A simple analysis of the influence of the solvent-induced electronic polarization on the N-15 magnetic shielding of pyridine in water

Electronic polarization induced by the interaction of a reference molecule with a liquid environment is expected to affect the magnetic shielding constants. Understanding this effect using realistic theoretical models is important for proper use of nuclear magnetic resonance in molecular characterization. In this work, we consider the pyridine molecule in water as a model system to briefly investigate this aspect. Thus, Monte Carlo simulations and quantum mechanics calculations based on the B3LYP/6-311++G (d,p) are used to analyze different aspects of the solvent effects on the N-15 magnetic shielding constant of pyridine in water. This includes in special the geometry relaxation and the electronic polarization of the solute by the solvent. The polarization effect is found to be very important, but, as expected for pyridine, the geometry relaxation contribution is essentially negligible. Using an average electrostatic model of the solvent, the magnetic shielding constant is calculated as -58.7 ppm, in good agreement with the experimental value of -56.3 ppm. The explicit inclusion of hydrogen-bonded water molecules embedded in the electrostatic field of the remaining solvent molecules gives the value of -61.8 ppm.

Functionalization of gold and silver nanoparticles with diphenyl dichalcogenides probed by surface enhanced Raman scattering

This paper describes a surface-enhanced Raman scattering (SERS) systematic investigation regarding the functionalization of gold (Au) and silver (Ag) nanoparticles with diphenyl dichalcogenides, i.e. diphenyl disulfide, diphenyl diselenide, and diphenyl ditelluride. Our results showed that, in all cases, functionalization took place with the cleavage of the chalcogenchalcogen bond on the surface of the metal. According to our density functional theory calculations, the molecules assumed a tilted orientation with respect to the metal surface for both Au and Ag, in which the angle of the phenyl ring relative to the metallic surface decreased as the mass of the chalcogen atom increased. The detected differences in the ordinary Raman and SERS spectra were assigned to the distinct stretching frequencies of the carbonchalcogen bond and its relative contribution to the ring vibrational modes. In addition, the SERS spectra showed that there was no significant interaction between the phenyl ring and the surface, in agreement with the tilted orientation observed from our density functional theory calculations. The results described herein indicate that diphenyl dichalcogenides can be successfully employed as starting materials for the functionalization of Au nanoparticles with organosulfur, organoselenium, and organotellurium compounds. On the other hand, diphenyl disulfide and diphenyl diselenide could be employed for the functionalization of Ag nanoparticles, while the partial oxidation of the organotellurium unit could be detected on the Ag surface. Copyright (C) 2011 John Wiley & Sons, Ltd.

Enhancement of optical absorption by modulation of the oxygen flow of TiO2 films deposited by reactive sputtering

Oxygen-deficient TiO2 films with enhanced visible and near-infrared optical absorption have been deposited by reactive sputtering using a planar diode radio frequency magnetron configuration. It is observed that the increase in the absorption coefficient is more effective when the O-2 gas supply is periodically interrupted rather than by a decrease of the partial O-2 gas pressure in the deposition plasma. The optical absorption coefficient at 1.5 eV increases from about 1 x 10(2) cm(-1) to more than 4 x 10(3) cm(-1) as a result of the gas flow discontinuity. A red-shift of similar to 0.24 eV in the optical absorption edge is also observed. High resolution transmission electron microscopy with composition analysis shows that the films present a dense columnar morphology, with estimated mean column width of 40nm. Moreover, the interruptions of the O-2 gas flow do not produce detectable variations in the film composition along its growing direction. X-ray diffraction and micro-Raman experiments indicate the presence of the TiO2 anatase, rutile, and brookite phases. The anatase phase is dominant, with a slight increment of the rutile and brookite phases in films deposited under discontinued O-2 gas flow. The increase of optical absorption in the visible and near-infrared regions has been attributed to a high density of defects in the TiO2 films, which is consistent with density functional theory calculations that place oxygen-related vacancy states in the upper third of the optical bandgap. The electronic structure calculation results, along with the adopted deposition method and experimental data, have been used to propose a mechanism to explain the formation of the observed oxygen-related defects in TiO2 thin films. The observed increase in sub-bandgap absorption and the modeling of the corresponding changes in the electronic structure are potentially useful concerning the optimization of efficiency of the photocatalytic activity and the magnetic doping of TiO2 films. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4724334]

Two-photon absorption in oxazole derivatives: An experimental and quantum chemical study

Experimental and theoretical studies on the two-photon absorption properties of two oxazole derivatives: 2,5-diphenyloxazole (PPO) and 2-(4-biphenylyI)-5-phenyl-1,3,4-oxadiazole (PBD) are presented. The two-photon absorption cross-section spectra were determined by means of the Z-scan technique, from 460 up to 650 nm, and reached peak values of 84 GM for PBD and 27 GM for PPO. Density Functional Theory and response function formalism are used to determine the molecular structures and the one- and two-photon absorption properties and to assist in the interpretation of the experimental results. The Polarizable Continuum Model in one-photon absorption calculations is used to estimate solvent effects. (C) 2011 Elsevier B.V. All rights reserved.

Very Intense Distinct Blue and Red Photoluminescence Emission in MgTiO3 Thin Films Prepared by the Polymeric Precursor Method: An Experimental and Theoretical Approach

MgTiO3 (MTO) thin films were prepared by the polymeric precursor method with posterior spin-coating deposition. The films were deposited on Pt(111)/Ti/SiO2/Si(100) substrates and heat treated at 350 degrees C for 2 h and then heat treated at 400, 450, 500, 550, 600, 650 and 700 C for 2 h. The degree of structural order disorder, optical properties, and morphology of the MTO thin films were investigated by X-ray diffraction (XRD), micro-Raman spectroscopy (MR), ultraviolet-visible (UV-vis) absorption spectroscopy, photoluminescence (PL) measurements, and field-emission gun scanning electron microscopy (FEG-SEM) to investigate the morphology. XRD revealed that an increase in the annealing temperature resulted in a structural organization of MTO thin films. First-principles quantum mechanical calculations based on density functional theory (B3LYP level) were employed to study the electronic structure of ordered and disordered asymmetric models. The electronic properties were analyzed, and the relevance of the present theoretical and experimental results was discussed in the light of PL behavior. The presence of localized electronic levels and a charge gradient in the band gap due to a break in the symmetry are responsible for the PL in disordered MTO lattice.

Rare-earth impurities in gallium nitride: The role of the Hubbard potential

We performed a first principles investigation on the electronic properties of 4f-rare earth substitutional impurities in zincblende gallium nitride (GaN:REGa, with RE=Eu, Gd, Tb, Dy, Ho, Er and Tm). The calculations were performed within the all electron methodology and the density functional theory. We investigated how the introduction of the on-site Hubbard U potential (GGA + U) corrects the electronic properties of those impurities. We showed that a self-consistent procedure to compute the Hubbard potential provides a reliable description on the position of the 4f-related energy levels with respect of the GaN valence band top. The results were compared to available data coming from a recent phenomenological model. (C) 2012 Elsevier B.V. All rights reserved.

Electronic and magnetic properties of SnO2/CrO2 thin superlattices

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2) n (SnO2) n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2) n (SnO2) n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2) n (SnO2) n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Atomistic Study of Water Confined in Silica.

In this work, we have used a combined of atomistic simulation methods to explore the effects of confinement of water molecules between silica surfaces. Firstly, the mechanical properties of water severe confined (~3A) between two silica alpha-quartz was determined based on first principles calculations within the density functional theory (DFT). Simulated annealing methods were employed due to the complex potential energry surface, and the difficulties to avoid local minima. Our results suggest that much of the stiffness of the material (46%) remains, even after the insertion of a water monolayer in the silica. Secondly, in order to access typical time scales for confined systems, classical molecular dynamics was used to determine the dynamical properties of water confined in silica cylindrical pores, with diameters varying from 10 to 40A. in this case we have varied the passivation of the silica surface, from 13% to 100% of SiOH, and the other terminations being SiOH2 and SiOH3, the distribution of the different terminations was obtained with a Monte Carlo simulation. The simulations indicates a lowering of the diffusion coefficientes as the diameter decreases, due to the structuration of hydrogen bonds of water molecules; we have also obtained the density profiles of the confined water and the interfacial tension.

Study of the Influence of Localized Vibrational Modes in Charge Transport Properties at Nanoscale Systems.

In molecular and atomic devices the interaction between electrons and ionic vibrations has an important role in electronic transport. The electron-phonon coupling can cause the loss of the electron's phase coherence, the opening of new conductance channels and the suppression of purely elastic ones. From the technological viewpoint phonons might restrict the efficiency of electronic devices by energy dissipation, causing heating, power loss and instability. The state of the art in electron transport calculations consists in combining ab initio calculations via Density Functional Theory (DFT) with Non-Equilibrium Green's Function formalism (NEGF). In order to include electron-phonon interactions, one needs in principle to include a self-energy scattering term in the open system Hamiltonian which takes into account the effect of the phonons over the electrons and vice versa. Nevertheless this term could be obtained approximately by perturbative methods. In the First Born Approximation one considers only the first order terms of the electronic Green's function expansion. In the Self-Consistent Born Approximation, the interaction self-energy is calculated with the perturbed electronic Green's function in a self-consistent way. In this work we describe how to incorporate the electron-phonon interaction to the SMEAGOL program (Spin and Molecular Electronics in Atomically Generated Orbital Landscapes), an ab initio code for electronic transport based on the combination of DFT + NEGF. This provides a tool for calculating the transport properties of materials' specific system, particularly in molecular electronics. Preliminary results will be presented, showing the effects produced by considering the electron-phonon interaction in nanoscale devices.

Spin Orbit Effects in the Electronic Transport Properties of Adsorbed Graphene Nanoribbons

Graphene has received great attention due to its exceptional properties, which include corners with zero effective mass, extremely large mobilities, this could render it the new template for the next generation of electronic devices. Furthermore it has weak spin orbit interaction because of the low atomic number of carbon atom in turn results in long spin coherence lengths. Therefore, graphene is also a promising material for future applications in spintronic devices - the use of electronic spin degrees of freedom instead of the electron charge. Graphene can be engineered to form a number of different structures. In particular, by appropriately cutting it one can obtain 1-D system -with only a few nanometers in width - known as graphene nanoribbon, which strongly owe their properties to the width of the ribbons and to the atomic structure along the edges. Those GNR-based systems have been shown to have great potential applications specially as connectors for integrated circuits. Impurities and defects might play an important role to the coherence of these systems. In particular, the presence of transition metal atoms can lead to significant spin-flip processes of conduction electrons. Understanding this effect is of utmost importance for spintronics applied design. In this work, we focus on electronic transport properties of armchair graphene nanoribbons with adsorbed transition metal atoms as impurities and taking into account the spin-orbit effect. Our calculations were performed using a combination of density functional theory and non-equilibrium Greens functions. Also, employing a recursive method we consider a large number of impurities randomly distributed along the nanoribbon in order to infer, for different concentrations of defects, the spin-coherence length.

Reaction intermediates of ethanol electro-oxidation on platinum investigated by SFG spectroscopy

Although electrochemical oxidation of simple organic molecules on metal catalysts is the basic ingredient of fuel cells, which have great technological potential as a renewable source of electrical energy, the detailed reaction mechanisms are in most cases not completely understood. Here, we investigate the ethanol-platinum interface in acidic aqueous solution using infrared-visible sum frequency generation (SFG) spectroscopy and theoretical calculations of vibrational spectra in order to identify the intermediates present during the electro-oxidation of ethanol. The complex vibrational spectrum in the fingerprint region imply on the coexistence of several adsorbates. Based on spectra in ultra-high-vacuum (UHV) and electrochemical environment from the literature and our density functional theory (DFT) calculations of vibrational spectra, new adsorbed intermediates, never before observed with conventional infrared (IR) spectroscopy, are proposed here: g2-acetaldehyde, g2-acetyl, ethylidyne, monodentate acetate, methoxy, tertiary methanol derivative, COH residue, g2-formaldehyde, mono and bidentate formate, CH3 and CH2 residues. In addition, we present new evidences for an ethoxy intermediate, a secondary ethanol derivative and an acetyl species, and we confirm the presence of previously observed adsorbates: a tertiary ethanol derivative, bidentate acetate, and COad. These results indicate that the platinum surface is much more reactive, and the reaction mechanism for ethanol electro-oxidation is considerably more complex than previously considered. This might be also true for many other molecule-catalyst systems.

Host-guest interactions between xanthones and water: the role of O-H center dot center dot center dot O, C-H center dot center dot center dot O, and pi center dot center dot center dot pi contacts in the channel- and cage-type frameworks

In this article were studied two xanthone derivatives known as 1,5-dihydroxy-8-methoxyxanthone (I) and 1,3,7-trihydroxy-8-methoxyxanthone (II), which show one water molecule into their crystal structures. In xanthone I, there are water wires contributing to build up channel-like cavities along the c axis, whereas in xanthone II the water is surrounded by three xanthone molecules forming a cage-type structure. The geometries of I and II were optimized using the density functional theory method with B3LYP functional, and the results were compared with crystal structure. Both theoretical and experimental investigations reveal a concordance between structural parameters, with the xanthone core presenting an almost flat conformation and substituents adopting the more stable orientations. In the two compounds, the hydroxyl group linked at position 1 is involved in a resonance-assisted hydrogen bond with the carbonyl group. Besides, the supramolecular arrangement of the host/guest systems are stabilized mainly by classical intermolecular hydrogen bonds (O-H center dot center dot center dot O) involving xanthone-to-water and xanthone-to-xanthone. In addition, C-H center dot center dot center dot O weak hydrogen bonds, as well as pi-pi interactions play an important role to stabilize the crystal self-assembly of xanthones I and II. The results reported here underline the role of inclusion of water molecules and their different arrangement into the crystal structure of two xanthone host/guest systems.

Theoretical models for the antitrypanosomal activity of thiosemicarbazone derivatives

Thiosemicarbazones are cruzain inhibitors which have been identified as potential antitrypanosomal agents. In this work, several molecular properties were calculated at the density functional theory (DFT)/B3LYP/6-311G* level for a set of 44 thiosemicarbazones. Unsupervised and supervised pattern recognition techniques (hierarchical cluster analysis, principal component analysis, kth-nearest neighbors, and soft independent modeling by class analogy) were used to obtain structureactivity relationship models, which are able to classify unknown compounds according to their activities. The chemometric analyses performed here revealed that 12 descriptors can be considered responsible for the discrimination between high and low activity compounds. Classification models were validated with an external test set, showing that predictive classifications were achieved with the selected variable set. The results obtained here are in good agreement with previous findings from the literature, suggesting that our models can be useful on further investigations on the molecular determinants for the antichagasic activity. (C) 2012 Wiley Periodicals, Inc.

Light emission and current rectification in a molecular device: Experiment and theory

We have investigated optical and transport properties of the molecular structure 2,3,4,5-tetraphenyl-1-phenylethynyl-cyclopenta-2,4-dienol experimentally and theoretically. The optical spectrum was calculated using Hartree-Fock-intermediate neglect of differential overlap-configuration interaction model. The experimental photoluminescence spectrum showed a peak around 470nm which was very well described by the modeling. Electronic transport measurements showed a diode-like effect with a strong current rectification. A phenomenological microscopic model based on non-equilibrium Green's function technique was proposed and a very good description electronic transport was obtained. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767457]

Surface-enhanced Raman spectroscopy studies of organophosphorous model molecules and pesticides

This work reports the analytical application of surface-enhanced Raman spectroscopy (SERS) in the trace analysis of organophosphorous pesticides (trichlorfon and glyphosate) and model organophosphorous compounds (dimethyl methylphosphonate and o-ethyl methylphosphonothioate) bearing different functional groups. SERS measurements were carried out using Ag nanocubes with an edge square dimension of ca. 100 nm as substrates. Density functional theory (DFT) with the B3LYP functional was used for the optimization of ground state geometries and simulation of Raman spectra of the organophosphorous compounds and their silver complexes. Adsorption geometries and marker bands were identified for each of the investigated compound. Results indicate the usefulness of SERS methodology for the sensitive analyses of organophosphorous compounds through the use of vibrational spectroscopy.