Piercarlo Fantucci

Density functional study of structural and electronic properties of bimetallic silver–gold clusters: Comparison with pure gold and silver clusters

Bimetallic silver–gold clusters offer an excellent opportunity to study changes in metallic versus “ionic” properties involving charge transfer as a function of the size and the composition, particularly when compared to pure silver and gold clusters. We have determined structures, ionization potentials, and vertical detachment energies for neutral and charged bimetallic AgmAun [3⩽(m+n)⩽5] clusters. Calculated VDE values compare well with available experimental data. In the stable structures of these clusters Au atoms assume positions which favor the charge transfer from Ag atoms. Heteronuclear bonding is usually preferred to homonuclear bonding in clusters with equal numbers of hetero atoms. In fact, stable structures of neutral Ag2Au2, Ag3Au3, and Ag4Au4 clusters are characterized by the maximum number of hetero bonds and peripheral positions of Au atoms. Bimetallic tetramer as well as hexamer are planar and have common structural properties with corresponding one-component systems, while Ag4Au4 and Ag8...

Effective core potential-configuration interaction study of electronic structure and geometry of small neutral and cationic Agn clusters : predictions and interpretation of measured properties

The ground state geometries of small neutral Agn(n=2–9) and cationic Agn+ (n=2–9) clusters have been determined in the framework of the Hartree–Fock and complete active space self‐consistent field (CASSCF) procedure employing a relativistic effective core potential (RECP) accounting for core–valence correlation effects (CVC). Similarities and differences between topologies of neutral and charged structures have been found. Large scale configuration interaction (CI) calculations for 5s electrons only have been carried out for determining stabilities and ionization potentials (IP). A comparison between predicted and measured observables allows tentative geometrical assignments. Structural and electronic properties of small Ia and Ib clusters are compared.

An accurate relativistic effective core potential for excited states of Ag atom: An application for studying the absorption spectra of Agn and Agn+ clusters

A new 11-electron relativistic effective core potential (11e-RECP) for Ag atom based on correlated level of theory and the associated atomic orbital (AO) basis set have been derived which allows for an accurate determination of excited states. This has been verified by comparing the calculated excited states of the dimer with experimental data. Therefore, we applied the new 11e-RECP in the framework of the linear response equation-of-motion coupled-cluster (EOM-CC) method to determine absorption spectra of small Agn=2−4 and Agn=2−4+ clusters. The correlation treatment of 11 electrons per atom and calculations of transition energies and oscillator strengths in a large energy interval allowed us to investigate the influence of d-electrons on the spectroscopic patterns. We have found that d-electrons play a crucial role for accurate predictions of absorption spectra in spite of the fact that they are not always directly involved in the leading excitations contributing to the intense transitions. The calculat...

Charge density topological study of bonding in lithium clusters

The topological behaviour of the electron density (ρ) derived from correlated wavefunctions is analyzed for Li2, Li4(D2h), Li5(C2v), and Li6(D3h) planar clusters considered in their optimal geometry. The topology ofρ of Li2 shows an unusual maximum located at the midpoint of the Li-Li equilibrium distance. The occurrence of maxima ofρ at positions other than nuclei (characteristic also for planar Li4, Li5, and Li6 clusters) implies the existence of molecular subspaces (bounded by zero-flux surfaces in the gradient ofρ at each point of the surface) which do not enclose a nucleus but still satisfy the virial theorem. This result provides a generalization of Baders quantum theory of atoms in molecules to systems in which electrons behave partially as mobile metallic electrons. Maxima ofρ preferentially occur within the triangles (two in Li4, two in Li5 and three in Li6), while the number of maxima at the Li-Li midpoint is minimized: they are present only when the existence of a maximum within a triangle is not allowed because of the non suitable formal valence of the Li atoms involved. All the cluster atoms are bonded to “attractors” associated with the unusualρ maxima, but they are not directly bonded to each other. The cluster stability is found to be dependent on the number and kind ofρ maxima. The topological analysis clearly differentiates between Li atoms which occupy different coordination positions within the cluster in terms of their local and average properties. In particular, the degree ofsp hybridization is markedly different for Li atoms with two, three or four nearest neighbors. This implies that a unique definition of a reference valence state for atoms in clusters is impossible. As a consequence, the use of standard electron density difference maps for the description of the charge accumulation and depletion process which ensues the chemical bonding, appears rather questionable.

Theoretical interpretation of the photoelectron detachment spectra of Na−2–5 and of the absorption spectra of Na3, Na4, and Na8 clusters

The configuration‐interaction (CI) study of excited states of alkali metal clusters accounts for spectroscopical patterns obtained from (i) the photoelectron detachment spectra of their anions and from (ii) the photodepletion spectra of the neutral species, reproduces observed excitation energies, intensities for allowed transitions, and permits an assignment of cluster structures. For Na−2–4 the linear anionic geometries are responsible for the photoelectron detachment spectra. In the case of Na−5, both planar and linear anionic isomers seem to contribute to the recorded spectrum. The calculation of optically allowed states for Na3(C2v) and Na4(D2h) structures and oscillator strengths yield rich spectra which have been fully assigned to the observed ones. In the case of Na8, the Td and the related D2d forms give rise to an intense transition located at ∼495 nm and the weak fine structure shifted to the red in full agreement with the measured spectrum. A molecular versus collective excitation interpretati...

Effective core potential‐configuration interaction study of electronic structure and geometry of small anionic Agn clusters: Predictions and interpretation of photodetachment spectra

The ground state geometries of small anionic Ag−n(n=2–9) clusters were determined in the framework of the Hartree–Fock procedure employing a relativistic effective core potential (RECP) accounting for core–valence correlation (CVC) effects. Large scale configuration interaction (CI) calculations for 5s electrons only were carried out in order to determine the ground state energies of anionic and neutral species as well as of excited states of the latter in the geometries of the former. The calculated vertical detachment and excitation energies account for the observed photodetachment spectroscopic patterns and permit an assignment of the cluster geometries. Structural and electronic properties of small Ia and Ib anionic clusters are compared.

Free-energy landscape, principal component analysis, and structural clustering to identify representative conformations from molecular dynamics simulations: The myoglobin case

Several molecular dynamics (MD) simulations were used to sample conformations in the neighborhood of the native structure of holo-myoglobin (holo-Mb), collecting trajectories spanning 0.22 micros at 300 K. Principal component (PCA) and free-energy landscape (FEL) analyses, integrated by cluster analysis, which was performed considering the position and structures of the individual helices of the globin fold, were carried out. The coherence between the different structural clusters and the basins of the FEL, together with the convergence of parameters derived by PCA indicates that an accurate description of the Mb conformational space around the native state was achieved by multiple MD trajectories spanning at least 0.14 micros. The integration of FEL, PCA, and structural clustering was shown to be a very useful approach to gain an overall view of the conformational landscape accessible to a protein and to identify representative protein substates. This method could be also used to investigate the conformational and dynamical properties of Mb apo-, mutant, or delete versions, in which greater conformational variability is expected and, therefore identification of representative substates from the simulations is relevant to disclose structure-function relationship.

Mutations in the ''lid'' region affect chain length specificity and thermostability of a Pseudomonas fragi lipase

The cold‐adapted Pseudomonas fragi lipase (PFL) displays highest activity on short‐chain triglyceride substrates and is rapidly inactivated at moderate temperature. Sequence and structure comparison with homologous lipases endowed with different substrate specificity and stability, pointed to three polar residues in the lid region, that were replaced with the amino acids conserved at equivalent positions in the reference lipases. Substitutions at residues T137 and T138 modified the lipase chain‐length preference profile, increasing the relative activity towards C8 substrates. Moreover, mutations conferred to PFL higher temperature stability. On the other hand, replacement of the serine at position 141 by glycine destabilized the protein.

Influence of the [2Fe]H Subcluster Environment on the Properties of Key Intermediates in the Catalytic Cycle of [FeFe] Hydrogenases: Hints for the Rational Design of Synthetic Catalysts†

Natures recipe: A theoretical study analyzes how the environment of the [FeFe] hydrogenases catalytic cofactor affects its chemical properties, particularly the relative stability of complexes with bridging and terminal hydride ligands (see picture; Fe teal, S yellow, C green, N blue, O red, H gray). The results help to elucidate key rules for the design of bioinspired synthetic catalysts for H(2) production.

Calculated properties of alkali metal clusters with fivefold symmetry

Ab initio calculations indicate a high stability for the pentagonal pyramid Li6 and the pentagonal bipyramid Li7 clusters. The pivotal role of the Jahn–Teller or pseudo‐Jahn–Teller effect in determining the optimal geometrical shape of small neutral and anionic Li clusters is demonstrated. The calculated electronic properties of the pentagonal bipyramids Li7, Na7, and K7 are in satisfactory agreement with recent ESR measurements.