Assignment of the EPR spectrum of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) superoxide spin adduct

[structure: see text] Spin trapping consists of using a nitrone or a nitroso compound to "trap" an unstable free radical as a long-lived nitroxide that can be characterized by electron paramagnetic resonance (EPR) spectroscopy. The formation of DMPO-OOH, the spin adduct resulting from trapping superoxide (O(2)(*)(-)) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), has been exploited to detect the generation of superoxide in a wide variety of biological and chemical systems. The 12-line EPR spectrum of DMPO-OOH has been either reported or mentioned in more than a thousand papers. It has been interpreted as resulting from the following couplings: A(N) approximately 1.42 mT, A(H)beta approximately 1.134 mT, and A(H)gamma(1H) approximately 0.125 mT. However, the DMPO-OOH EPR spectrum has an asymmetry that cannot be reproduced when the spectrum is calculated considering a single species. Recently, it was proposed that the 0.125 mT splitting was misassigned and actually results from the superimposition of two individual EPR spectra associated with different conformers of DMPO-OOH. We have prepared 5,5-dimethyl-[3,3-(2)H(2)]-1-pyrroline N-oxide (DMPO-d(2)), and we showed that the EPR spectrum of the corresponding superoxide spin adduct is composed of only six lines, in agreement with the assignment of the 0.125 mT splitting to a gamma-splitting from a hydrogen atom bonded to carbon 3 of DMPO. This result was supported by DFT calculations including water solvation, and the asymmetry of the DMPO-OOH EPR spectrum was nicely reproduced assuming a chemical exchange between two conformers.     

Application of water as a solvent in microwave-assisted extraction for analysis of PCBs and CBzs in fly ash

Polychlorinated biphenyls (PCBs) and chlorobenzenes (CBzs) are two classes of dioxin precursors formed in municipal solid waste incinerators (MSWIs); they produce negative health effects similar to those of dioxins. Reducing the analytical time required for determining the concentrations of these compounds in MSWIs is important for quickly evaluating their importance and assessing associated health risks. In the present study, water is used as a safe and environmentally friendly solvent in microwave-assisted extraction (MAE) for PCB and CBz analyses. MAE is compared with traditional Soxhlet extraction (SE) to determine the extraction efficiencies. The evaluation of extraction efficiencies shows that MAE has a high extraction efficiency compared with that of SE when water content is lower than 60%. Furthermore, the extraction time and organic solvent consumption are reduced with MAE compared with SE.     

Conjugate addition of organocuprates to chiral bicyclic delta-lactams. Enantioselective synthesis of cis-3,4-disubstituted and 3,4,5-trisubstituted piperidines

[reaction: see text] Chiral nonracemic bicyclic lactam 3, easily accessible by cyclodehydration of (R)-phenylglycinol and racemic methyl 4-formylhexanoate, was converted to the unsaturated lactams 5, which undergo the stereoselective conjugate addition of lower order cyanocuprates to ultimately lead to enantiopure cis-3,4-disubstituted and 3,4,5-trisubstituted piperidines.     

Use of Silicon Substituents in the Syntheses of Natural and Non-natural Molecules

Trimethylsilyl groups have been used in our research as a director as well as a bulky and lipophilic group in our quest for natural and non-natural molecules.     

A series of neutral radical CpNi(dithiolene). complexes

The heteroleptic neutral radical dithiolene complexes CpNi(dmit)., CpNi(dsit). and CpNi(dmid).(dmit=1,3-dithiole-2-thione-4,5-dithiolate; dsit=1,3-dithiole-2-thione-4,5-diselenolate; dmid=1,3-dithiole-2-one-4,5-dithiolate) are obtained from the reaction of (Cp2Ni)BF4 with either (n-Bu4N)[Ni(dmit)2] and (n-Bu4N)[Ni(dmid)2] or PhSb(dmit) and PhSb(dsit), respectively. The three complexes reduce reversibly to the corresponding Ni(II) anions and oxidize reversibly to the cationic state. As deduced from DFT calculations performed on CpNi(dmit)., the SOMO of these complexes is essentially localized on the dithiolene moiety with little metal contribution. CpNi(dsit). is isostructural with CpNi(dmit). and crystallizes in the monoclinic system, space group P2(1). In the solid-state structures of both CpNi(dmit). and CpNi(dsit)., molecules interact through a three-dimensional set of intermolecular interactions mediated by short SS, SeSe and SSe contacts, as confirmed from the temperature and field dependence of the magnetic susceptibility by the observation of an antiferromagnetic ground state below T(Neel)=27 K in CpNi(dmit)., 18 K in CpNi(dsit).. Finally, CpNi(dmid). crystallizes in the orthorhombic system, space group Pnma. Molecules organize into uniform chains through the stacking of the dmid moieties in a sigma-type face-to-face overlap.     

Synthesis and capsule formation of upper rim substituted tetra-acrylamido calix[4]arenes

Upper rim substituted tetraiodo calix[4]arenes are coupled to a variety of acrylamides using the palladium catalysed Heck reaction. Tetra-acrylamido upper rim substituted calix[4]arenes are obtained in good yields with exceptionally high stereoselectivity, to produce the all-trans isomers. Tetra-acrylamido calix[4]arenes derived from secondary acrylamides are shown to dimerise via eight hydrogen bonds to form dimeric capsules, which are able to include small organic molecules.     

Inclusion behavior of thiacalix[4]arenetetrasulfonate toward water-miscible organic molecules studied by salting-out and X-ray crystallography

[reaction: see text] Inclusion ability of thiacalix[4]arenetetrasulfonate (TCAS) toward water-miscible organic molecules such as alcohols, ketones, cyclic ethers, and so on was studied by salting-out of the inclusion complex. NMR spectra of the recovered precipitates showed size selectivity to include the guest molecules. X-ray crystallographic analyses of TCAS salts including acetone and 1,4-dioxane suggested that guests are retained with the aid of cation coordination and H-bonding.     

Flow injection system with in-line Winkler's procedure using 16-way valve and spectrofluorimetric determination of dissolved oxygen in environmental waters

Flow injection spectrofluorimetry with in-line Winklers procedure was developed for the dissolved oxygen (DO) determination. 2-Thionaphthol reacted with iodine produced by Winkler’s method to form fluorescence inactive disulfide compound. To automate the process completely, a 5-channel flow system with a newly designed 16-way valve was assembled. The system consisted of a dispersion coil (DC), a precipitate formation coil (PFC), a precipitate dissolving coil (PDC), and extraction coil (EC). A calibration can be constructed by using a standard iodine solution for dissolved oxygen. The calibration graph was linear over the range 1.2×10⁻⁴∼6.0×10⁻⁴ mol l⁻¹ iodine (1.96∼9.80 mg O l⁻¹)). The relative standard deviation (n=6) was below 0.3% for the 4×10⁻⁴ mol l⁻¹ iodine (6.27 mg O l⁻¹) determination. The sample throughput was 12/h.     

Rapid analysis of triterpenic acids by liquid chromatography using porous graphitic carbon and evaporative light scattering detection

An original system which uses Porous Graphitic Carbon as support and a mixture of organic solvents as mobile phase is proposed for the analysis of triterpenic acids by liquid chromatography. The separation of betulinic acid, ursolic acid, oleanolic acid, and 18alpha- and 18beta-glycyrrhetinic acids was carried out within a short time and monitored by evaporative light scattering detection as universal detection method. Molecular modelling studies show that the main contribution to the selectivity comes from the electrostatic interaction characterised by the dipole moment of the products.