England, U.K.), where he obtained his M.Sc. degree in Mathematics under the supervision of Professor C. A. Coulson and his D.Phil. degree in the Faculty of Physical Sciences (Theoretical Chemistry) under Professor P. W. Atkins. Since 1976, he has been a Professor at the Department of Chemistry of the University of Porto. His major contributions have been in the topology of the magnetic currentdensity field and in several aspects of the simulation of liquid/metal and liquid/liquid interfaces. He has served at different levels of responsibility in the University of Porto and as an evaluator for the national research financing agency, Fundac ¸a ˜o para a Cie ˆncia e Tecnologia. He is currently the Vice President of the Portuguese Chemical Society and the Vice Rector of the University of Porto.

X-ray photoelectron spectroscopy and first-principles density-functional calculations were used to study the interaction of thiophene, H(2)S, and S(2) with Ni(2)P(001), alpha-Mo(2)C(001), and polycrystalline MoC. In general, the reactivity of the surfaces increases following the sequence MoC < Ni(2)P(001) < alpha-Mo(2)C(001). At 300 K, thiophene does not adsorb on MoC. In contrast, Ni(2)P(001) and alpha-Mo(2)C(001) can dissociate the molecule easily. The key to establish a catalytic cycle for desulfurization is in the removal of the decomposition products of thiophene (C(x)H(y) fragments and S) from these surfaces. Our experimental and theoretical studies indicate that the rate-determining step in a hydrodesulfurization (HDS) process is the transformation of adsorbed sulfur into gaseous H(2)S. Ni(2)P is a better catalyst for HDS than Mo(2)C or MoC. The P sites in the phosphide play a complex and important role. First, the formation of Ni-P bonds produces a weak "ligand effect" (minor stabilization of the Ni 3d levels and a small Ni --> P charge transfer) that allows a high activity for the dissociation of thiophene and molecular hydrogen. Second, the number of active Ni sites present in the surface decreases due to an "ensemble effect" of P, which prevents the system from deactivation induced by high coverages of strongly bound S. Third, the P sites are not simple spectators and provide moderate bonding to the products of the decomposition of thiophene and the H adatoms necessary for hydrogenation.

No abstract

Although the majority of the ion pairs found in proteins consists of two charges of opposite sign, the observation of some unusual arrangements of two arginines led us to a search of such occurrences in the Brookhaven Protein Data Bank. We have found 41 Arginine-Arginine interactions with a C zeta ... C zeta distance less than 5 A. Computer graphics analysis of these structures shows that most of the Arg-Arg pairs are found in the vicinity of the surface of the proteins, in an easily hydrated region. In order to determine which factors could stabilize such arrangements of species of similar charge, we have carried out AM1 semi-empirical calculations on a model of two guanidinium ions surrounded by several water molecules. The results show the existence of stable clusters with six or more water molecules, with distances between C zeta atoms around 3 A. The bridging role of the water molecules is an important structural and energetic feature and we find bridges of two and three molecules between the guanidinium ions. These results are in good agreement with the structures found in our search of the experimental data. Enhancement of the electrostatic potential around these clusters, when compared to one of the guanidinium ions alone, is also demonstrated.

Molecular dynamics simulations were performed to study the structural and dynamic properties of the water/2-heptanone (HPT2) liquid/liquid interface. It was found that HPT2 forms a bilayer structure at the interface, pointing its polar heads into the aqueous phase. Water molecules penetrate the hydrophilic headgroup region but not the hydrophobic core. At the hydrophilic region water molecules establish hydrogen bonds with the ketone oxygen of the HPT2 molecule. Behind that zone, the water molecules show a preference in keeping their dipoles in the interfacial plane and these orientations remain in two or three molecular layers. The water dipole distribution is slightly asymmetric, having an average excess in the resulting component normal to the interfacial plane. The water dipoles point toward the aqueous phase for waters in the aqueous side of the interface and into the organic phase for water molecules in the organic side of the interface. The water structure remains almost unchanged at the Gibbs dividing surface. The HPT2 structure is not so robust, and near the interface it is distorted by the presence of the aqueous phase. Self diffusion exhibits long range anisotropy, diffusion toward the interface being slower than diffusion in the interfacial plane. The water orientational dynamics is slowed down near the interface. The HPT2 reorientation becomes anisotropic at the interface as reorientations perpendicular to the interface are slower than those in the interfacial plane. The interface was found to be sharp, highly corrugated, and broadened by capillary waves.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

This work focuses on the study of the properties of two liquid/liquid interfaces, the H2O/2-heptanone and the H2O/iso-octane interfaces, and on the transfer of the iodide ion across them. A detailed study of the properties of the first interface was already reported (J. Phys. Chem. B, 1999, in press). The iso-octane liquid is a hydrophobic analog of the very hydrophilic 2-heptanone, and the properties of the H2O/iso-octane interface are analyzed here and compared with the ones obtained for the H2O/2-heptanone system. It is shown that the basic features characterizing the interface structure (such as the non-existence of a mixed solvent region or the broadening of the sharp interface by capillary waves) are almost unaffected by the change of the hydrophilic nature of the organic solvent. A new method is proposed to calculate more accurately properties which depend on the distance to the interface. In the case of density profiles, the application of this method reveals that both liquids are packed in layers against the interface. This structural pattern, not detectable using classical methods, allows us to understand the reason for the oscillations in the density profiles calculated perpendicularly to the interfacial plane, an unsolved problem for more than one decade. The free energy profiles for the transfer of iodide across the two interfaces are computed and compared. In both cases they show a monotonous decrease in the free energy as the ion moves from the organic solvent into water. The value obtained for the Gibbs free energy of transfer is in good agreement with the available experimental data. In addition, the mechanism of the ion transfer is investigated. The process of transfer from the water phase to the organic one and the reverse process involve, in both cases, the formation of a water cone that connects the hydration sphere of the ion to the water phase. This water cone is a chain of molecules that can be as long as 10 Å. After the disruption and retraction of the water cone, the ion in the organic solvent retains part of its first hydration shell. The mechanism of the transfer through both interfaces is, in qualitative terms, very similar, although the ion transfer free energies are very different, as expected considering the relative hydrophilicity of the present solvents.

No abstract