystal ionic radii.

Hydration of lanthanoids(III) and actinoids(III): an experimental/theoretical saga.

The latest experimental and theoretical studies on structural and dynamical properties of lanthanoid(III) and actinoid(III) ions in water have been reviewed. In the last years, most of the issues about lanthanoid(III) hydration have been resolved combining X-ray absorption experiments and different theoretical methods. Since 2008 an effort has been made to treat the entire series thus obtaining coherent sets of experimental and theoretical results that were lately put together in such a way that it was possible to derive new basic properties, such as effective ionic radii, across the series. While for the hydration of lanthanoids(III) many experiments and simulations have been reported, the hydration of actinoids(III) was less investigated. There are some experiments performed by different research groups and few simulations that we discuss in this review. Currently, there are enough results that it is possible to gain some understanding of the hydration behavior of lanthanoids(III) and actinoids(III). The ultimate goal of this review is to provide clues on the analogies and differences between the two series. These aspects are connected to several issues: 1) technological: the separation of these elements that is necessary for recycling and stocking of nuclear waste, 2) practical: because experiments on actinoids need particular care, the definition of possible analogies will give the possibility to use the correct lanthanoid when the information on a specific actinoid is needed, 3) fundamental: related to chemical similarities between the two series.

A first-principles method to model perturbed electronic wavefunctions : the effect of an external homogeneous electric field

In this Letter, we show that with the use of matrix notation to express the time-independent Schroedinger equation, it is possible to model perturbed electronic wavefunctions. Such a method makes use of first principles of the quantum mechanical theory and hence is rigorous within the only approximation due to the truncation of the perturbed Hamiltonian matrix used. Results show that for three different molecules in vacuo under an electric field, the proposed method provides reliable perturbed electronic wavefunctions at a low computational costs.

Building a polarizable pair interaction potential for lanthanoids(III) in liquid water: a molecular dynamics study of structure and dynamics of the whole series.

In this work we have extended our previously presented polarizable pair interaction potential for La(3+)-water [Duvail et al., J. Chem. Phys. 127, 034503 (2007)] to the whole lanthanoid(III) series (Ln(3+)) interacting with water. This was performed taking into account known modification of ionic radius and atomic polarizability across the series and thus changing potential parameters according to that. Our procedure avoids the hard task of doing expensive high level ab initio calculations for all the atoms in the series and provides results in good agreement with experimental data and with ab initio calculations performed on the last atom in the series (Lu(3+), the atom for which the extrapolation should be in principle much crude). Thus we have studied the hydration properties of the whole Ln(3+) series by performing classical molecular dynamics in liquid phase. This systematic study allows us to rationalize from a microscopic point of view the different experimental results on Ln(3+)-water distances, first shell coordination numbers and first shell water self-exchange reactivity. In particular, we found that across the series the coordination number decreases from 9 for light lanthanoids to 8 for heavy lanthanoids in a continuous shape. This is due to the continuous changing in relative stability of the two forms that can be both populated at finite temperature with different probabilities as a function of the Ln(3+) atomic number. The changeover of the Ln(3+) ionic radius across the series resulted to be the main driving physical properties governing not always the Ln(3+)-water distance changing across the series but also the observed coordination number and consequently ligand dynamics.

A dynamic model to explain hydration behaviour along the lanthanide series.

From nine to eight : Molecular dynamics simulations of all the lanthanide cations in water show that the change in first shell coordination number from nine to eight water molecules (see figure) is not a sudden change in behaviour. Instead, it results from a statistical predominance of one first hydration shell structure containing nine to eight water molecules.

Hydration of lanthanide chloride salts: A quantum chemical and classical molecular dynamics simulation study

We present the results of a quantum chemical and classical molecular dynamics simulation study of some solutions containing chloride salts of La(3+), Gd(3+), and Er(3+) at various concentrations (from 0.05 to 5 M), with the purpose of understanding their structure and dynamics and analyzing how the coordination varies along the lanthanide series. In the La-Cl case, nine water molecules surround the central La(3+) cation in the first solvation shell, and chloride is present only in the second shell for all solutions but the most concentrated one (5 M). In the Gd(3+) case, the coordination number is ∼8.6 for the two lowest concentrations (0.05 and 0.1 M), and then it decreases rapidly. In the Er(3+) case, the coordination number is 7.4 for the two lowest concentrations (0.05 and 0.1 M), and then it decreases. The counterion Cl(-) is not present in the first solvation shell in the La(3+) case for most of the solutions, but it becomes progressively closer to the central cation in the Gd(3+) and Er(3+) cases, even at low concentrations.

Extension of the perturbed matrix method: application to a water molecule

In this Letter we extend the perturbed matrix method by explicitly including the nuclear degrees of freedom and showing how to treat a non-homogeneous electric field. In a previous Letter we showed that this method provides reliable perturbed energies. In the present Letter we evaluate a more sophisticated property such as molecular polarizability for a water molecule.

Protonated Urea Collision-Induced Dissociation. Comparison of Experiments and Chemical Dynamics Simulations

Quantum mechanical plus molecular mechanical direct chemical dynamics were used, with electrospray tandem mass spectrometry experiments, potential energy surface calculations, and RRKM analyses, to study the gas-phase collision-induced dissociation (CID) of protonated urea. The direct dynamics were able to reproduce some of the experimental observations, in particular the presence of two fragmentation pathways, and, thus, to explain the dynamical origin of the two fragmentation ions observed in the CID spectra. A shattering dissociation mechanism takes place during the collision, and it becomes more important as the collision energy increases, thus explaining the linear increase of the high-energy reaction path (loss of ammonia) versus collision energy. By combining the different theoretical and experimental findings, a complete dynamical picture leading to the fragmentation was identified: (i) Oxygen-protonated urea, the most stable structure in the gas phase, must first isomerize to the nitrogen-protonated form. This can happen by multiple CID collisions or in the electrospray ionization process. (ii) Once the nitrogen-protonated isomer is formed, it can dissociate via two mechanisms: i.e, a slow, almost statistical, process forming a NH(4)(+)--NHCO intermediate that rapidly dissociates or a fast nonstatistical process which may lead to the high-energy products.

Hydration properties and ionic radii of actinide(III) ions in aqueous solution

Ionic radii of actinide(III) cations (from U(III) to Cf(III)) in aqueous solution have been derived for the first time starting from accurate experimental determination of the ion-water distances obtained by combining extended X-ray absorption fine structure (EXAFS) results and molecular dynamics (MD) structural data. A strong analogy has been found between the lanthanide and actinide series concerning hydration properties. The existence of a contraction of the An-O distance along the series has been highlighted, while no decrease of the hydration number is evident up to Cf(III).

Dynamical Friction Effects on the Photoisomerization of a Model Protonated Schiff Base in Solution

Photoisomerization involving a conical intersection (CI) for a model protonated Schiff base (PSB) in modeled water and acetonitrile solvents is examined with the inclusion of energy- and momentum-transfer effects described via a generalized Langevin equation (GLE) frictional approach and surface-hopping dynamics. Short-time GLE frictional effects on the models three coordinates, the intramolecular bond length alternation and torsional PSB coordinates and a solvent coordinate, eliminate several unphysical features associated with a no-friction inertial description and have the general feature of accelerating nonadiabatic transitions to the ground electronic state. The inertial prediction of equal probability formation of ground-state trans and cis isomer products subsequent to the Franck-Condon excitation of the ground cis isomer is replaced by the GLE prediction of a preferential higher proportion of ground-state trans isomer, that is, a successful cis to trans photoreaction. This preference is solvent-dependent and is enhanced in water solvent with its higher friction intensity and short time scales. For the fast water solvent motion, the nonadiabatic transitions to the S(0) ground state are centered around the CI seam (which is due to the solvent coordinates role as a tuning coordinate), facilitating direct transitions to the ground-state trans isomer. In contrast, for the slower acetonitrile solvent motion, the decay occurs, on average, away from the CI seam in regions with a finite free-energy gap between the excited and ground states, resulting in reduced trans isomer production. Some directions for the extension of the model description are also discussed.