Adsorption of two pesticides on a clay surface: a theoretical study

The contamination of water resources with many organic xenobiotic compounds poses a challenge to environmental sciences and technologies [1]. Although in many cases these contaminants are present only in small concentrations, the large variety of such compounds (some of which are classified as priority pollutants) is a matter of concern. Adsorption, alone or as part of a more complex water or wastewater treatment process, has been seen as playing a very important role in the removal of many of these pollutants [2]. In this regard, the choice of adsorbent materials is crucial, which requires an understanding of the details involved in the adsorption of more or less complex organic molecules by a variety of surfaces of different types. In addition to laboratory studies, computational studies may be valuable in this study [3]. MCPA (2-methyl-4-chlorophenoxyacetic acid, a herbicide) and Clofibric acid (2-(4-chlorophenoxy)-2-methylpropanoic, the metabolite of a pharmaceutical, clofibrate, and also a herbicide) are two phenoxy acids that differ only slightly in their structures. However, a quite distinct behavior in adsorption phenomena on clay materials has been observed in past studies [4]. By relating those differences with the molecules' structural features through atomistic computational studies, some insight may be gained into the respective adsorption processes of this type of compounds. In the present work quantum chemical calculations at density functional theory level have been performed to study the adsorption of MCPA and Clofibric acid by a clay surface model. Since hydration plays an important role for the adsorption process of these species, solvent effects were considered by inclusion of water molecules explicitly into the quantum chemical calculations. The deprotonated negatively charged species were found to strongly interact with the surface and the distinct behavior of both species upon adsorption was compared with experimental evidences

Density Functional Theory Calculations on η5-Monocyclopentadienylmetal Complexes with Conjugated Nitrile Chromophores

During the past decade, the interest on organometallic compounds for the development of novel nonlinear optical (NLO) materials with large second-order nonlinearities has considerably increased in view of their potential application in the area of integrated optics [1]. Experimental and computational investigations utilizing quantum theory afforded useful insights concerning the molecular structural requirements necessary to induce large second-order nonlinearity response. The strategy to obtain these properties has been the synthesis of compounds with highly polarizable asymmetric structures through the existence of delocalized electronic system bridge (chromophore) between an electron-withdrawing and electron-donating group. Recent experimental work on 5-monocyclopentadienylmetal complexes with p-substituted benzonitrile and oligothiophene nitrile chromophores showed a correlation between the first hyperpolarizability () and the communicability between the chromophores and the organometallic fragments [2]. In addition, some EHMO calculations on nickel and iron complexes had corroborated that the larger values of experimental hyperpolarizability correspond to lower HOMO-LUMO gaps [3]. In order to get a better understanding on the electronic factors that may be responsible for the second-order nonlinear optical behavior of [CoCp(dppe)(p-NCC6H4R)]2+ and [FeCp(dppe)(NC{SC4H2}nNO2)]+ complexes and their correlation with experimental spectroscopic and electrochemical data, density functional theory calculations were made in the model complexes [CoCp(H2PCH2CH2PH2)(p-NCC6H4R)]2+ and [FeCp(H2PCH2CH2PH2) (NC{SC4H2}nNO2)]+ (n=1,2). Especial emphasis was given to the HOMO-LUMO gaps and their character. Spatial localization of electron charge by means of topological analysis of the electron localization function (ELF) as been performed to gain insight into the nature of the chromophores binding to the metal centers

Nonlinear Optical Properties of η5-Monocyclopentadienyliron Complexes From DFT Calculations

Recently, organometallic complexes have emerged as potential building blocks for second-order nonlinear optical (SONLO) materials in view of their potential application in the area of integrated optics [1]. Unlike organic molecules, whose optical nonlinearities have been extensively studied by computational methodologies using both semi-empirical and ab initio methods, organometallic systems have received much less attention due to the difficulty in the calculation of reliable hyperpolarizabilities in the presence of transition metal atoms. ZINDO has been the most widely used program to calculate SONLO coefficients of organometallic compounds. A more reliable approach, using the density functional theory (DFT) method is less explored but has been increased in the recent years. Experimental work on 5-monocyclopentadienyliron complexes with p-nitro benzonitrile ligands showed that the first hyperpolarizability decreases with increasing conjugation length of the chromophores [2]. EHMO calculations performed on model complexes [FeCp(PH3)2(p-NCR)]+ confirms this behavior and showed that the larger values of experimental hyperpolarizabilities correspond to lower HOMO-LUMO gaps [3]. For similar complexes possessing a thiophene based conjugated backbone, a dramatic increase in experimental first hyperpolarizability with increasing conjugation length was observed [4]. The explanation of this different behaviour was not definitively established. In order to get a better understanding on the electronic factors that may be responsible for the SONLO behavior of these compounds and their correlation with experimental spectroscopic and electrochemical data, high accuracy DFT calculations using GAMESS-US were made in model complexes [FeCp(H2PCH2CH2PH2)(NC{SC4H2}nNO2)]+ (n=1-3). Spatial localization of electron charge as been performed to gain insight into the nature of the chromophores binding to the metal center. First static hyperpolarizability was calculated, compared with experimental results and correlated to the HOMO-LUMO gaps

Recent Developments in the Study of the Behavior of Fluorescent Membrane Probes in Lipid Bilayers: Molecular Dynamics Approach

We present a review of recent developments in the study of the behavior of fluorescent membrane probes in lipid bilayers by molecular dynamics simlations

Palladium catalysed sequential imine arylation/Suzuki–Miyaura coupling: synthesis of a-(biarylyl)benzylamines

report an innovative, high yielding one-pot sequential catalytic imine arylation/SuzukieMiyaura cross-coupling reaction, which converts suitably activated imine substrates to various biarylarylmethyl amine products using several commercial Pd catalysts. Many biarylarylmethyl amine molecules are biologically active. Insightful computational studies detail the mechanism of the imine arylation process. The sequence of reactions is likely to be dependent on the reaction conditions

Mono and Binuclear Complexes for Nonlinear Optical Switching: A DFT Study

The search for materials exhibiting switchable second-order nonlinear optical (SONLO) properties has attracted a great deal of attention owing to their potential application as key nanoscale components for digital processing and data storage. Our research in organometallic complexes with SONLO properties resulted in the development of some new promising thienyl-acetylide 5-monocyclopentadieniliron(II) and ruthenium(II) complexes [1,2]. These compounds present typical push-pull architecture (crucial for maximizing the molecular quadratic hyperpolarizability, β), were an electron donor is linked to an electron acceptor group by a conjugated system. Changing the donor/acceptor abilities of any of these end-groups, by redox means for example, gives the chance to control the magnitude of β value, and hence obtain a SONLO switch. In this presentation, we show the application of Density Functional Theory (DFT) in the prediction of SONLO switching properties of mono and bimetallic complexes bearing two organometallic fragments, 5-monocyclopentadienyliron(II) and 5-monocyclopentadienylnickel(II) moieties, in different formal oxidation states. The obtained hyperpolarizabilities will be correlated with structural and electronic data. Results show that redox changes provide a feasible way to obtain good SONLO switches

Assessing the importance of Van der Waals interactions on the adsorption of azobenzene on the rutile TiO2(110) surface

The role of dispersion in the adsorption of cis- and trans-azobenzene on rutile TiO2(110) has been investigated by means of density functional theory methods. Dispersion is crucial to properly describe the relative values of the adsorption energy since it differently affects each isomer. Dispersion terms are also crucial to describe the energy reaction corresponding to the cleavage of the N-N azo bond. However, the main features of the interaction remain unchanged since the density of states, electron density difference plots and Bader charges are only slightly affected by the inclusion of dispersion terms

Molecular simulation of C60 adsorption onto a TiO2 rutile (110) surface

Monte Carlo molecular simulation study is presented on the adsorption and growth of C60 films on the surface of the (1 1 0) face of rutile. Simulations are performed for a temperature of 600 K using atomistic models both for the fullerene molecules and the TiO2 surface. It is found in this work that C60 is adsorbed preferably in an ordered arrangement along the surface depressions over the exposed undercoordinated Ti cations. At low densities adsorption occurs preferably at alternate rows, with locations in consecutive rows being occupied appreciably only at higher C60 densities. At low densities, the fullerene molecules tend to aggregate into islands in the surface plane. Additional layers of C60 form only as the density increases, and do so before a monolayer is completed in all consecutive rows. Full monolayer capacity obtained at the highest densities is about 0.9 C60 molecules per nm2, but this is only achieved by completing the packing of molecules in interstices at a slightly upper level. The fraction of the molecules that lie closest to the surface only amounts to 0.6 molecules per nm2

Adsorption of Xe atoms on the TiO2(110) surface: A density functional study

The interaction of xenon atoms with the TiO2(1 1 0) surface of rutile has been studied by density functional theory methods. Five different possible adsorption sites on the relaxed and clean TiO2(1 1 0) surface and on two different type of oxygen vacancies possible on this oxide substrate have been considered. In the case of the defect-free substrate, and when compared with a previous study concerning the adsorption of Ar atoms also on TiO2(1 1 0), the xenon atom, as a larger and easier polarizable species, is shown to have a deeper physisorption well, as expected. Likewise, Xe atoms prefer to be bounded to positions nearby the outermost titanium atoms as found previously for Ar. This is in agreement with most studies concerning rare gases adsorbed on transition metal surfaces. In the case of the reduced surfaces, it is found that the interaction is more favourable in the protruding rows. The interaction is dominated by dispersion forces and DFT + dispersion energies are 3.5–5 times larger than the non-corrected DFT values and Xe-surface distances are smaller. Finally, an interesting correlation is obtained for the calculated interaction energies and the Xe–Ti distance