Studies of the naturally occurring biomethylation of selenium and the determination of the products

A systematic study of the biomethylation of selenium and the determination of the methylated species indicates preliminarily that selenium is susceptible to natural biomethylation under certain environmental conditions. Detectable levels of methylated selenium species, including dimethyl selenide [(CH₃)₂Se], dimethyl diselenide [(CH₃)₂Se₂] and dimethylselenone [(CH₃)₂SeO₂] have been detected by gas chromatography – graphite furnace atomic absorption spectrophotometry (GC – GF AA) from a variety of environmental samples. Findings of naturally methylated selenium species from both soil samples and related air samples suggest that there may exist a localized cycle of selenium between ground soil and the ambient air. Factors that influence the sensitivity and accuracy for the determination of alkyl selenide compounds by GC – GF AA have also been investigated. Flashlike injection mode and addition of about 10% of hydrogen gas to the argon carrier gas provide for highly sensitive detection. Reproducible determination can be obtained with a precision of about 6% and the detection limits are 0.3 ng Se m^?3.     

Classification of the distribution of trace impurities by spark source mass spectrometry analysis

Spark source mass spectrometry in combination with principal component analysis and clustering analysis was used to investigate the trace element distributions in metallic samples. The analysis of Zn and Cu samples and a comparison with direct imaging secondary ion microscopy demonstrated the consistency of the approach.On leave from Department of Chemistry, Nanjing Normal University, Nanjing, People's Republic of China.     

Largeness of the set of finite products in a semigroup

We investigate when the set of finite products of distinct terms of a sequence 〈x _n 〉 _n=1^∞ in a semigroup (S,⋅) is large in any of several standard notions of largeness. These include piecewise syndetic, central, syndetic, central*, and IP*. In the case of a “nice” sequence in (S,⋅)=(ℕ,+) one has that FS(〈x _n 〉 _n=1^∞) has any or all of the first three properties if and only if {x _n+1−∑ _t=1 ⁿ x _t :n∈ℕ} is bounded from above. N. Hindman acknowledges support received from the National Science Foundation via Grant DMS-0554803.     

Verschiedene Gasbrennerconstructionen

Ohne Zusammenfassung     

An unexpected aminocyclopropane reductive rearrangement

A reductive rearrangement of aminocyclopropanes is described for the synthesis of cis- or trans-fused bicyclic 1,2-diaminocyclobutanes. Ionization of a cyclic aminal using BF₃·OEt₂ induces rearrangement to a cyclobutyl iminium ion, which is subsequently reduced by Et₃SiH. Substitution with allyltrimethylsilane allows carbon incorporation, giving a quaternary center. Silyloxy-substituted cyclopropanes rearrange rapidly to cyclobutanones which react with NaBH₄ to provide 1,2-aminohydroxycyclobutanes. These aminals were generated by the reduction of a Boc-imide with DIBAL-H or LiBH₄.