Comparative analysis of the state of lithium and sodium atoms in water clusters

Structures and energetic characteristics of Li(H₂O) _n and Li⁺(H₂O) _n clusters with n = 1–6, 19, and 27 determined in the second order of the Møller-Plesset perturbation theory with 6–31++G(d,p) basis set are analyzed. The electron density redistribution, which takes place upon the electron addition to a Li⁺(H₂O) _n cluster, is found to be provided by hydrogen-bonded water molecules: initially almost neutral molecules, which are most distant from lithium, become negatively charged. The calculated energies of the electron capture by Li⁺(H₂O) _n clusters are approximated with the appropriate electrostatic model, and estimates of the lithium ionization energy in water clusters of various sizes are found. Similar estimates obtained earlier for sodium are made more accurate.     

Thermal investigation of some heterocyclic ligand complexes of tin(IV) in the solid state

1,10-phenanthroline (phen), 2,2′-bipyridyl (bipy), pyridine (py) and 4-picoline (4-pic) complexes of dibutyltindichloride (Bu₂SnCl₂) and dimethyltindichloride (Me₂SnCl₂) were synthesized. The complexes were characterized with the help of elemental analyses, IR spectra and thermal analyses. The complexes were found to have the compositions [Bu₂SnCl₂·phen], [Bu₂SnCl₂·bipy], [Me₂SnCl₂·phen], [Me₂SnCl₂·bipy], [Me₂SnCl₂·2py] and [Me₂SnCl₂·2(4-pic)]·H₂O. All these complex compounds appeared to posses octahedral structures. Thermodynamic parameters, such as activation energyE _a ^* enthalpy change ΔH and entropy change ΔS, for the dehydration and sublimation of the complexes were evaluated using some standard methods.     

Immersion of hydrogen atoms in aluminium clusters

The immersion of a H atom at the centre of Al _N clusters with 8≦N≦21 has been studied using the density functional formalism and a pseudopotential averaged about the cluster centre. The immersion energy is negative forN≦12, due to the fact that the corresponding pure Al clusters contain a central vacancy, which shows a strong affinity for the H atom. For larger sizes the immersion energy is positive (with a few exceptions), since the H atom must displace a central Al atom to the cluster surface, and it shows non negligible size oscillations.     

Evaporation of atoms from metal clusters

The evaporation of atoms from metal clusters following photon absorption has been investigated by comparing the predictions of the statistical Weisskopf approach with the one of macroscopic kinetic theory. The consequences on the evaporation rate due to finite size effects in the separation energy and to the initial temperature of the cluster are also discussed.     

Glycoluril dimers bearing hydrogen atoms on their convex face and their self-assembly in the solid state

A selective method for the synthesis of 1,6-substituted glycolurils has been developed. The glycolurils have been used for the synthesis of methylene-bridged glycoluril dimers bearing hydrogen atoms on their convex face. Depending on the side walls of the dimers, different modes of self-assembly in the solid state have been described using X-ray crystallography.     

Metal atoms and clusters in fullerene cages

Fullerenes containing metal atoms and clusters can be formed by the arc-vaporization method. The electronic structure of these metallofullerenes can be probed using magnetic resonance techniques. Electron paramagnetic resonance (EPR) spectra of LaC₈₂, YC₈₂, ScC₈₂ and Sc₃C₈₂ have been obtained. Metallofullerenes containing a single metal atom (MC₈₂ with M = La, Y, or Sc), have small hyperfine couplings and g-values close to 2, implying that they can be described as + 3 metal cations within — 3 fullerene radical anion cages. In the La and Y cases, additional EPR active MC₈₂ species have been found. The EPR spectrum of Sc₃C₈₂ shows that the metal atoms are equivalent, suggesting that they may form a triangular molecule. No EPR spectrum is found for Y₂C₈₂ or for Sc₂C_2n species (with 2n = 82,84,86), suggesting that they are diamagnetic. Sc NMR spectra of a solution containing Sc₂C_2n species have been obtained which confirm the diamagnetism of the discandium metallofullerenes.     

Solvent free reactions in the solid state: solid state metathesis

Transition Metal Chemistry -     

Naked Au55 clusters: dramatic effect of a thiol-terminated dendrimer

Reaction of the thiol-terminated fourth-generation dendrimer 2-G4 (96 SH groups) with the gold cluster compound Au55(PPh3)12Cl6 in a 3:1 molar ratio in dichloromethane results in the formation of bare Au55 clusters. The cuboctahedrally shaped Au55 particles coalesce to well-formed microcrystals (Au55) infinity. The role of the dendrimer is not only to remove the phosphine and chlorine ligands but also to act as an ideal matrix for perfect crystal growth. Transmission electron microscopy (TEM), small- and wide-angle X-ray diffraction (SAXRD and WAXRD) measurements indicate a structure where rows of edge-linked Au55 building blocks form a distorted cubic lattice. The X-ray data fit best if a 5% reduction of the Au-Au bond length in the Au55 clusters is assumed, in agreement with previous extended X-ray absorption fine structure (EXAFS) measurements. Energy-dispersive X-ray spectroscopy (EDX) analyses and IR investigations show the absence of PPh3 and Cl in the microcrystals.     

Unusual electronic state of tin dopant atoms in surface layers of NiTiO<Subscript>3</Subscript>

The ¹¹⁹Sn Mössbauer spectra of polycrystalline NiTiO₃ samples impregnated with a solution containing 0.3 at % Sn⁴⁺ are evidence that annealing in H₂ converts tin into the state with the electron density |Ψ(0)|² on ¹¹⁹Sn nuclei corresponding to “Sn³⁺” ions. The stabilization of tin atoms in such an untypical formal oxidation state occurs at a depth of no more than 2–3 nm from the surface of titanate crystallites. It was revealed that the Sn³⁺ ions are not subjected to spin polarization even at temperatures considerably lower than the Néel temperature of NiTiO₃, which can be explained by their location in the Ni²⁺ positions. The formation of Sn3+ prevents the further reduction of tin to the divalent state and, hence, precludes localization of ¹¹⁹Sn probe cations in positions at the interface.     

Electronic shells and shells of atoms in metallic clusters

Intensity anomalies (magic numbers) have been observed in the mass spectra of sodium clusters containing up to 22 000 atoms. For small clusters (Na _n ,n≤1500) the anomalies appear to be due to the filling of electronic shells (groups of subshells having the same energy). The shells can be characterized rather well by a pseudoquantum-number, indicating the possible existence of a symmetry higher than spherical. The mass spectra of larger clusters (1500≤n≤22 000) are well explained by the completion of icosahedral or cuboctahedral shells of atoms. The fact that the two types of shells (electron and atom) occur in distinct and non-overlapping size intervals might indicate the existence of a “liquid” to “solid” transition in going from small to large clusters.     

The use of solid state detectors for the quantitative determination of tin and molybdenum in alloys by gamma-excited X-ray fluorescence

The potential of solid state detectors in the quantitative determination of medium weight elements by γ-excited X-ray fluorescence was evaluated. As criteria, sensitivity, precision and detection limits have been determined for the quantitative analysis of tin and molybdenum in a series of realistic and well characterized samples within a concentration range of 0.04–9%. The results demonstrate that a non-destructive and fast determination is possible with a precision of a few per cent, a sensitivity of at least 10⁴ cpm/per cent concentration and a detection limit below 100 ppm.     

The melting behavior of small clusters of atoms

The melting transition of small clusters composed of 13 particles interacting via the Lennard-Jones 6-12 potential has been investigated by means of extensive Metropolis Monte Carlo simulations. The results indicate that whereas the cluster evaporates in vacuum, in a confined pore the cluster undergoes a smooth melting transition from an icosahedral microcrystal to an inhomogeneous liquid phase, with a specific heat peak centered at 39 K. No evidence was found to support the suggestion of a solid-liquid transition in vacuum at 29 K.     

Semiempirical estimates of the correlation energy in small clusters of hydrogen atoms

The Pamuk EPCE-F2σ method is applied to neutral and charged clusters composed from 2–9 hydrogen atoms. The range of applicability of the method is demonstrated with H₂, H ₃ ⁺ , and H₃ by comparing the results with the reported rigorous SCF and CI calculations. Predictions of the correlation energy were made for larger hydrogen atom systems, the emphasis being laid in the discussion on H₄, H ₅ ⁺ , and H₆.     

Heteronuclear NOE in the solid state

A series of small molecules and polymers was examined in order to investigate the heteronuclear NOE effect in solids. Contrary to a previously reported interpretation it was found that caution has to be used in interpreting NOE rates purely in terms of correlation times. The NOE rate of a methyl group was not solely dependent upon correlation times but also the distance (r⁻⁶) from the nearest methyl neighbour.     

Conformation-specific photochemistry in the solid state

The unsymmetrical ene-dione $\text{1b}$ exists in solution as a mixture of two conformational isomers, A and B, in rapid equilibrium. Irradiation of $\text{1b}$ in this medium leads to four products, two from each conformer. In the solid state, however, compound $\text{1b}$ adopts a single conformation (A), and photolysis of crystals of $\text{1b}$ leads to a single product. The formation of one rather than two photoproducts in this instance is attributed to a unique solid state steric effect, termed steric compression.