<p> A continuation of studies of main group compounds carried out in this laboratory has led to the investigation of the structural and oxidative chemistry of several compounds and mixtures of the elements phosphorus, arsenic, sulfur and selenium. A number of questions and a lack of data regarding known compounds of these elements lead to an examination of the series P4-X^AsX^S3-Y^SeY, X = 0-4, Y = 0-3. Raman spectra of several members of the series have been recorded. The 31P and natural abundance 77Se nmr spectra of these compounds were also recorded, including a reinvestigation and complete assignment of the 31P nmr spectrum of P4Se3 . Several trends in the coupling constants and chemical shifts were noted and an empirical equation was devised for the 31P nmr spectral assignments for quaternary members of the series. The compound formerly believed to be P2As2S3 was shown to be PAs3S3. The crystal structure of an occupationally disordered crystal of stoichiometry P2As2S3 is also reported in this thesis.</p> <p> The compound As4S4 and 1:1 As:Se fused mixtures were separately oxidized with the Lewis acids AsF5 and SbF5 to produce the new cations As3S4+ and As3Se4+. The cations were characterized as the salts As3S4 (AsF6), As3S4 (SbF6), As3Se4 (AsF6 ) and As3Se4 (SbF6) by Raman spectroscopy and infrared spectroscopy. The determination of the crystal structures of the first three salts is also reported.</p> <p> Reactions of As4S4 with PF5, PCl5, BCl3, SO3, NbF5 , TaF5 and WF6 were shown to not proceed or, in the case of PF5, to not produce a stable adduct. The action of AsF5 on compounds and mixtures of heavy main group elements (e.g., Sb2Te3) of groups V and VI produced no new compounds that were identified and generally just gave known homo-polyatomic cations of the chalcogens.</p> <p> Oxidation of As4S4 with the halogens (X) chlorine and bromine produced AsX3 and S2X2 while the reaction of SbCl5 with As4S4 or 1:1 As:Se fused mixtures produced SbCl3, AsCl3 and the salts SCl3 (SbCl6) and SeCl3 (SbCl6),respectively. The crystal structure of SCl3 (SbCl6) is reported here along with unit cell data for SeCl3 (SbCl6) and SBr1.2Cl1.8 (SbCl6) and the Raman data for all three compounds. Only AsSI was produced by the oxidation of As4S4 by I2 in SO2 while a reaction between molten P4Se3 and I2 gave the new compound αP4Se3I2. Raman spectra for both AsSI and αP4Se3I2 were recorded as well as the 31P nmr spectrum for the latter.</p> / Thesis / Doctor of Philosophy (PhD)

<p> The mass spectra of disubstituted 1,4-dithiane, 1,4-oxathiane, and 1,4-oxaselenane, as well as the mass spectra of some disubstituted bicyclic and tricyclic sulfur and selenium compounds, were studied. The exact composition of the major ions of many of these compounds was determined. Fragmentation mechanisms are proposed to account for the major peaks in their spectra.</p> / Thesis / Master of Science (MSc)

Determination of arsenic and selenium compounds in water samples organotin compounds in fish samples by LC-ICP-MS

Includes bibliographical references.

No description available.

<p> A 19F nmr study has shown that SeO2F2, SeOF2 and SeOCl2 behave as bases (B) towards SbF5 forming the adducts, (SbF5)n•B where n = 1-5 and in which they are bonded to antimony through an oxygen. Structural information about these adducts was also obtained. Solutions of SbF5 in SeOF2 and SeOCl2 were also shown to contain the SbF-6 and cis and trans [SbF4 (B)2]+ ions. The order of basicity towards SbF5 for the following bases is, SeOCl2 > SeOF2 > SbF-6 > SOF2 > SeO2F2 > SO2ClF.</p> <p> A 77Se nmr study of the SeOCl2 solvent system has shown that the order of Lewis acidity for the following acids is, SbF5 ~ SO3 > SbCl5 > SnCl4 > SbCl3 > AsCl3.</p> <p> A new selenium oxyfluoride, SeOF4 has been identified and is shown to form an ionic adduct SeOF+3SbF-6 with SbF5.</p> <p> Polyselenium oxyfluorides, F(SeO3)nSeO2F, where n=1-3, were prepared and are found to have acyclic structures. The 77Se spectra of Se4^2+ and Se8^2+ were studied. </p> <p> Redistribution reactions between selenium and phosphorus halides and oxyhalides were studied using the nmr resonances of 19F, 31P and 77Se.</p> / Thesis / Doctor of Philosophy (PhD)

 Molecular Beam Epitaxy is an advanced method for the synthesis of single-crystal thin-film structures. However, the growth behavior varies case by case due to the complicated kinetic process. In this thesis, the epitaxial growth processes of AlxIn1-xN alloy and Bi2Se3 thin-films are studied. Heteroepitaxial growth of AlxIn1-xN alloy on GaN(0001) substrate is carried out in the Nitrogen-rich flux conditions. A series of transient growth stages are identified from the initiation of the deposition. A significant effect of source beam-flux on the incorporation rate of Indium atoms is observed and measured. A correlation between the incorporation rate and the growth conditions (flux ratio and growth temperature) is revealed by the dependence of the growth-rate of the film on beam fluxes. A mathematic model is then suggested to explain the effect, through which the measured results indicating a surface diffusing and trapping process is indicated. Unexpected behavior of the lattice-parameter evolution of the growth front during deposition is also observed, indicating a complex strain-relaxation process of the epilayers. For three-dimensional (3D) topological insulator of Bi2Se3, growths are attempted on various substrate surfaces, including clean Si(111)-(7x7), Hydrogen terminated Si(111), Bismuth induced Si(111) reconstructed surfaces, GaN(0001), and some selenide “psudo-substrates”. The specific formation process of this quintuple-layered material in MBE is investigated, from which the Van der Waals epitaxy growth characteristics inherent to deposition of Bi2Se3 is determined, and the mechanism of the “two-step growth” technique for this material is further clarified. Among the various substrates, those that are inert to chemical reaction with Bi/Se are important for the growth. The epilayers’ lattice-misfit with the substrate is also a crucial factor to the structural quality of the Bi2Se3 epifilms, such as the defects density and the single-crystalline domain size. The effect of a vicinal substrate on suppressing the twin-defects in film is also addressed. Using a suitable substrate and adapting an optimal condition, ultra-thin films of Bi2Se3 with a superior structural quality have been achieved. Multilayered Bi2Se3 structures with ZnSe and In2Se3 spacers are attempted. Finally the high-quality superlattices of Bi2Se3/In2Se3 are successfully synthesized. The hetero-interfaces in the superlattice structure of Bi2Se3/In2Se3 are sharp, and the individual layers are uniform with thicknesses being strictly controlled. The behaviors of strain evolution during the hetero-growth process are finally investigated. An exponential relaxation of misfit strain is observed. And the correlation between the residual strain and the starting surface in the initial growth stage is also identified. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 1977 / Includes bibliographical references. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Chemistry

During the attempts to carry out Suzuki coupling reactions, the σ-bonded Pd−Caryl benzochalcogenadiazolyl complexes trans-[ClPd(PPh3)2(C6H2BrN2E)] (E = S, Se) were isolated. The corresponding bromo derivatives were also synthesized on purpose to investigate their activity in Stille coupling reactions. A head-to-tail dimer trans- [{ClPd(PPh3)(μ-C6H2BrN2Se)}2] was synthesized from the thermolysis of trans- [ClPd(PPh3)2(C6H2BrN2Se)] in the presence of SeO2. The reduction potentials of the mononuclear and dinuclear complexes were measured by cyclic voltammetry (CV) and square wave voltammetry (SWV). 4,7-bis(2/4-pyridyl)benzochalcogenadiazole ligands were synthesized by Stille coupling reactions and the 1,5-bis(4-pyridyl)naphthalene ligand was prepared by a Suzuki coupling reaction. Reactions of the labile complex [BrRe(CO)4(NCMe)] with 4,7-bis(4- pyridyl)benzochalcogenadiazole ligands in a 2:1 ratio afforded self-assembled molecular rods [{ReBr(CO)4}2(μ-4,7-bis(4-pyridyl)benzochalcogenadiazoles)]. Palladium directed molecular squares [(enPd)(μ-4,7-bis(4-pyridyl)benzochalcogenadiazole)]4[PF6]8 were prepared by reactions of enPd(PF6)2 and 4,7-bis(4-pyridyl)benzochalco-genadiazoles in a 1:1 ratio. The optoelectronic properties of the ligands and the molecular rods were investigated by CV and SWV, and by luminescence spectroscopy. The optical properties of the square complexes were also studied by luminescence spectroscopy. / xvii, 152 leaves : ill. (some col.) ; 29 cm

Alzheimer s (DA) is a neurodegenerative disease evidenced by cognitive disorders and attention deficit and learning, and is the main cause of dementia in the elderly. The amyloid hypothesis posits that extracellular amyloid-β (Aβ) deposits are the fundamental etiological factor of the disease. However, the AD etiology has yet to be fully understood and common treatments remain largely non-efficacious. Caernorhabditis elegans transgenic strains expressing toxic Aβ has been employed as AD in vivo model in order to elucidate mechanisms and verifying the effectiveness of pharmacological compounds. The organoselenium compound tested in this study, Diphenyl-diselenide (PhSe)2, has shown efficacy in ameliorate several parametres in neurodegenerative disease models. In the present study, we analyzed the effects of (PhSe)2 chronic treatment on Aβ peptide-induced toxicity in C. elegans. This data shows that chronic exposure to (PhSe)2 attenuated oxidative stress induced by Aβ with concomitant recovery of associative learning memory in worms. In addition, (PhSe)2 decreased Aβ transgene expression, suppressing the Aβ peptide and down-regulating hsp-16.2 by reducing the need of this chaperone under Aβ toxicity. This observations suggest that (PhSe)2 plays an important role in protection against oxidative stress-induced toxicity, this representing a promising potential pharmaceutical modality by attenuating Aβ expression. / A Doença de Alzheimer (DA) é uma doença neurodegenerativa evidenciada por distúrbios cognitivos e déficit de atenção e aprendizagem, sendo a principal causa de demência em idosos. A Hipótese Amilóide postula o acúmulo de depósitos extracelulares do peptídio β-amilóide (Aβ) no cérebro como o principal fator da doença. Entretanto, sua etiologia ainda não está completamente elucidada e seu tratamento visa apenas a melhora dos sintomas. Cepas transgênicas do nematódeo Caernorhabditis elegans que expressam as espécies tóxicas Aβ, têm sido utilizadas como modelos in vivo de DA para elucidar mecanismos e verificar a eficácia de novas moleculas. O disseleneto de difenila ((PhSe)2), composto orgânico de selênio utilizado nesse estudo, tem demonstrado eficácia em melhorar diversos parâmetros em modelos de doenças neurodegenerativas. No presente estudo foram analisados os efeitos do tratamento crônico com (PhSe)2 na toxicidade induzida pela Aβ em C. elegans. Os resultados mostraram que a exposição crônica ao (PhSe)2 atenuou o estresse oxidativo induzido pela Aβ, além de recuperar a memória associativa no nematódeo. Além disso, o (PhSe)2 diminuiu a expressão do gene Aβ, levando a supressão do peptídio Aβ e reduzindo a expressão do gene hsp-16.2, por diminuir a necessidade desta chaperona frente a toxicidade Aβ. Estes dados sugerem que o (PhSe)2 desempenha um importante papel na proteção contra a toxicidade induzida por estresse oxidativo, além de representar um promissor agente farmacológico por atenuar a expressão do Aβ.