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The role of surface oxygen vacancies in the optical properties of tin dioxide nanobelts is investigated in this paper. Using a first-principles approach, based on the density functional theory combined to a very accurate exchange correlation functional, we characterize SnO(2) (101), that is the nanobelt largest surface. We show that the presence of surface oxygen vacancies leads to the appearance of (i) occupied states located at about 1 eV above the valence band and (ii) unoccupied states lying in resonance with the conduction band. Photoluminescence characterization performed on samples of SnO(2) nanobelts at low temperature shows that the basic spectral features of luminescence are in excellent agreement with theoretical predictions.

Vibrational spectroscopy and Density Functional Theory (DFT) quantum chemical calculations, have been used to investigate structure and chemical properties of some fluorinated ethers. A good agreement between experimental and theoretical results has been obtained. Interesting conformational behaviour has been observed and interpreted for C-H bonds in difluoromethyl end groups. These effects help in understanding the chemical behaviour of these compounds, since C-H equilibrium bond length and bond dissociation energy (BDE) can be correlated with vibrational properties. (C) 1998 Elsevier Science S.A. All rights reserved.

The effect of the Madelung field corresponding to formal point charges ± e arranged according to the fcc lattice of LiF upon CO, N2 and NO molecules is studied in order to isolate the purely electrostatic interaction from the other forces acting in the related physisorption processes. The polarization which is produced in those molecules by the Madelung field is analyzed in terms of dipole and quadrupole moments, as well as in terms of electron charge distributions. The influence of the electron correlation is taken into account by using the QCISD technique, which requires the semi-infinite lattice being simulated with a suitable cluster of point charges.

In magnetic resonance imaging (MRI), the degree of contrast can be improved by using suitable contrast agents. The diethylenetriaminepentaacetic acid (DTPA) complexes of paramagnetic ions have been proposed for this purpose. This paper deals with extensive simulations of the distribution of species in blood plasma when solutions of manganese(II), copper(II), iron(III) and gadolinium(III) ions (as their soluble salts and as DTPA complexes) are injected. The various interaction equilibria, for which formation constants are known, are considered in order to assess toxic side-effects associated with their use in MRI. The data obtained support from the thermodynamic point of view, the use of GdDTPA, and suggest that the administration of a slight excess of ligand would guarantee complete coordination of the toxic gadolinium ion, and only minor interaction with the metal ions naturally present in blood plasma.

The regular adsorption of CO at various concentrations on an infinite three-layer slab of LiF has been studied by using the CRYSTAL ab initio program. The CO molecules are adsorbed vertically above the surface cations by purely electrostatic attraction counterbalanced by the Pauli repulsion. The induced polarization has been analysed through the changes occurring in the molecular orbitals of the adsorbed species. The adsorption via the oxygen atom appears to be preferred, which contradicts IR experimental data. Such contradiction arises from the correlation error in the Hartree-Fock SCF method which is applied in the CRYSTAL program. Consideration of simple correlated clusters enables a semi-quantitative evaluation which additively corrects the above contradictory results.

The electronic structure of corundum (α-Al 2 O 3 ) is calculated at the ab initio Hartree-Fock level. Cohesive energy, total and projected densities of states, atomic multipoles, bond populations and electron charge density distribution maps are given. The oxygen-aluminium bond is found to be partially covalent in nature; the atomic charges are −0.73 e and +1.09 e for O and Al respectively.