Synthesis of anionic hypervalent cyclic selenenate esters: relevance to the hypervalent intermediates in nucleophilic substitution reactions at the selenium(II) center

The synthesis of a diaryl diselenide that contains 2,6‐dicarboxylic acid groups, 2,2′‐diselanediylbis(5‐tert‐butylisophthalic acid) ( 10 ), is described. Diselenide 10 undergoes intramolecular cyclization in methanol to form a cyclic selenenate ester, 5‐tert‐butyl‐3‐oxo‐3H‐benzo[c][1,2]oxaselenole‐7‐carboxylic acid ( 11 ). The cyclization reaction proceeds more rapidly in the presence of organic bases, such as pyridine, adenine, and 4,4′‐bipyridine, to form pyridinium 5‐tert‐butyl‐3‐oxo‐3H‐benzo[c][1,2]oxaselenole‐7‐carboxylate ( 14 ), adeninium 5‐tert‐butyl‐3‐oxo‐3H‐benzo[c][1,2]oxaselenole‐7‐carboxylate ( 15 ), and 4,4′‐bipyridiniumbis(5‐tert‐butyl‐3‐oxo‐3H‐benzo[c][1,2]oxaselenole‐7‐carboxylate) ( 16 ), respectively. However, 2,2′‐diselanediyldibenzoic acid ( 22 ) does not undergo cyclization under similar conditions. Structural studies on cyclic selenenate esters 14 – 16 revealed that the Se???O (COO^?) secondary distances (2.170, 2.075, and 2.176 Å) were significantly shorter than the corresponding Se???O distances (2.465, 2.472, and 2.435 Å) observed for the selenenate esters stabilized by the neutral donors (CHO, COOH, and COOEt). ¹H, ¹³C, and ⁷⁷Se NMR spectroscopy of compounds 11 and 14 – 16 reveal that the aryl protons of compound 11 and the organic cations of compounds 14 – 16 exchange between the two carboxylate groups via a hypercoordinate intermediate. The corresponding hypercoordinate intermediate ( 14 b , pyridinium selenuranide) for compound 14 was detected at low temperatures using ⁷⁷Se NMR spectroscopy. The presumed hypercoordinate intermediates in the carboxylate‐exchange reactions at the selenium(II) center for a set of model reactions were optimized using DFT‐B3LYP/6–311+g(d) calculations and their structural features compared with the X‐ray structure of anionic selenenate esters 14 – 16 .     

Correcting for a density distribution: particle size analysis of core-shell nanocomposite particles using disk centrifuge photosedimentometry

Many types of colloidal particles possess a core-shell morphology. In this Article, we show that, if the core and shell densities differ, this morphology leads to an inherent density distribution for particles of finite polydispersity. If the shell is denser than the core, this density distribution implies an artificial narrowing of the particle size distribution as determined by disk centrifuge photosedimentometry (DCP). In the specific case of polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, we demonstrate that the particle density distribution can be determined by analytical ultracentrifugation and introduce a mathematical method to account for this density distribution by reanalyzing the raw DCP data. Using the mean silica packing density calculated from small-angle X-ray scattering, the real particle density can be calculated for each data point. The corrected DCP particle size distribution is both broader and more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction, and dynamic light scattering. Artifactual narrowing of the size distribution is also likely to occur for many other polymer/inorganic nanocomposite particles comprising a low-density core of variable dimensions coated with a high-density shell of constant thickness, or for core-shell latexes where the shell is continuous rather than particulate in nature.     

Synthesis of new ESIPT-fluorescein: photophysics of pH sensitivity and fluorescence

ESIPT-inspired benzimidazolyl substituted fluorescein dyes were synthesized. PH-sensitivity was determined by the photophysical property measured at a physiological possible pH range. Fluorescence quantum efficiency values were calculated independently at two different emissions. A rational relationship is defined between fluorescence quantum efficiency and calculated HOMO energy.     

Preparation,X-ray crystallography and thermolysis of transition metal nitrates of 2,2′-bipyridine (Part 63)

Recent work has described the preparation and characterization of the two complexes [Fe₂(C₁₀H₈N₂)₄O(OH₂)₂](NO₃)₄ and [Co(C₁₀H₈N₂)₃]₂[Co(OH₂)₆]·7(OH₂) (NO₃)₈ in which both the nitrogen atoms of 2,2′-bipyridine are directly bonded with the metals. Their structures were determined by single-crystal X-ray diffraction at 296 K. Thermolysis of these complexes has been detailed by the use of TG–DTA and ignition delay measurements. Kinetics of thermal decomposition has also been established. Model free isoconversional and model fitting kinetic approaches have been applied to isothermal TG data for the decomposition of these complexes.     

Structural analysis of some sodium and alumina rich high-level nuclear waste glasses

Sodium- and aluminum-rich high-level nuclear waste glasses are prone to nepheline (NaAlSiO₄) crystallization. Since nepheline removes three moles of glass-forming oxides (Al₂O₃ and SiO₂) per mole of Na₂O, the formation of this phase can result in severe deterioration of the chemical durability in a given glass. The present study aims to investigate the relationships between the molecular-level structure and the crystallization behavior of sodium alumino-borosilicate-based simulated high-level nuclear waste glasses with infrared spectroscopy (FTIR) and X-ray diffraction, respectively. The molecular structure of most of the investigated glasses comprise a mixture of Q² and Q³ (Si) units while aluminum and boron are predominantly present in tetrahedral and trigonal coordination, respectively. The increasing boron content has been shown to suppress the nepheline formation in the glasses. The structural influence of various glass components on nepheline crystallization is discussed.     

Synthesis, characterization, and antifungal activity of biaryl-based bis(1,2,3-triazoles) using click chemistry

Abstract  

Click chemistry was used to synthesize a series of biaryl-based bis(1,2,3-triazoles). Their antifungal activity was evaluated against three soil-borne plant pathogenic fungi, viz. Rhizoctonia bataticola, Sclerotium rolfsii, and Fusarium oxysporum, using the food poison technique at concentrations of 62.5–500 μg/cm³.     

Convenient and simple synthesis of 2-aminothiazoles by the reaction of α-halo ketone carbonyls with ammonium thiocyanate in the presence of N-methylimidazole

Substituted 2-aminothiazole derivatives were obtained as a result of N-methylimidazole catalyzed cyclization of α-halo ketone carbonyls with ammonium thiocyanate in water-alcoholic media. The generality of the method has been demonstrated by screening a series of aromatic/heteroaromatic/aliphatic α-halo ketones, α-halo β-diketones, and α-halo β-ketoesters. The developed method is simple, mild, and general route for the preparation of diversely functionalized 2-aminothiazoles in good to moderate yields from readily available starting materials.     

The synthesis and structural characterization of N-ortho-ferrocenyl benzoyl amino acid esters. The X-ray crystal structure of N-{ortho-(ferrocenyl)benzoyl}-l-phenylalanine ethyl ester

A series of N-ortho-ferrocenyl benzoyl amino acid ethyl esters 3-9 have been prepared by coupling ortho-ferrocenyl benzoic acid 2 to the amino acid ethyl esters of glycine, l-alanine, l-leucine, l-phenylalanine, β-alanine, 4-aminobutyric acid and (±)-2-aminobutyric acid using the conventional 1,3-dicyclohexylcarbodiimide, 1-hydroxybenzotriazole protocol. The compounds were fully characterized by a range of NMR spectroscopic techniques and by mass spectrometry (MALDI-MS, ESI-MS). The X-ray crystal structure of the l-phenylalanine derivative 6 has been determined.     

Microenvironment in the corona region of triblock copolymer micelles: temperature dependent solvation and rotational relaxation dynamics of coumarin dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements using coumarin-153 (C153) and coumarin-151 (C151) as the fluorescence probes have been carried out in aqueous poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123) and poly(ethylene oxide)100-poly(propylene oxide)70-poly(ethylene oxide)100 (F127) block copolymer micelles with an aim to understand the water structures and dynamics in the micellar corona region. It has been established that the probes reside in the micellar corona region. It is indicated that the corona regions of P123 and F127 micelles are relatively less hydrated than the Palisade layers of neutral micelles like Triton-X-100 and Brij-35. From the appraisal of total Stokes' shift values for the probes in the two block copolymer micelles, it is inferred that the F127 micelle is more hydrated than the P123 micelle. It is observed that the dynamic Stokes' shift values for both of the probes remain more or less similar at all the temperatures studied in the P123 micelle. For C153 in F127, however, the observed Stokes' shift is seen to decrease quite sharply with temperature, though it remains quite similar for C151. Moreover, the fraction of the unobserved initial dynamic Stokes' shift is appreciably higher for both the probes in the F127 micelle compared to that in P123. Over the studied temperature range of 293-313 K, the spectral shift correlation function is described adequately by a bi-exponential function. Rotational relaxation times for C153 in both the micelles show a kind of transition at around 303 K. These results have been rationalized assuming collapse of the poly(ethylene oxide) (PEO) blocks and formation of water clusters in the corona region due to dehydration of poly(ethylene oxide) blocks with an increase in temperature. A dissimilar probe location has been inferred for the differences in the results with C153 and C151 probes in F127. Comparison of the microviscosity and the hydration of the block copolymer micelles has also been made with those of the other commonly used neutral micelles, for a better understanding of the results in the block copolymer micelles.     

Energy and photoinduced electron transfer in a wheel-shaped artificial photosynthetic antenna-reaction center complex

Functional mimics of a photosynthetic antenna-reaction center complex comprising five bis(phenylethynyl)anthracene antenna moieties and a porphyrin-fullerene dyad organized by a central hexaphenylbenzene core have been prepared and studied spectroscopically. The molecules successfully integrate singlet-singlet energy transfer and photoinduced electron transfer. Energy transfer from the five antennas to the porphyrin occurs on the picosecond time scale with a quantum yield of 1.0. Comparisons with model compounds and theory suggest that the F?rster mechanism plays a major role in the extremely rapid energy transfer, which occurs at rates comparable to those seen in some photosynthetic antenna systems. A through-bond, electron exchange mechanism also contributes. The porphyrin first excited singlet state donates an electron to the attached fullerene to yield a P(*+)-C(60)(*-) charge-separated state, which has a lifetime of several nanoseconds. The quantum yield of charge separation based on light absorbed by the antenna chromophores is 80% for the free base molecule and 96% for the zinc analogue.