H NMR Characterization of the Product from Single Solid-Phase Resin Beads Using Capillary NMR Flow Probes

A capillary NMR flow probe was designed to generate high-resolution ¹H NMR spectra at 600 MHz from the cleaved product of individual 160-μm Tentagel combinatorial chemistry beads. By injecting a dissolved sample sandwiched between an immiscible, perfluorinated organic liquid directly into the probe, NMR spectra of the product cleaved from single beads were acquired in just 1 h of spectrometer time without diffusional dilution. Sample handling efficiency on the single bead scale was comparable to that obtained with a bulk sample. Using the relative intensity of the DMSO-d₅H versus the analyte signals in a fully relaxed CPMG spectrum, the amount of product cleaved from a single bead was determined to be 540±170 pmol in one of the samples. Following the NMR data collection, the samples were examined with electrospray ionization mass spectrometry to provide additional structural information. By coupling with microliter-volume fluidic capabilities, the capillary flow probe described here will enable multidimensional characterization of single solid-phase resin products in an online manner.     

Sol-Gel Synthesis of Protoenstatite and a Study of the Factors That Affect Crystallization

Three different magnesium silicate sols were prepared from Mg(OMe)₂ and TEOS for which hydrolysis with H₂O₂ and H₂O was under stoichiometric, stoichiometric, or over stoichiometric. Xerogels were prepared from the sols by simple evaporation, spray-drying, or freeze-drying. The freeze-dried precursor formed mostly protoenstatite, a high-temperature polymorph of enstatite (MgSiO₃) that is generally not stable at room temperature; the other precursors formed mixtures of protoenstatite and clinoenstatite. The three xerogels and their calcined products were studied with XRD, HTXRD, BET, carbon analysis, TG/DTA and²⁹ Si-NMR. Residual carbon was found to be an important factor in the crystallization, and ²⁹Si-NMR spectroscopy was found to be a better tool than XRD to assess the phase purity of protoenstatite.     

Hexamethylsilirane : I. Preparation,characterization and thermal decomposition

Hexamethylsilirane has been prepared by the action of magnesium on dimethyl-bis(α-bromoisopropyl)silane in tetrahydrofuran (THF) solution. It was found to be highly reactive toward atmospheric oxygen and moisture and to decompose when heated in solution at 60–75°C. Its decomposition results in the extrusion of dimethylsilylene which may add to the tetramethylethylene produced in the decomposition to regenerate the silirane, insert into the reactive SiC₂ ring of the silirane to give octamethyl-1,2-disilacyclobutane or oligomerize to give (Me₂Si)_n oils.Dimethyldiisopropyl-, tetraisopropyl- and tert-butyltriisopropylsilane were prepared by catalytic hydrogenation of the corresponding isopropenylsilanes. Bromination of dimethyldiisopropylsilane at 65°C resulted in exclusive formation of dimethyl-bis(α-bromoisopropyl)silane.     

Interpreting Negative Results in an Angle Maximization Problem

The correct interpretation of positive and negative results in a situation involving mathematical modeling poses problems for some students. This article presents a situation in which differential calculus is used with inverse trigonometric tangentfunctions to maximize an angle measure. A negative distance measure ultimately results, thus requiring a reconsideration of assumptions inherent in the initial figure.     

The determination of hopping rates and carrier concentrations in ionic conductors by a new analysis of ac conductivity

The a.c. conductivity σ(ω) of ionic materials takes the form, σ(ω) = σ(0) + Aωⁿ. The carrier hopping rate, ω_p, is obtained from the new expression σ(0) = A ω_pⁿ and the carrier concentration is estimated from σ(0). The contribution of creation and migration terms to the activation energy for conduction may be determined from the thermal activation of σ(0) and ω_p and the corresponding entropy terms quantified. Data have been analyzed for four widely different ionic materials: single crystal Na β-alumina, polycrystalline Li₄SiO₄, Ag₇l₄AsO₄ glass and Ca(NO₃)₂/KNO₃ glass and melt. For each, the carrier concentration and hopping rates have been obtained.     

Mesomorphic N-alkylpyridinium dodecylsulphates

Reaction of pyridine with 1-bromoalkanes leads to N-alkylpyridinium bromides which can be further reacted with silver dodecylsulphate to give N-alkylpyridinium dodecylsulphates in good yield. These salts are mesomorphic and their mesophases have been characterized by optical microscopy, DSC and X-ray scattering techniques.