Organic Arsenic in the Soil Environment: Speciation, Occurrence, Transformation, and Adsorption Behavior

An attempt is made to describe the fate and behavior of organic arsenic (As) compounds in the soil environment, based on an extensive literature researches. The objective of this review is to provide an overview on the state of knowledge to date about the occurrence and potential transformation of organic As, including methylation, degradation, and hydration, in soils and their uptake and accumulation in plants and animals. Accordingly, the biogeochemical cycle of organic As in the soil environment is propose

Polarizing agents for high-frequency DNP : development and applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2006.Vita.Includes bibliographical references (p. 269-273).Dynamic Nuclear Polarization (DNP) is utilized to enhance NMR signal intensities in systems including metals, ceramics, polymers and biological solids. The enhancement results from polarization transfers from unpaired electrons, usually carried by a polarizing agent such as TEMPO (2,2,6,6-tetramethylpiperydin-l-oxyl), to the target nuclei. In this thesis, polarizing agents were developed for efficient polarization mechanisms at 5 Tesla. DNP using biradicals yielded improvements of proton enhancement by about fourfold compared to an identical amount of monomeric TEMPO as used previously. The polarizing mechanism involved was the cross effect (CE), which relies on three-spin processes involving two electrons and one nucleus. Optimization of the CE requires the appropriate electron-electron interaction and the correct EPR frequency separation matching the nuclear Larmor frequency. Due to the relatively short inter-radical distance in interesting biradicals, multi-frequency EPR lineshape analyses are suitable to characterize, the distance and relative g-tensor orientations between electrons, revealing spectral parameters that explain the improvement of DNP efficiency. Alternatively, radical mixtures of TEMPO and Trityl, methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[ 1 ,2-d:4,5-d']bis( 1 ,3)dithiol-4-yl, improve the probability of the correct EPR frequency separation compared to TEMPO by itself.(cont.) A 1:1 radical mixture produced a combined EPR spectrum with the required frequency separation and gave an improvement of the DNP enhancement by about threefold relative to TEMPO alone. In addition, a quantum mechanical theory of the CE was developed to provide sound explanations of the improved polarizing mechanism using the above polarizing agents. The soluble biradical-TOTAPOL, yielding proton enhancements of 160-290-was developed and applied to a wide range of aqueous systems from amyloid peptide nanocrystals to liquid samples. Polarizing nanocrystals relies on nuclear spin diffusion that transfers enhanced nuclear polarization from solvent into crystals that are isolated from paramagnetic species. This requires efficient polarizing agents that produce and maintain a high level of nuclear polarization surrounding the nanocrystals. In a second application, efficient polarizing agents that reduce the required radical concentration are important for temperature-jump DNP experiments involving a cycle of freezing, polarization, melting and observation of the liquid-state NMR spectrum of samples of interest. During melting, preservation of the nuclear polarization benefits from reduced paramagnetic relaxation at low radical concentrations.by Kan-Nian Hu.Ph.D

Organic Arsenic in the Soil Environment: Speciation, Occurrence, Transformation, and Adsorption Behavior

An attempt is made to describe the fate and behavior of organic arsenic (As) compounds in the soil environment, based on an extensive literature researches. The objective of this review is to provide an overview on the state of knowledge to date about the occurrence and potential transformation of organic As, including methylation, degradation, and hydration, in soils and their uptake and accumulation in plants and animals. Accordingly, the biogeochemical cycle of organic As in the soil environment is proposed

Zero-quantum frequency-selective recoupling of homonuclear dipole-dipole interactions in solid state nuclear magnetic resonance

We describe a method for measuring magnetic dipole-dipole interactions, and hence distances, between pairs of like nuclear spins in a many-spin system under magic-angle spinning (MAS). This method employs a homonuclear dipolar recoupling sequence that creates an average dipole-dipole coupling Hamiltonian under MAS with full zero-quantum symmetry, including both secular and flip-flop terms. Flip-flop terms are then attenuated by inserting rotor-synchronized periods of chemical shift evolution between recoupling blocks, leaving an effective Hamiltonian that contains only secular terms to a good approximation. Couplings between specific pairs of nuclear spins can then be selected with frequency-selective π pulses. We demonstrate this technique, which we call zero-quantum shift evolution assisted homonuclear recoupling, in a series of one-dimensional and two-dimensional 13C NMR experiments at 17.6 T and 40.00 kHz MAS frequency on uniformly 13C-labeled L-threonine powder and on the helix-forming peptide MB(i+4)EK, synthesized with a pair of uniformly 13C-labeled L-alanine residues. Experimental demonstrations include measurements of distances between 13C sites that are separated by three bonds, placing quantitative constraints on both sidechain and backbone torsion angles in polypeptides

Multichromophore Donor Materials Derived from Diketopyrrolopyrrole and Phenoxazine: Design, Synthesis, and Photovoltaic Performance

Herein two conjugated donor molecules, TDPP-POCN and FDPP-POCN, with planar diketopyrrolopyrrole (DPP) as the core building-block acceptor unit and phenoxazine-capped acrylonitriles as arms are designed and synthesized. Solution-processed bulk-heterojunction organic solar cells based on blends of the small-molecule donors and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) exhibit promising photovoltaic device performance with a maximum power conversion efficiency up to 4.8 % for TDPP-POCN and 3.4 % for FDPP-POCN under the illumination of AM1.5G, 100 mW cm(-2). To the best of our knowledge, this is the highest efficiency reported so far in DPP-phenoxazine-based systems and among acrylonitrile-bridged donor-pi-acceptor-pi-donor-structured small-molecule donors for solution-processed small-molecule-based organic solar cells