Differential non-destructive image current detection in a fourier transform quadrupole ion trap

Dual-detector differential non-destructive Fourier transform detection in a quadrupole ion trap is shown to improve signal intensity and reduce noise compared with spectra recorded using a single detector. A larger area detector in each end-cap electrode is machined to fit its hyperbolic shape and so minimize field imperfections on the z-axis. Argon, acetophenone and bromobenzene spectra were recorded to allow a comparison between single- and dual-detector (differential) modes of detection and to demonstrate the improvement achieved with differential detection. Copyright 1999 John Wiley & Sons, Ltd.     

Pseudo-steady-state solutions for solidification in a wedge

Polubarinova-Kochina's analytical differential equation methodis used to determine the pseudo-steady-state solution to problemsinvolving the freezing (solidification) of wedges of liquidwhich are initially at their fusion temperature. In particular,we consider four distinct problems for wedges which are: freezingwith the same constant boundary temperature, freezing with thesame constant boundary heat fluxes, freezing with distinct constantboundary temperatures and freezing with distinct constant fluxesat the boundaries. For the last two problems, a Heun's differentialequation with an unknown singularity is derived, which in bothcases admits a particularly elegant simple solution for thespecial case when the wedge angle is . The moving boundariesobtained are shown pictorially.     

Ester dienolate [2,3]-Wittig rearrangement in natural product synthesis: diastereoselective total synthesis of the triester of viridiofungin A, A2, and A4

[structure: see text] An ester dienolate [2,3]-Wittig rearrangement was utilized to access the alkylated citric acid skeleton 6 that is characteristic for the viridiofungins and other members of the alkyl citrate family of secondary natural products. The [2,3]-sigmatropic rearrangement of (Z,Z)-15 provided the rearrangement product (+/-)-syn-16 in moderate yield and with very good diastereoselectivity. A Julia-Kocienski olefination efficiently served to connect the polar head (+/-)-syn-26 with the lipophilic tail (32a-c) of the viridiofungins. Amide formation between the racemic viridiofungin precursors 35a-c and the enantiomerically pure amino acid L-tyrosine methyl ester followed by preparative reversed-phase HPLC provided the isopropyl dimethyl ester of viridiofungin A ((+)-39a), A2 ((+)-39b), and A4 ((+)-39c) as well as the nonnatural diastereomers (-)-38a-c.     

A thallium-205 NMR study of thallium acetate ion association in 2,2,2-trifluoroethanol and dimethylsulfoxide

The association of thallium acetate ion in 2,2,2-trifluoroethanol and dimethyl-sulfoxide has been investigated by thallium-205 NMR spectroscopy. Analysis of the association constants as a function of temperature indicates that in trifluoroethanol the association enthalpy and entropy are, respectively, 1.6 kcal-mol^–1 and 19.3 cal-mol^–1-K^–1 and in dimethylsulfoxide they are-0.99 kcal-mol^–1 and 18.0 cal-mol^–1-K^–1. Examination of the temperature dependence of the chemical shift of the ion-pair reveals that in dimethylsulfoxide the ion-pair exists as a contact species, while in trifluoroethanol the solvent-separated ion-pair is more likely.