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

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

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

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

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

Second Order Hyperpolarizabilities Dependence on the Benzenic Ring position of Organic and Organometallic Benzo[c]thiophene species: an assessment by DFT methods

Nonlinear optical (NLO) active organic chromophores find application in a wide range of technological applications, from molecular electronics to optical sensors and switches. Thus, the establishment of methods that can predict a priori the optical properties of organic or organometallic chromophores is indeed a crucial topic of current research. Density Functional Theory (DFT) methods are by far the most used computational methods for estimation of optical properties of molecules due to their low computational effort and accurate results. [1] Furthermore, they allow the study of structure/activity correlations in molecules without the need to synthesize all of them. DFT calculations can be used for a screening of NLO chromophores prior to the synthetic labor. Among all the available DFT functionals, B3LYP is by far the most common. Others, like CAM-B3LYP are now being evaluated and they show similar or improved results concerning the prediction of optical properties. [2] In the present work a combination of DFT with the Finite Field (FF) method was used to investigate the influence on the benzenic ring position on the magnitude of the static first hyperpolarizibility of several nitro acetylene organic fragments based on benzo[c]thiophenes. Time Dependent – DFT methods (TD-DFT) were employed to determine the spectral data of all the compounds. After the screening of the organic chromophores was complete, coordination to an iron (II) and ruthenium (II) monocyclopentadienyl fragment was investigated. The chromophores for coordination were chosen according to available procedures in literature to further synthetic achievement. Results show that better NLO responses should be obtained when the benzene ring is close to the electron donor group, whilst the presence of such aromatic ring close to the acceptor group should lead to a decreasing on the NLO response. This behavior is justified, among other possibilities, in terms of HOMO-LUMO band gaps by correlation of the DFT and TD-DFT methodologies