Characterization of novel optically active conjugated polyarylenes and poly(aryleneethnylene)s by a combination of off-line static and dynamic light scattering with gel permeation chromatography

By combining the offline static and dynamic laser light scattering (LLS) and gel permeation chromatography (GPC) results of a broadly distributed polymer sample, we were able to characterize a series of chiral binaphthyl-based polyarylenes and poly(aryleneethnylene)s in THF at 25°C. For each of the samples, we obtained not only the weight-average molar mass M_w, the second virial coefficient A₂ and the z-average translational diffusion coefficient 〈D〉, but also two calibrations: V = A + Blog(M) and D = k_D M^−αD, where V, D, and M are the elution volume, the translational diffusion coefficient and the molar mass for monodisperse polymer chains, respectively, and A, B, k_D, and α_D are four calibration constants. Using these calibrations, we estimated the molar mass distributions of these novel polymers. We showed that using polystyrene to calibrate the GPC columns could lead to a smaller M_w. Our results indicate that all the polymers studied have a rigid chain conformation in THF at 25°C and the introduction of the —NO₂ groups into the monomer can greatly promote the polymer solubility in THF.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2615–2622, 1998     

Structure and Bonding in CE5− (E=Al–Tl) Clusters: Planar Tetracoordinate Carbon versus Pentacoordinate Carbon

The structure, bonding, and stability of clusters with the empirical formula CE₅^? (E=Al–Tl) have been analyzed by means of high‐level computations. The results indicate that, whereas aluminum and gallium clusters have C_2v structures with a planar tetracoordinate carbon (ptC), their heavier homologues prefer three‐dimensional C_4v forms with a pentacoordinate carbon center over the ptC one. The reason for such a preference is a delicate balance between the interaction energy of the fifth E atom with CE₄ and the distortion energy. Moreover, bonding analysis shows that the ptC systems can be better described as CE₄^?, with 17‐valence electrons interacting with E. The ptC core in these systems exhibits double aromatic (both σ and π) behavior, but the σ contribution is dominating.     

Determination of carbonyl compounds in the atmosphere by DNPH derivatization and LC-ESI-MS/MS detection

A method of determination of 32 carbonyl compounds by high performance liquid chromatography (HPLC) and electrospray ionization (ESI) tandem mass spectrometry (MS/MS) after derivatization with 2,4-dinitrophenylhydrazine (DNPH) was developed and successfully applied to the atmosphere sample of a residential area of Liwan District (S1) and a research institute of Tianhe District (S2) in Guangzhou, China. Some operation conditions of ESI-MS/MS in the negative ion mode including selection of parent and daughter ions, declustering potential (DP), entrance potential (EP), collision energy (CE), collision cell exit potential (CXP) and effect of buffer in ESI-MS/MS process were optimized. The regression coefficient of the calibration curves (R²), recovery, reproducibility (R.S.D., n = 5) and limit of detection (LOD) were in the range of 0.9938-0.9999, 90-104%, 1.7-11% and 0.4-9.4 ng/m³, respectively. Among most of the samples, acetone was the most abundant carbonyl in two sampling sites and formaldehyde, acetaldehyde and butyraldehyde/2-butanone were also abundant carbonyls. In contrast to LC-UV method, the LOD, the separation of some co-eluting compounds and the precision (mainly to higher molecular weight carbonyls) are all improved by LC-ESI-MS/MS.     

“One-shot” analysis of polybrominated diphenyl ethers and their hydroxylated and methoxylated analogs in human breast milk and serum using gas chromatography-tandem mass spectrometry

The presence of polybrominated diphenyl ethers (PBDEs) and their hydroxylated (OH-BDE) and methoxylated (MeO-BDE) analogs in humans is an area of high interest to scientists and the public due to their neurotoxic and endocrine disrupting effects. Consequently, there is a rise in the investigation of the occurrence of these three classes of compounds together in environmental matrices and in humans in order to understand their bioaccumulation patterns. Analysis of PBDEs, OH-BDEs, and MeO-BDEs using liquid chromatography-mass spectrometry (LC-MS) can be accomplished simultaneously, but detection limits for PBDEs and MeO-BDEs in LC-MS is insufficient for trace level quantification. Therefore, fractionation steps of the phenolic (OH-BDEs) and neutral (PBDEs and MeO-BDEs) compounds during sample preparation are typically performed so that different analytical techniques can be used to achieve the needed sensitivities. However, this approach involves multiple injections, ultimately increasing analysis time. In this study, an analytical method was developed for a “one-shot” analysis of 12 PBDEs, 12 OH-BDEs, and 13 MeO-BDEs using gas chromatography with tandem mass spectrometry (GC-MS/MS). This overall method includes simultaneous extraction of all analytes via pressurized liquid extraction followed by lipid removal steps to reduce matrix interferences. The OH-BDEs were derivatized using N-(t-butyldimethylsilyl)-N-methyltrifluoroacetamide (TBDMS-MTFA), producing OH-TBDMS derivatives that can be analyzed together with PBDEs and MeO-BDEs by GC-MS/MS in “one shot” within a 25-min run time. The overall recoveries were generally higher than 65%, and the limits of detection ranged from 2 to 14 pg in both breast milk and serum matrices. The applicability of the method was successfully validated on four paired human breast milk and serum samples. The mean concentrations of total PBDEs, OH-BDEs, and MeO-BDEs in breast milk were 59, 2.2, and 0.57 ng g⁻¹ lipid, respectively. In serum, the mean total concentrations were 79, 38, and 0.96 ng g⁻¹ lipid, respectively, exhibiting different distribution profiles from the levels detected in breast milk. This “one-shot” GC-MS/MS method will prove useful and cost-effective in large-scale studies needed to further understand the partitioning behavior, and ultimately the adverse health effects, of these important classes of brominated flame retardants in humans.     

Nanoarchitectured Graphene‐Based Supercapacitors for Next‐Generation Energy‐Storage Applications

Tremendous development in the field of portable electronics and hybrid electric vehicles has led to urgent and increasing demand in the field of high‐energy storage devices. In recent years, many research efforts have been made for the development of more efficient energy‐storage devices such as supercapacitors, batteries, and fuel cells. In particular, supercapacitors have great potential to meet the demands of both high energy density and power density in many advanced technologies. For the last half decade, graphene has attracted intense research interest for electrical double‐layer capacitor (EDLC) applications. The unique electronic, thermal, mechanical, and chemical characteristics of graphene, along with the intrinsic benefits of a carbon material, make it a promising candidate for supercapacitor applications. This Review focuses on recent research developments in graphene‐based supercapacitors, including doped graphene, activated graphene, graphene/metal oxide composites, graphene/polymer composites, and graphene‐based asymmetric supercapacitors. The challenges and prospects of graphene‐based supercapacitors are also discussed.     

Facile S(N)2 reaction in protic solvent: quantum chemical analysis

We study the effects of protic solvent (water, methanol, ethanol, and tert-butyl alcohol) and cation (Na+, K+, Cs+) on the unsymmetrical SN2 reaction X- + RY --> RX + Y- (X = F, Br; R = CH3,C3H7;Y = Cl, OMs). We describe a series of calculations for the S(N)2 reaction mechanism under the influence of cation and protic solvent, presenting the structures of pre- and postreaction complexes and transition states and the magnitude of the activation barrier. An interesting mechanism is proposed, in which the protic solvent molecules that are shielded from the nucleophile by the intervening cation act as a Lewis base to reduce the unfavorable Coulombic influence of the cation on the nucleophile. We predict that the reaction barrier for the S(N)2 reaction is significantly lowered by the cooperative effects of cation and protic solvent. We show that the cation and protic solvent, each of which has been considered to retard the SN2 reactivity of the nucleophile, can accelerate the reaction tremendously when they interact with the fluoride ion in an intricate, combined fashion. This alternative S(N)2 mechanism is discussed in relation to the recently observed phenomenal efficiency of fluorination in tert-alcohol media [Kim, D. W.; et al. J. Am. Chem. Soc. 2006, 128, 16394].     

Self-organization of Gold Nanoparticles Protected by 9-(5-Thiopentyl)-carbazole

Introduction During the past decade, nanometer-sized metal clusters, special the gold clusters, possessing unusual physical and chemical properties have been extensively investigated1-5. In recent years, a new effort has been focused on fabricating ordered metal nanoparticles with well-defined two or three dimensions using nanometer-sized metal clusters as building blocks6-9. This will provide a new horizon to develop novel optical and nanoelectronic devices. Ordered nanoparticle assemblie…     

Synthesis,Characterization, Self‐Assembly,Gelation, Morphology and Computational Studies of Alkynylgold(III) Complexes of 2,6‐Bis(benzimidazol‐2′‐yl)pyridine Derivatives

A novel class of alkynylgold(III) complexes of the dianionic ligands derived from 2,6‐bis(benzimidazol‐2′‐yl)pyridine (H₂bzimpy) derivatives has been synthesized and characterized. The structure of one of the complexes has also been determined by X‐ray crystallography. Electronic absorption studies showed low‐energy absorption bands at 378–466 nm, which are tentatively assigned as metal‐perturbed π–π* intraligand transitions of the bzimpy^2? ligands. A computational study has been performed to provide further insights into the nature of the electronic transitions for this class of complexes. One of the complexes has been found to show gelation properties, driven by π–π and hydrophobic–hydrophobic interactions. This complex exhibited concentration‐ and temperature‐dependent ¹H NMR spectra. The morphology of the gel has been characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).     

Structure and stability of hexa-aqua V(III) cations in vanadium redox flow battery electrolytes

The vanadium(III) cation structure in mixed acid based electrolyte solution from vanadium redox flow batteries is studied by (17)O and (35/37)Cl nuclear magnetic resonance (NMR) spectroscopy, electronic spectroscopy and density functional theory (DFT) based computational modelling. Both computational and experimental results reveal that the V(III) species can complex with counter anions (sulfate/chlorine) depending on the composition of its solvation sphere. By analyzing the powder precipitate it was found that the formation of sulfate complexed V(III) species is the crucial process in the precipitation reaction. The precipitation occurs through nucleation of neutral species formed through deprotonation and ion-pair formation process. However, the powder precipitate shows a multiphase nature which warrants multiple reaction pathways for precipitation reaction.