Anna Maria Ferrari

Cluster and band structure ab initio calculations on the adsorption of CO on acid sites of the TiO2(110) surface

The interaction of CO with the cationic sites of the TiO2(110) surface has been investigated with cluster models and band structure calculations. The analysis of Hartree-Fock and correlated wavefunctions has shown that CO adsorbs at a distance of 4.4–4.5 bohr and a binding energy of 0.7–0.8 eV at low coverage. The bond strength is determined by the CO polarization and the CO б-donation to the surface while the electrostatic attraction is almost exactly cancelled by the Pauli repulsion. The adsorption is accompanied by two important measurable features, a considerable blue shift of the CO vibrational frequency and an increase of the CO 5б ionization potential. Both these effects have been analyzed in detail. We found that the CO ω shift is largely due to a combination of the electrostatic Stark effect and of the repulsion occurring when the CO molecule stretches in the presence of the rigid surface. The change in the 1π–5б binding energies, on the other hand, has an entirely electrostatic origin. Neither the ω shift nor the 5б binding energy shift are determined by the б-donation mechanism. Nevertheless, the occurrence of a charge transfer from CO to the empty levels of the Ti centers is well documented by (a) the energy and dipole moment change associated to this mechanism, (b) the expectation value of a projection operator which measures the charge associated with a given orbital, and (c) the CO dynamic dipole moment. The same analyses also rule out the occurrence of a Ti-to-CO back donation.

Ab initio study of the adducts of carbon monoxide with alkaline cations

The interaction between CO (either via the C or the O end) and the alkaline cations (Li+, Na+, K+, Rb+, and Cs+) has been studied by means of six ab initio methods, featuring the classical Hartree–Fock, the second order Mo/ller–Plesset treatment of electron correlation, one local density functional and two gradient‐corrected methods as well as a quadratic configuration interaction inclusive of single and double substitutions with a noniterative triples contribution to the energy. Basis sets adopted for CO, Li+, Na+, and K+ and the corresponding adducts are of triple‐ζ valence quality augmented with a double set of polarization functions (d on C and O; p on the cations). For Rb+ and Cs+, Hay–Wadt effective core potential basis sets have been adopted. Calculated features are the binding energy, the frequency and intensity of the CO stretch, the bending mode, the cation‐carbon (or oxygen) stretch, and the equilibrium geometry. Gradient‐corrected density functional methods yield results nearly as good as the ...

Pd and Ag dimers and tetramers adsorbed at the MgO(001) surface: a density functional study

We have studied computationally the adsorption properties of small Ag and Pd clusters deposited on the MgO(001) surface. The calculations were carried out employing a gradient-corrected density functional approach; the oxide surface was represented by model clusters embedded in a large array of point charges. Supported Ag and Pd dimers and tetramers were investigated in order to identify the preferred adsorption sites and the modifications induced in the cluster by the interaction with the substrate. All metal clusters adsorb in proximity of oxygen centers. An adsorption mode with the molecular axis parallel to the surface is the most stable one for Pd2 while Ag2 prefers an upright adsorption mode. Various isomer structures of the supported metal tetramers have been considered. In general, the most stable gas phase structure is also the preferred one upon adsorption. This suggests that the metal–metal bonding prevails over the metal–MgO interaction.

Cluster models of O2− adsorption on regular and defect sites and Fs centers of the MgO (100) surface

Abstract We have investigated the interaction of O 2 − with Mg 2+ ions at terraces, edges and corners and with the F s centers of the MgO (100) surface by means of ab i3itio cluster model wavefunctions. While O 2 is unbound or weakly bound at surface cations, O 2 − is electrostatically bound with adsorption energies from 2 to 4 eV depending on the local coordination. O 2 interacts with the electrons trapped at the F s centers to form O 2 − and F s + . In a similar way, O 2 becomes O 2 − on F s + centers with the formation of F s 2+ O 2 − surface complexes. The high cost for the ionization of the F s centers is compensated by the electrostatic attraction between O 2 − and the charged vacancies. The effect of the local electric field on the g -tensor is discussed.

Surface reactivity of MgO oxygen vacancies: electrostatic mechanisms in the formation of O2− and CO− species

The interaction of O2 and CO gas-phase molecules with oxygen vacancies on the MgO (100) surface has been studied by means of cluster models and ab initio wave functions. It is found that the surface oxygen vacancies, or Fs centers, exhibit a high reactivity toward O2 and CO at variance with the regular MgO surface. The reaction proceeds through the formation of radical anions, O2− and CO−, via the transfer of one electron trapped in the surface cavity to the empty levels of the adsorbed molecule. The resulting surface complexes, X−/Fs+ or X−/Fs2+ (X=O2 or CO), are bound by electrostatic forces. Although the mechanism of the interaction is the same for the two molecules, the details of the energetics are different. O2 spontaneously removes the electrons trapped in the MgO oxygen vacancies to form the stable O2− superoxide anion. On the contrary, CO− forms only at finite temperatures and is a metastable species. The different behavior can be rationalized in terms of electron affinities of the two molecules....

Cationic and anionic vacancies on the NiO(100) surface: DFT+U and hybrid functional density functional theory calculations

The electronic structure of oxygen and nickel vacancies at the surface of NiO(100) has been investigated theoretically by means of density functional theory (DFT) comparing plane wave density functional theory with the Hubbard correction DFT+U with atomic-orbital-hybrid-functional (20% exact-exchange) calculations. The two approaches provide a similar description of the nature of the oxygen vacancy. At variance with the same defect center in MgO, where two electrons are trapped in the vacancy, on NiO the charge is more delocalized, partly over the Ni ions around the vacancy, the rest either trapped in the vacancy or delocalized over other Ni ions. Concerning the nickel vacancy, both methods clearly show that the removal of a neutral Ni atom does not result in the oxidation of other nickel ions from Ni2+ to Ni3+ but rather in the formation of two holes in the O 2p valence band. However, the description is slightly different with the approaches, the hybrid functional indicates that the holes are essentially localized on the oxygen ions nearest to the vacancy, while they result much more delocalized from the DFT+U calculation. Comparison with the corresponding results, obtained with the two methods for the case of the Mg vacancy in MgO, suggests that the DFT+U approach does not adequately correct for the self-interaction of the unpaired electrons in this case. However, the overall picture that emerges clearly from the present calculations is that both defects affect the electronic structure in a much wider region in NiO than in MgO.

Discovery of new inhibitors of aldose reductase from molecular docking and database screening.

Aldose reductase (ALR2) is a target enzyme for the treatment of diabetic complications. Owing to the limited number of currently available drugs for the treatment of diabetic complications, the discovery of new inhibitors of ALR2 that can potentially be optimized as drugs appears highly desirable. In this study, a molecular docking analysis of the structures of more than 127,000 organic compounds contained in the National Cancer Institute database was performed to find and score molecules that are complementary to ALR2. Besides retrieving several carboxylic acid derivatives, which are known to generally inhibit aldose reductase, docking proposed other families of putative inhibitors such as sulfonic acids, nitro-derivatives, sulfonamides and carbonyl derivatives. Twenty-five compounds, chosen as the highest-scoring representatives of each of these families, were tested as aldose reductase inhibitors. Five of them were found to inhibit aldose reductase in the micromolar range. For these active compounds, selectivity with respect to the closely-related aldehyde reductase was determined by measuring the corresponding inhibitory activities. The structures of the complexes between the new lead inhibitors and aldose reductase, here refined with molecular mechanics and molecular dynamics calculations, suggest that new pharmacophoric groups can bind aldose reductase very efficiently. In the case of the family of the nitro-derivative inhibitors, a class of particularly interesting compounds, a round of optimizations was performed with the synthesis and biological evaluation of a series of derivatives aimed at testing the proposed binding mode and at improving interaction with active site residues. Starting from a hit compound having an IC(50) of 42 microM, the most potent compound synthesized showed a 10-fold increase in inhibitory activity and 10-fold selectivity with respect to ALR1, and structure--activity relationships of the designed compounds were in agreement with the proposed mode of binding at the active site.

Reactions of silica strained rings: an experimental and ab-initio study

Abstract Ab-initio techniques inclusive of electron correlation (accounted for through both the perturbative MP2 method and the local and gradient-corrected density functional methods) are used in conjunction with IR spectroscopy of surface species to study the reaction of a few molecules with the strained four-atom rings present at the surface of amorphous silica outgassed at high temperature, mimicked in the calculations by cyclodisiloxane. Slightly acidic molecules like water are known to react under mild conditions; attempts have been made to activate N 2 , CO 2 , H 2 and H 2 CO in a microwave discharge. Only with H 2 reaction has been observed. In full agreement with the experiment, calculated standard free enthalpies of reaction and activation energies show that reaction is: (i) facile with water; (ii) possible though somewhat activated with hydrogen; (iii) thermodynamically not allowed with carbon dioxide; (iv) by no means possible with molecular nitrogen.

CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

Abstract The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal–CO binding energy and the C–O stretching frequency change as function of the substrate site where the metal atom is bound.

Acetylene polymerization on supported transition metal clusters

A review. The polymn. of acetylene, studied exptl. and theor. on nano catalysts consisting of nanoscale clusters of different size and elemental compn., is reviewed. As on bulk systems palladium is the most active transition metal for this reaction. More important, however, is the changing selectivity as function of size and elemental compn. As an example, palladium atoms, dimers, and trimers, as well as nanoscale copper clusters are highly selective for the cyclotrimerization reaction. In the case of palladium, the I€-bonding of acetylene and a charge transfer from the substrate to the atom/cluster are responsible for the high selectivity. In addn. calcns. revealed the whole reaction path of this reaction on palladium atoms and it could be shown that in contrast to bulk systems the rate-detg. step is the formation of benzene from the Pd(C4H4)C2H2 complex. [on SciFinder(R)]