Theoretical study of hydrolysis mechanism of khellin

This paper describes khellin hydrolysis mechanism using semiemperical PM3 implemented in Gaussian 03 package. The calculations show that in the presence of an acidic media, an enolate molecular ion leads directly to ω-acetokhellinone while in the basic media it leads to khellinone.     

Concurrent Covalent and Supramolecular Polymerization

Covalent and supramolecular polymerizations, both of which offer their own unique advantages, have emerged as popular strategies for making artificial materials. Herein, we describe a concurrent covalent and supramolecular polymerization strategy—namely, one which utilizes 1) a bis‐azide‐functionalized diazaperopyrenium dication that undergoes polymeriation covalently with a bis‐alkyne‐functionalized biphenyl derivative in one dimension as a result of a rapid and efficient β‐cyclodextrin(CD)‐accelerated, cucurbit[6]uril(CB)‐templated azide–alkyne cycloaddition, while 2) the aromatic core of the dication is able to dimerize in a criss‐cross fashion by dint of π–π interactions, enabling simultaneous supramolecular assembly, resulting in an extended polymer network in an orthogonal dimension.     

A full multiple scattering model for the analysis of time-resolved X-ray difference absorption spectra

A full multiple theoretical model (MXAN) is applied to fit picosecond difference X-ray absorption spectra at the ruthenium L(3) edge upon photoexcitation of aqueous [RuII(bpy)3]2+. We show that fitting difference spectra allows an increase in sensitivity, such that slight structural changes can be retrieved, which are not detected in fitting full spectra. The Ru-N bond distances of the excited complex in the (3)MLCT state are in good agreement with recently published values. The implementation of the present approach to L-edge spectra and its high sensitivity opens opportunities for its extension to a large class of experiments where difference X-ray absorption spectra are recorded.     

Aqueous solvation dynamics at metal oxide surfaces

Broadband transient absorption (TA) spectroscopy, three-pulse photon echo peak shift (3PEPS), and anisotropy decay measurements were used to study the solvation dynamics in bulk water and interfacial water at ZrO(2) surfaces, using Eosin Y as a probe. The 3PEPS results show a multiexponential behavior with two subpicosecond components that are similar in bulk and interfacial water, while a third component of several picoseconds is significantly lengthened at the interface. The bandwidth correlation function from TA spectra exhibits the same behavior, and the TA spectra are well reproduced using the doorway-window picture with the time constants from PEPS. Our results suggest that interfacial water is restricted to a thickness of less than 5 A. Also the high-frequency collective dynamics of water does not seem to be affected by the interface. On the other hand, the increase of the third component may point to a slowing down of diffusional motion at the interface, although other effects, may play a role, which are discussed.     

Probing the transition from hydrophilic to hydrophobic solvation with atomic scale resolution

Picosecond and femtosecond X-ray absorption spectroscopy is used to probe the changes of the solvent shell structure upon electron abstraction of aqueous iodide using an ultrashort laser pulse. The transient L(1,3) edge EXAFS at 50 ps time delay points to the formation of an expanded water cavity around the iodine atom, in good agreement with classical and quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. These also show that while the hydrogen atoms pointed toward iodide, they predominantly point toward the bulk solvent in the case of iodine, suggesting a hydrophobic behavior. This is further confirmed by quantum chemical (QC) calculations of I(-)/I(0)(H(2)O)(n=1-4) clusters. The L(1) edge sub-picosecond spectra point to the existence of a transient species that is not present at 50 ps. The QC calculations and the QM/MM MD simulations identify this transient species as an I(0)(OH(2)) complex inside the cavity. The simulations show that upon electron abstraction most of the water molecules move away from iodine, while one comes closer to form the complex that lives for 3-4 ps. This time is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform an H-bond network. Only then is the interaction with the solvation shell strong enough to pull the water molecule of the complex toward the bulk solvent. Overall, much of the behavior at early times is determined by the reorientational dynamics of water molecules and the formation of a complete network of hydrogen bonded molecules in the first solvation shell.     

Ultrafast fluorescence studies of dye sensitized solar cells

Time-resolved fluorescence spectra from the RuN719 dye exhibit very short lifetimes (<30 fs) in solutions, on non-injecting substrates and on injecting ones. This reveals <10 fs intramolecular energy redistribution competing with the injection. We conclude that injection proceeds on a sub-10 fs time scale from non-thermalized levels of the dye.     

The Effects of Formulation on Imidacloprid Dissipation in Grapes and Vine Leaves and on Required Pre-Harvest Intervals under Lebanese Climatic Conditions

In this study, imidacloprid, a systemic insecticide, currently having a specified European Commission MRL value for vine leaves (2 mg kg⁻¹), was applied on a Lebanese vineyard under different commercial formulations: as a soluble liquid (SL) and water dispersible granules (WDG). In Lebanon, many commercial formulations of imidacloprid are subject to the same critical good agricultural practice (cGAP). It was, therefore, important to verify the variability in dissipation patterns according to matrix nature and formulation type. Random samplings of grapes and vine leaves were performed starting at 2 days until 18 days after treatment. Residue extractions were performed according to the QuEChERS method and the analytical determination using liquid chromatography coupled to tandem mass spectrometry (LC-MS-MS). The SL formulation yielded significantly higher initial deposit than the WDG formulation on grapes and vine leaves. The formulation type did not significantly affect the dissipation rates; the estimated half-lives in grapes and vine leaves were 0.5 days for all imidacloprid formulations. No pre-harvest intervals were necessary on grapes. PHIs of 3.7 days for the SL formulation and 2.8 days for the WDG formulation were estimated on vine leaves. The results showed that the type of formulation and the morphological and physiological characteristics of the matrix had an effect on the initial deposits, and thus residue levels, but not on the dissipation patterns.     

Relativistic effects in spectroscopy and photophysics of heavy-metal complexes illustrated by spin–orbit calculations of [Re(imidazole)(CO)3(phen)]+

Spin–orbit coupling (SOC) is an essential factor in photophysics of heavy transition metal complexes. By enabling efficient population of the lowest triplet state and its strong emission, it gives rise to a very interesting photophysical behavior and underlies photonic applications such as organic light emitting diodes (OLED) or luminescent imaging agents. SOC affects excited-state characters, relaxation dynamics, radiative and nonradiative decay pathways, as well as lifetimes and reactivity. We present a new photophysical model based on mixed-spin states, illustrated by relativistic spin–orbit TDDFT and MS-CASPT2 calculations of [Re(imidazole)(CO)₃(1,10-phenanthroline)]⁺. An excited-state scheme is constructed from spin–orbit (SO) states characterized by their energies, double-group symmetries, parentages in terms of contributing spin-free singlets and triplets, and oscillator strengths of corresponding transitions from the ground state. Some of the predictions of the relativistic SO model on the number and nature of the optically populated and intermediate excited states are qualitatively different from the spin-free model. The relativistic excited-state model accounts well for electronic absorption and emission spectra of Re^I carbonyl diimines, as well as their complex photophysical behavior. Then, we discuss the SO aspects of photophysics of heavy metal complexes from a broader perspective. Qualitative SO models as well as previous relativistic excited-state calculations are briefly reviewed together with experimental manifestations of SOC in polypyridine and cyclometallated complexes of second- and third row d⁶ metals. It is shown that the relativistic SO model can provide a comprehensive and unifying photophysical picture.     

Acid-Promoted Oxygen-Atom Transfer from a Novel Dihydroisoquinoline-Derived Oxaziridine Substituted at Position 1

N-Alkyl-oxaziridines react in the presence of a nucleophile as an oxygenating agent; in the absence of a nucleophile, an isomerization reaction is observed. This study describes the synthesis of two new dihydroisoquinoline oxaziridines and their reactivity in an acid-promoted reaction in the presence and absence of sulfides.     

Removal of Cyanide from Aqueous Solutions by Adsorption on Activated Carbon Prepared from Lignocellulosic By-products

Cyanide is considered one of the most dangerous compounds for the environment. They are discharged by various industries: chemical and metallurgical processes (extraction of gold and silver) and food industries. Adsorption is among the most used processes for elimination of cyanides particularly for the low concentrations. In this work, the cyanide removal is carried out by adsorption onto activated carbons prepared from olive stones and coffee ground. So we can promote this by-product as an inexpensive adsorbent. The prepared activated carbons are characterized by scanning electron micrograph and by determination of the physicochemical properties and specific surface area. All the adsorption experiments were performed in batch mode on synthetic water cyanide (KCN) at pH 10.8–11.0 to avoid volatilization of very toxic HCN. To describe the adsorption kinetics, the kinetic models of pseudo-first-order, pseudo-second-order, and intra-particle diffusion were applied. The experimental equilibrium data for adsorption of free cyanide were analyzed by the Langmuir, Freundlich, and Temkin isotherm models.