Shelton Bank

Some principles of NMR spectroscopy and their novel application

Nuclear magnetic resonance spectroscopy often uses complex methodologies in complex applications. In this review, some of the details of NMR spectroscopy are made more accessible and more interesting. Determinations of components of everyday interest, such as beer, peanuts, corn, and bananas, illustrate recent advances in overcoming sensitivity problems and highlight fascinating applications of relaxation methods. Examples are given of the selection of windows for the application of chemical shifts and T2 relaxation times. The innovative and unusual applications of NMR techniques demonstrate and highlight important principles of NMR spectroscopy.

Steric effects in the metalations of 1,1-di-p-tolylethane. Kinetic vs. equilibrium control

The reactions of 1,1-di-p-tolylethane with metalating agents in tetrahydrofuran were examined. With lithio derivatives the nature of the anion formed depended upon the base. The 1H and 13C NMR spectra as well as the products of methylation of the anions are compared with those of p-xylyllithium and 1,1-diphenylethyllithium. Each anion is distinct in spectra and product of methylation. When a deficiency of 1,1-di-p-tolylethane was used, anion equilibration was effected.

13 C CPMAS nuclear magnetic resonance study of the adsorption of 2-phenethylamine on clays

Abstract2-Phenethylamine 13C-enriched in the beta position was adsorbed on four different aluminum-exchanged clays: hectorite, Barasym, Laponite RD, and lithium taeniolite. The sites for adsorption were characterized by 13C high-resolution cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS-NMR) spectroscopy. Using differences in chemical shift values and linewidths, three different types of bound ammonium compounds and a motionally restricted bound compound were identified. Correlation with charge effects indicated that one of the clay sites was extremely acidic.The important catalytic sites for the different clays, the edge or platelet face, interlamellar and some combination of both, were probed by using the trimethylsilyl group as a clay-blocking agent. Silylation of aluminum-exchanged Laponite RD and Li taeniolite had little effect on amine adsorption. This indicates that, for these clays, amine adsorption occurred mainly at interlamellar sites. For hectorite, amine adsorption occurred at both surface and interlamellar sites, and silylation had the effect of reducing surface adsorption. Silylation of Barasym resulted in a very interesting shift of adsorption from one kind of surface site to a more acidic surface site.

ON THE MECHANISM OF THE CLAY CATALYZED REACTION OF BENZYLAMINE

Abstract The mechanism of the aluminum exchanged Montmorillonite catalyzed reaction of benzylamine was studied with deuterium labelled benzylamine. From the analysis of the product dibenzylamine and the recovered benzylamine extensive hydrogen exchange is observed. An SN 2 process is thus unlikely and a process involving an imine is suggested.

Solid 13C CPMAS NMR spectroscopy studies of biosynthesis in whole cells of Methanosphaera stadtmanae

Whole cells of Methanosphaera stadtmanae were grown in media containing [13C]CO2, [2-13C]acetate, [1-13C]acetate, [3-13C]serine and [1-13C]formate. The label incorporation was determined using solid state 13C CPMAS NMR spectroscopy. The incorporation of serine hydroxymethyl carbon into the purine rings of nucleic acids and most probably the methyl group of thymine is demonstrated. The one carbon atom pathway shown in our previous work is operative in the biosynthesis of purines and pyrimidines. In addition, these studies clearly identified signals not observed in solution NMR spectroscopy and revealed an important pathway not previously known. The reversibility of formyl-containing one carbon atom carriers is demonstrated. The pattern of labeled carbon atoms in sugars confirms the biosynthetic route from pyruvate, which is formed from acetic acid and carbon dioxide. Finally, a preliminary lipid assignment is indicated. The solid state 13C CPMAS NMR of these intact cells proved to be a facile method to follow specific pathways.

Effects of Steric Strain on the Preparation, Stability and Reactions of Aryl Anions and Amines

Steric effects as important contributors to the behavior of organic compounds were first identified by Kehrmann less than 100 years ago.1 The subsequent major developments were outlined by Brown in 1956.2 Since that time steric effects have become an important part of the fabric of organic chemistry and recent advances are highlighted in chapters in advanced textbooks.3,4 Steric effects have not always occupied a position of high regard, and the classic work since 1940 has altered indelibly the then prevailing view. The present-day organic chemist values and utilizes steric factors along with polar and resonance factors to account for chemical effects.