Statistical-overlap theory for elliptical zones of high aspect ratio in comprehensive two-dimensional separations

The low prediction by statistical-overlap theory of the numbers of singlets and peaks in two-dimensional separations containing zones represented by either circles of small number or eccentric ellipses of any number is shown to result from use of probability expressions for unbound spaces of infinite extent. An exact theory is derived for the probability of singlet formation in a reduced two-dimensional space of unit length, width, and area. The probability is a weighted sum of the probabilities of singlet formation in the interior, edge, and corner regions of the space, which depend only on saturation. The weighting factors are the fractions of area associated with each region and depend on the number of zones, the aspect ratio, the saturation, and the ellipse's spatial orientation. The average numbers of doublets, triplets, and peaks in the space are approximated by combining these results with Roach's equations describing the clustering of circles in an unbound two-dimensional space. Simulations show that theory predicts the number of singlets, doublets, triplets, and peaks, when the number of zones is 25 or more, the aspect ratio is 100 or less, and the saturation is 2 or less. The relationship is derived between the aspect ratios of ellipses in the reduced space and actual separation space. Calculations are presented for comprehensive two-dimensional gas chromatography.     

Elucidation of the structure of quindolinone,a minor alkaloid of cryptolepis sanguinolenta: Submilligram 1H-13c and 1H-15N heteronuclear shift correlation experiments using micro inverse-detection

Elucidation of minor natural product structures has been significantly augmented by inverse-detection; further improvement has been afforded by the development of micro inverse-detection probes. We report here the elucidation of the structure of a new alkaloid, quindolinone (5H, 10H-indolo[3,2-b]quinolin-11-one), from the West African plant Cryptolepis sanguinolenta. All nmr data for this minor, preparative hplc-isolated alkaloid, including ¹H-¹⁵N one? bond heteronuclear shift correlation (HMQC) data, were recorded on an 800 μg sample of the alkaloid dissolved in 140 μl of 100% d₆-DMSO using a 400 MHz spectrometer.     

INAA determination of holmium in submilligram samples of cosmochemical and geochemical interest and the second-order activation interference

Second-order neutron activation becomes a serious interference in NAA determinations of REE in submilligram samples of meteoritic and terrestrial materials when neutron fluences reach levels of 10²⁰ n·cm^–2 or more. The Dy-interference to Ho determination and the Eu-interference to Gd determination are quite, large. We discuss the importance of second-order activation interferences in REE determinations, and present an experimental method to correct the Dy-interference to Ho determination by INAA. Ho concentrations in splits of less than 0.2 mg of BCR-1 and SP can be accurately determined by INAA with a neutron fluence of 2·10²⁰ n·cm^–2 in spite of large Dy interferences of up to 200%. It is necessary for NAA determinations of REE with high neutron fluences that each REE standard be separated from its neighboring REE, in order to correct the second-order activation interferences experimentally.     

Asymmetric synthesis of α-hydroxy carboxylic acids: direct oxidation of chiral amide enolates using 2-sulfonyloxaziridines

The direct oxidation of chiral amide enolates to optically active mandelic acid using 2-sulfonyloxaziridine $\text{1}$ is described. The diastereoselectivity is counterion dependent.     

Unsymmetrical selenides from selenocyanates and methyl grignard

The addition of methyl Grignard to diethyl acetamido(cyanoselenobenzyl)malonates 3 and 4 at ?78° followed by hydrolysis yields the 3-(4-and 3-methylselenophenyl)alanines 1 and 2.     

The chromatographic properties of a mixed-bed stationary phase combining reversed-phase and strong anion exchange properties

Summary The feasibility of using columns prepared by mixing together ion exchange and reversed-phase packing materials, for the simultaneous separation of ionised and unionised molecules, has been examined using a range of test solutes. Columns prepared in this way showed chromatographic properties which were intermediate between those of the individual phases. The dual nature of the retention mechanism allowed the retention of ionisable molecules to be adjusted using pH whilst that of uncharged compounds was unaffected. The simultaneous chromatography of model compounds and their glucuronide, sulphate and glycine conjugates, under a variety of conditions, was demonstrated.     

The carbonyl frequency of cyclopentanone as a function of its distortion,a vibrational model for 2- and 7-norbornanone

Using normal coordinate analysis, the infrared spectrum of cyclopentanone has been computed as the geometry is modified from the planar form to those in which the geometry about the 5-membered ring looks like 2-norbornanone and also like 7-norbornanone. The computed v _co of cyclopentanone is 1739.9 cm^–1 compared to the experimental value of 1740 cm^–1.As the cyclopentanone ring is deformed to resemble either the 2-norbornanone or the 7-norbornanone ring system, the computed v _co is too low using the force fields that give good agreement with experimental value for cyclopentanone. The carbonyl force constant had to be increased from 9.7 to 10.0 mdynes/Å for the 2-norbornanone-like model and to 10.4 mdynes/Å for the 7-model system to give v _co values in agreement with the experimental values.

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Zusammenfassung Mit Hilfe der Normalkoordinaten-Analyse wird das Infrarotspektrum des Zyklopentanons berechnet, wobei die Geometrie von der planaren Form bis zu der Form, bei der die Geometrie am Fünfring derjenigen des 2-Norbornanons oder auch des 7-Norbornanons ähnlich wird, variiert wird. Der berechnete Wert von v _co des Zyklopentanons ist 1739,9 cm^–1, während der experimentelle Wert 1740 cm^–1 beträgt. Bei den angegebenen Deformationen des Zyklopentanonrings wird der berechnete Wert von v _co zu niedrig, wenn man die Kraftfelder zugrunde legt, die für das Zyklopentanon gute Übereinstimmung mit dem Experiment liefern. Die Carbonylkraftkonstanten mußten von 9,7 auf 10,0 m dyn/Å für das 2-Norbornanon-Modell sowie auf 10,4 m dyn/Å für das 7-Norbornanon erhöht werden, um den Wert von v _co in Übereinstimmung mit dem Experiment zu bringen.

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Résumé Le spectre infra-rouge de la cyclopentanone a été calculé en employant l'analyse en coordonnées normales lorsque l'on modifie la géométrie depuis la forme plane jusqu'aux formes o u les configurations sur le noyau pentagonal ressemblent à celles de la 2-norbornanone et de la 7-norbornanone. La fréquence _co calculée pour la cyclopentanone est 1739.8 cm^–1 comparée à la valeur expérimentale de 1740 cm^–1. Lorsque l'on déforme le cycle de la cyclopentanone comme indiqué ci-dessus, la valeur calculée de _co est trop faible si l'on emploie les champs de force qui donnent un bon accord avec la valeur expérimentale pour la cyclopentanone. La constante de force du carbonyle doit etre augmentée de 9,7 à 10,0 mdynes/Å pour le modèle analogue à la 2-norbornanone et de 9,7 à 10,4 mdynes/Å pour le modèle analogue à la 7-norbornanone.

     

The effect of platinum oxide films on reaction kinetics at platinum electrodes

It has been known for some time that the pre-treatment of platinum electrodes often effects subsequent electrochemical reactions. Part of the effect of pre-treatment is due to the fact that anodized or chemically oxidized platinum electrodes become coated to some degree with a film of platinum oxide. This work was concerned with quantitative measurement of kinetic parameters as a function of the extent of oxide film formation. Whenever possible, variations in reaction mechanisms are proposed.

Most of the experimental evidence has been taken from current-potential curves but the techniques of chronopotentiometry and microscopy were also used.

The reduction of vanadium^V chromium^VI, arsenic^V, iodate and oxygen were investigated as well as the oxidation of vanadium^IV, arsenic^III, oxalic acid, and formic acid. The presence of the platinum oxide film effected the reactions studied in a variety of ways but in every case some variation in the kinetic parameters of the reactions studied was recorded. For a number of cases, a modified oxygen bridge theory was found useful.     

Forces in molecular recognition: Comparison of experimental data and molecular mechanics calculations

Summary NMR studies of the rotation barrier of the disaccharide of the glycopeptide antibiotic vancomycin have been used to test the performance of computer simulation techniques using molecular mechanics. In the absence of any solvated water, no correlation could be found between experiment and calculation. By introducing solvent water molecules into the binding region of the antibiotic, the NMR results could be simulated both qualitatively and quantitatively within experimental error without using massive computational resources.     

Simple correction for perturbations of peak amplitudes in statistical models of overlap

Summary The application of statistical models of overlap (SMOs) to saturated separations is made possible by theory that addresses variable peak amplitudes. These amplitudes cause peak widths to vary, and this variation can be modeled by a random variable whose effect on the probability of overlap is expressed by a convolution integral. Modified probabilities of overlap are derived for both homogeneous and nonhomogeneous one-dimensional separations, and the new probabilities are compared to results determined from published computer simulations. The new theory can describe overlap at saturations that are 3 to 4 times larger than before. Previously reported experimental chromatograms are reinterpreted to show the capabilities of theory. The theoretical extension is an important step towards making SMOs into practical tools for screening analytical separations.