Near-infrared (near-IR) Fourier transform vibrational circular dichroism (FT-VCD) spectroscopy has been used to monitor the epimerization of (S)-(+)-2,2-dimethyl-1,3-dioxolane-4-methanol (S-DDM). The near-IR-VCD spectra display clear isolated VCD bands at the range of 4700-5050 cm(-1) resulting from the OH stretch-bend combination bands of S-DDM, which were found to decrease in intensity with increasing reaction time. The near-IR-VCD spectra of 10 reference samples obtained were subjected to partial least-squares (PLS) regression analysis, and the results were used to build predictive models for enantiomeric excess (EE) determination. Multivariate regression was carried out on three different sets of spectra, corresponding to the epimerization of S-DDM in three different solvents: methylcyclohexane, carbon tetrachloride and tetrahydrofuran. The effects of solvent in DDM epimerization are discussed in terms of the relative stabilization of the reaction intermediate of the DDM epimerization reaction. The results of these near-IR-VCD studies for the determination of EE highlights the potential of VCD for in situ real-time process monitoring of the reaction kinetics of chiral molecules in solution.
The construction and performance of an electron diffraction camera suitable for the examination of crystalline regions of a few hundred Ångstroms extension is described. Goniometric control of the angle of incidence is available under conditions of low contamination. Brief descriptions are given of applications to (a) problems of symmetry including the direct determination of noncentrosymmetric groups, (b) the determination of thickness to an accuracy of one unit cell, (c) the refinement of structure potentials, (d) the measurement of the angular dependence of absorption, (e) the collection of structural data from thin crystals, and (f) the study of imperfect crystals.
An X-ray fluorescence method is described for the direct determination, without pretreatment, of up to 55 trace elements in coal and coal-derived materials. A single assay specimen is prepared by blending finely crushed sample with a binder and briquetting. Calibration is effected by a suite of synthetic calibration standards prepared from spectrographically pure materials blended into graphite. Variations in ash and sulphur content required matrix absorption corrections to analyte and background intensities, which were achieved by use of relationships with the measured Compton scattered radiation. Prior knowledge of or assumptions concerning the composition of the coal are not required. Accurate results are presented for three NBS coal standard reference materials. Certain elements (including T1, Hg, Te, In, Cd, Ag and most rare earths) are normally present in coal at levels below the detection capability of the described method and require quantification by alternative techniques. 0 Heyden
A CGR cold cathode tube for X‐ray anlysis of very light elements has been attached to a sequential Siemens XRF spectrometer. Economic consideraions justify a rapid changeover attachment rather than the purchase of a separate spectrometer which can be applied to a few elements. The essential conceppts developed for successful integration of the attachment with the spectrometer are described. An automic activated alumina trap system to remove backstreaming oil vapours was incorporated with the existing vacuum logic. The current stabilization circuit of the generator was redesigned in order to acheive quantitative analyses. The developed attachment has competitive applicability to the quantitative analyses of magnesium and lighter elements. For magnesium the cold cathode tube achieves a large deduction in both matrix absorption effects and high order spectral interferences when compared with a typical chromium tube.
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