João B. L. Martins

Electronic and structural properties of the (10(1)over-bar0) and (11(2)over-bar0) ZnO surfaces

The structural and electronic properties of ZnO (1010) and (1120) surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The stability and relaxation effects for both surfaces were analyzed. The electronic and energy band properties were discussed on the basis of band structure as well as density of states. There is a significant relaxation in the (1010) as compared to the (1120) terminated surfaces. The calculated direct gap is 3.09, 2.85, and 3.09 eV for bulk, (1010), and (1120) surfaces, respectively. The band structures for both surfaces are very similar.

New potential AChE inhibitor candidates.

We have theoretically studied new potential candidates of acetylcholinesterase (AChE) inhibitors designed from cardanol, a non-isoprenoid phenolic lipid of cashew Anacardium occidentale nut-shell liquid. The electronic structure calculations of fifteen molecule derivatives from cardanol were performed using B3LYP level with 6-31G, 6-31G(d), and 6-311+G(2d,p) basis functions. For this study we used the following groups: methyl, acetyl, N,N-dimethylcarbamoyl, N,N-dimethylamine, N,N-diethylamine, piperidine, pyrrolidine, and N,N-methylbenzylamine. Among the proposed compounds we identified that the structures with substitution by N,N-dimethycarbamoyl, N,N-dimethylamine, and pyrrolidine groups were better correlated to rivastigmine, and represent possible AChE inhibitors against Alzheimer disease.

Theoretical study of ZnO (1 0 1 0) and Cu/ZnO (1010) surfaces

Periodic HF/6-31G and a hybrid density functional, B3LYP/6-31G, calculations have been carried out in order to determine the geometric and electronic structure of bulk ZnO. The lattice parameters, bulk modulus, charge distribution and band structure are reported. Surface energy and charge distribution of the ZnO (1010) surface are obtained, while top site adsorption of Cu atoms on Zn or O atoms on the ZnO (1010) surface are considered. Optimized distances, charge transfers, vibrational frequencies and binding energies associated with both types of adsorption processes are calculated. The theoretical results are compared with previous theoretical studies and available experimental data.

A theoretical analysis on electronic structure of the (110) surface of TiO2-SnO2 mixed oxide

Mixed oxide compounds, such as TiO2– SnO2 system are widely used as gas sensors and should also provide varistor properties modifying the TiO2 surface. Therefore, a theoretical investigation has been carried out characterizing the effect of SnO2 on TiO2 addition on the electronic structure by means of ab initio SCF-LCAO calculations using all electrons. In order to take into account the finite size of the cluster, we have used the point charge model for the (TiO2)15 cluster to study the effect on electronic structure of doping the TiO2 (110) surface. The contracted basis set for titanium (4322/42/3), oxygen (33/3) and tin (43333/4333/43) atoms were used. The charge distributions, dipole moments, and density of states of doping TiO2 and vacancy formation are reported and analysed. q 2003 Elsevier B.V. All rights reserved.

Lateral interaction of CO and H2 molecules on ZnO surfaces: an AM1 study

Abstract We have studied the effects of lateral interactions for CO and H 2 adsorbed on large (ZnO) 60 cluster models. The calculations were performed with the AM1 semi-empirical method. The geometric parameters of the adsorbed molecules were fully optimized. CO interacts with the zinc cation located at the site having the lowest coordination at the edge sites between the (0001) and (10 1 0) surfaces. The binding energy is increased as we increase the number of adsorbed CO molecules on the ZnO surface. For H 2 molecular interaction, the calculated energy gaps and ionization potentials are modified relative to the bare cluster. We have analyzed the optimized geometric parameters, charge transfer as well as the density of states and compared our results with available experimental data such as density of states, vibrational frequencies, adsorption energies and surface charge.

Ab initio study of CO and H2 interaction on ZnO surfaces using a small cluster model

Abstract We have studied the adsorption of H 2 and CO molecules, as well as the dissociation of H 2 , on the (ZnO) 6 cluster model using the ab initio Hartree-Fock method. The effective core potential was used for Zn, C, and O atoms at double-zeta-type valence basis set level, whereas for H we used Dunnings basis set. We have also added polarization and diffuse functions to the O, C, and H basis set. The CO molecule interacts with the lowest coordination zinc sites which are located on the edge between the (0001) and (1010) surfaces. The decrease in CO bond length upon adsorption on ZnO surfaces is associated with the charge transfer from CO to the surface. Our calculations indicate the 5σ orbital from adsorbed CO stabilized to a 1.56 eV deeper energy. Of all the configurations investigated, the molecular H 2 interaction has the lowest binding energy with a decrease in H 2 bond strength. The H 2 molecule also dissociates on the zinc and oxygen sites of the ZnO cluster, and the preferential dissociation site is the oxygen which has a coordination number of two. The H 2 dissociation shows a large stabilization energy for the most stable adsorption site which is the lowest coordination site. Molecular CO and H 2 adsorption yields a smaller change in the estimated energy gaps and ionization potentials. We have also analysed the geometry of the adsorbed molecules, the Mulliken charge, the orbital SCF energies, and also the molecular orbital densities and contour plots. Our results are compared with the available experimental data.

CO2 adsorption on single-walled boron nitride nanotubes containing vacancy defects

The adsorption of a CO2 molecule on the vacancy defect type of armchair (5,5) and zigzag (10,0) single-walled boron nitride nanotubes was studied based on Density Functional Theory (DFT). Vacancy defects were studied and the geometrical modifications implemented on the original hexagonal lattice yielded a considerable level of changes in the electronic properties. These changes are reflected in a greater level of CO2 reactivity in relation to the adsorption over a pristine structure. For all types of studied CO2 molecule interaction, we have found a chemical adsorption process based on binding energy. Furthermore, the CO2 adsorption takes place on the top of the vacancy region. A decomposition state was observed when the CO2 molecule interacted with the armchair nanotube with a vacancy on the nitrogen site. By comparing the values of the adsorption energies with those from other defect approaches present in the literature, we conclude that the proposed protocol presents a possible tool to develop stable and sensible carbon dioxide sensors.

Carbon dioxide adsorption on doped boron nitride nanotubes

Boron nitride (BN) nanotubes are promising structures as far as the gas adsorption process is concerned. The electronic and vibrational properties of pristine and cobalt doped single walled boron nitride nanotubes of different chiralities interacting with a carbon dioxide molecule are investigated through the use of density functional theory (DFT) and the discrete variable representation method. When compared to similar simulations concerning carbon nanotubes, a stronger interaction is observed between the carbon dioxide molecule and the functionalized BN nanotube. A density of state investigation suggests that the doping induces major changes in the electronic structure pattern in the sense of critically reducing the original gap. From the vibrational point of view, we note that the zig-zag chirality tends to present higher values of vibrational frequencies for most of the states considered, regardless of the nanotubes being doped or not. Our results suggest that doped zig-zag BN nanotubes are among the best possible candidates for adsorption purposes.

The H + Li2 bimolecular exchange reaction: Dynamical and kinetical properties at J = 0

For the first time in the literature, rigorous time-independent quantum scattering formalism was applied, by means of the ABC program, to the H + Li(2) → LiH + Li reaction. The state-to-state probabilities as a function of the total energy have been computed at zero total angular momentum (J = 0) allowing us to evaluate the effect of vibrational/rotational excitation on the reaction promotion/inhibition, the energetic distribution of products, and the temperature dependence of the J-shifting thermal rate coefficients.

Electronic structure and PCA analysis of covalent and non-covalent acetylcholinesterase inhibitors

Hartree-Fock and density functional methods were used to analyze electronic and structural properties of known drugs to evaluate the influence of these data on acetylcholinesterase inhibition. The energies of the frontier orbitals and the distances between the more acidic hydrogen species were investigated to determine their contributions to the activity of a group of acetylcholinesterase inhibitors. Electrostatic potential maps indicated suitable sites for drugs-enzyme interactions. In this study, the structural, electronic and spatial properties of nine drugs with known inhibitory effects on acetylcholinesterase were examined. The data were obtained based on calculations at the B3LYP/6-31 + G(d,p) level. Multivariate principal components analysis was applied to 18 parameters to determine the pharmacophoric profile of acetylcholinesterase inhibitors. Desirable features for acetylcholinesterase inhibitor molecules include aromatic systems or groups that simulate the surface electrostatic potential of aromatic systems and the presence of a sufficient number of hydrogen acceptors and few hydrogen donors. PCA showed that electronic properties, including the HOMO-1 orbital energy, logP and aromatic system quantity, as well as structural data, such as volume, size and H-H distance, are the most significant properties.