Preparation and certification of the new reference materials; plastics (disk form, JSAC 0621-0625) for determination of mercury using x-ray fluorescent analysis.

The Japan Society for Analytical Chemistry has developed some new plastic certified reference materials (CRMs) for the analysis of mercury in polyester disks using XRF analysis. These CRMs (named as JSAC 0621-0625) were prepared by casting polyesters including a toluene solution of organometallic compounds as a standard. Concentrations of the five levels of mercury ranged from 0 to 250 mg/kg. Homogeneity tests of prepared disks had shown excellent results. Interlaboratory comparison study for the certification was performed by 15 laboratory participants. The z-scores in robust statistical method was applied for the evaluation of outliers. The certified values were assigned after discarding outliers. The uncertainties of certified values were determined as the confidence levels of 95%.     

Chemical ionization mass spectrometry of hydrofluorocarbons, hydrofluorocarbon ethers and perfluoroalkenes

Positive and negative chemical ionization (CI) mass spectrometry is studied for hydrofluorocarbons (HFCs), hydrofluorocarbon ethers (HFEs), and perfluoroalkenes (PFCs) using various kinds of reagent gas. While no quasi-molecular ion was observed under electron impact ionization for saturated MFCs, [M-F]⁺ is detected under CI conditions using methane as a reagent gas. Mechanisms for the generation of [M-F]⁺ are discussed. Furthermore, nitrogen monoxide can be used as a reagent gas to observe [M + NO]⁺ for many HFCs and HFEs. In negative mode chloroform is also available to generate [M + Cl]⁻ for HFCs and HFEs containing -CHF- groups.     

Formation of Periodic γ‐Turns in α/β‐Hybrid Peptides: DFT and NMR Experimental Evidence

Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen‐bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ‐turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ‐turns in the oligomers of 1:1 natural‐α/unnatural trans‐β‐norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8‐membered (pseudo γ‐turn)/7‐membered (inverse γ‐turn) hydrogen bonds in both polar and non‐polar solvent media. It is observed that the stereochemistry and local conformational preference of the β‐amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side‐chains.     

Picosecond time-resolved fluorescence study on solute-solvent interaction of 2-aminoquinoline in room-temperature ionic liquids: aromaticity of imidazolium-based ionic liquids

Time-resolved fluorescence spectra and fluorescence anisotropy decay of 2-aminoquinoline (2AQ) have been measured in eight room-temperature ionic liquids, including five imidazolium-based aromatic ionic liquids and three nonaromatic ionic liquids. The same experiments have also been carried out in several ordinary molecular liquids for comparison. The observed time-resolved fluorescence spectra indicate the formation of pi-pi aromatic complexes of 2AQ in some of the aromatic ionic liquids but not in the nonaromatic ionic liquids. The fluorescence anisotropy decay data show unusually slow rotational diffusion of 2AQ in the aromatic ionic liquids, suggesting the formation of solute-solvent complexes. The probe 2AQ molecule is likely to be incorporated in the possible local structure of ionic liquids, and hence the anisotropy decays only through the rotation of the whole local structure, making the apparent rotational diffusion of 2AQ slow. The rotational diffusion time decreases rapidly by adding a small amount of acetonitrile to the solution. This observation is interpreted in terms of the local structure formation in the aromatic ionic liquids and its destruction by acetonitrile. No unusual behavior upon addition of acetonitrile has been found for the nonaromatic ionic liquids. It is argued that the aromaticity of the imidazolium cation plays a key role in the local structure formation in imidazolium-based ionic liquids.     

A new approach toward light-harvesting reverse micelles from donor-acceptor miscible blends

In the present paper, we report a new approach toward light-harvesting reverse micellar systems from molecular blends of anthracene and perylene building blocks. The self-assembly initiated by protonation of the molecular blends gave rise to the mixed reverse micelles, in which intermolecular energy transfer from the anthracene to the perylene chromophores was observed. The atomic force microscope (AFM) studies on the reverse micelles prepared from the donor and acceptor blends at a range of the feed ratios showed a number of nanoscale-sized spherical objects homogeneously dispersed on the highly oriented pyrolytic graphite (HOPG) substrate. The critical micelle concentration (cmc) values of the reversed micelles at the donor:acceptor ratios of 100:0, 50:50, and 0:100 were estimated to be 7, 3, and 10 μM by fluorescence batch titrations, respectively, indicating that the cmc values should be almost equivalent regardless of the constitution of each chromophoric component. Attempt to generate the mixed reverse micelles through pairwise mixing of the donor- and acceptor-based reverse micelles resulted in spectral behaviors identical with those obtained by the self-assembly employing the donor-acceptor blends. This suggests that these two reverse micelles undergo thermodynamic exchange of the surfactant molecules to afford the mixed reverse micelles when mixing the two discrete reverse micellar systems.     

Real‐time homonuclear broadband and band‐selective decoupled pure‐shift ROESY

Unambiguous spectral assignments in ¹H solution‐state NMR are central, for accurate structural elucidation of complex molecules, which is often hampered by signal overlap, primarily because of scalar coupling multiplets, even at typical high magnetic fields. The recent advances in homodecoupling methods have shown powerful means of achieving high resolution pure‐shift ¹H spectra in 1D and also in 2D J‐correlated experiments, by effectively collapsing the multiplet structures. The present work extends these decoupling strategies to through‐space correlation experiments as well and describes two new pure‐shift ROESY pulse schemes with homodecoupling during acquisition, viz., homodecoupled broadband (HOBB)‐ROESY and homodecoupled band‐selective (HOBS)‐ROESY. Furthermore, the ROESY blocks suppress the undesired interferences of TOCSY cross peaks and other offsets. Despite the reduced signal sensitivity and prolonged experimental times, the HOBB‐ROESY is particularly useful for molecules that exhibit an extensive scalar coupling network spread over the entire ¹H chemical shift range, such as natural/synthetic organic molecules. On the other hand, the HOBS‐ROESY is useful for molecules that exhibit well‐separated chemical shift regions such as peptides (NH, Hα and side‐chain protons). The HOBS‐ROESY sensitivities are comparable with the conventional ROESY, thereby saves the experimental time significantly. The power of these pure‐shift ROESY sequences is demonstrated for two different organic molecules, wherein complex conventional ROE cross peaks are greatly simplified with high resolution and sensitivity. The enhanced resolution allows deriving possibly more numbers of ROEs with better accuracy, thereby facilitating superior means of structural characterization of medium‐size molecules. Copyright © 2014 John Wiley & Sons, Ltd.     

New method for the spectrophotometric determination of ultramicro amounts of copper

A deep yellow colour is formed by the reaction with phenol and chloramine-T in the presence of copper, and this reaction is used for the photometric determination of copper. An aqueous solution of copper and the reagents at pH 11.5-11.6 is heated. The molar absorptivity at 410 mmu, is 2.32 x 10(6). The method has been used satisfactorily to determine ultram amounts of copper in high-purity silicon.     

Non‐Uniform‐Sampling Ultrahigh Resolution TOCSY NMR: Analysis of Complex Mixtures at Microgram Levels

Non‐uniform sampling in combination with homonuclear broadband decoupling along an indirect dimension, and indirect covariance processing are used to record ultrahigh resolution two‐dimensional TOCSY spectra in less than half an hour, for typical sample concentrations in the mm range. TOCSY correlations belonging to protons separated by as little as ≈2 Hz can be distinctly discerned. The utility of the technique for low concentrations has been demonstrated.