Nuclear magnetic resonance temperature

At room temperature, nuclear magnetic resonance cannot used to distinguish between R2Mg and RMgX. What does this signify Propose a mechanism for a process to explain this observation. 

Shaji, T. et al., Isolation and structure elucidation of some procyanidins from apple by low-temperature nuclear magnetic resonance, J. Agric. Food Chem., 51, 3806, 2003. 

Fig. 5.4.16 Typical room temperature nuclear magnetic resonance spectra of human BA and standards of glycine- and taurine-conjugated bile acid, (reprinted from [40])...

Whilst this chapter is not comprehensive, attention is focused on those reports of photooxygenation where peroxide yields approached or exceeded 50%. The isolation of peroxides involves meticulous and potentially dangerous work [1]. Thus, percentage yields are not always reported in the same way, and may relate to the feasibility of purification and safety and, perhaps because of this issue, safety information is often incomplete. Analytical methods such as low-temperature nuclear magnetic resonance (NMR) spectroscopy has allowed the characterization of unstable peroxides in reaction mixtures. Some peroxides have been isolated in traps under vacuum or by low-temperature silica gel chromatography, while others were recrystallized at room temperature [2—4]. 

Compared to reactions with methyl halides, reactions of [M(sdt)2]2 (M = Ni, Pd, Pt) with benzyl halides are more complex (64). Three interconvertible isomers (9-11) have been observed in solution by low-temperature nuclear magnetic resonance (NMR) spectroscopy (Scheme 5). The trans-anti isomers (9) for all three metals have been isolated and are isomorphous (64). 

J. A. Jackson, also of this laboratory, has made room temperature nuclear magnetic resonance studies of the Knight shift of cadmium in slowly cooled CeCd, 45 alloys with different compositions and different histories. All CeCd 4 5 samples tested showed a major peak at almost the same position and shifted from that of metallic cadmium. One sample showed only this peak, while others clearly showed satellite peaks either at larger or at smaller shift. Possibly some samples had small amounts of both satellite peaks, and there was apparently some further difference in the shapes of satellite peaks and of the major peak these latter observations are tenuous, however, since they were near the resolution limit of the apparatus. The differences apparently do not correlate simply with composition however, they may correlate with differences in microphase structures. 

An operational description is that one reactant (the more ionic compound with the more electropositive metal) transfers alkyl anions to the other. Thus the four methyl groups in Li2BeMe4 form a distorted tetrahedron around the beryllium, with longer distances to the lithium ions. However, this description is oversimplified. The low-temperature nuclear magnetic resonance (NMR) spectrum of Li3MgMe5 has three different methyl resonances, suggesting structure (14), related to the MeLi tetramer. Ate complexes with zinc and aluminum compounds also form. Electron-deficient bridge-bonded structures, exemplified by the X-ray structure of... 

Golubev, N. S., Smirnov, S. N., Gindin, V. A., Denisov, G. S., Benedict, H., and Limbach, H.-H., Formation of charge relay chains between acetic acid and pyridine observed by low-temperature nuclear magnetic resonance, J. Am. Chem. Soc. 116, 12055-12056 (1994). 

Reaction of hydrogen bromide-acetic acid with the diacetal of 5,6-an-hydro-aZZo-inositol (76) gives the meto-dibromotetrol (77) tetraacetate, crystals of which are isolable from the reaction mixture. This result is surprising because the diastereomeric anhydroinositol diacetal (96) gave, under the same conditions, only monobromo products. The structure and configuration dl(1234/56) (77) were established by variable-temperature, nuclear magnetic resonance and spin-decoupling (see p. 56). ... 

The mechanism of peroxyoxalate chemiluminescence centers on the nature of the postulated key intermediate and its mode of interaction with, and excitation of, the fluorophore. An early proposal for this key intermediate, the highly strained 1,2-dioxetanedione, was confirmed more than three decades later using low-temperature nuclear magnetic resonance spectroscopy in combination with ab initio calculations. As shown in Scheme 2, the formation of the key intermediate is subject to both nucleophilic and general-base catalysis by concurrent mechanisms. 

The twisted structure of 4,5-dimethylphenanthrene (1) contributes to the chirality of the molecule. It was recognized early on that the ability to resolve the two enantiomers could provide supporting evidence for the nonplanarity of the aromatic system [1]. Determinations of the rates of racemization and the activation barriers of twisted chiral polyarenes have been actively pursued. Resolutions of the enantiomers to allow these investigations to proceed were achieved in several cases. In other cases, variable-temperature nuclear magnetic resonance (NMR) experiments were employed to provide insights into the configurational stabilities of the molecules. For practical applications, such as using these twisted compounds... 

R. A. Widenhoefer developed a mechanistic study on the cyclization-hydrosilylation of various functionalized dienes catalyzed by cationic palladium phenanthroline or oxazoline complex (60). Kinetic, deuterium-labeling, and low temperature nuclear magnetic resonance studies permitted establishing a... 

The 2-methylallyluranium complexes can be prepared by an analogous approach [18]. None of these complexes is stable above 0°C for a significant period of time. Low-temperature nuclear magnetic resonance (NMR) spectroscopy reveals an A2B2X pattern characteristic of >y -allyl ligation (e.g. structure (7)),... 

The sy/j-stereochemistry, unambiguously determined by a combination of chemical correlation and spectroscopic methods, presumably results from a chair-like transition state as it has been proposed for related reactions of homo-ally he carbonates. The intermediacy of chloromethyl ether 44, generated by selective reactivity at site a, was determined by following the reaction progress with low-temperature nuclear magnetic resonance (NMR) in toluene-Js (Scheme 37.11). 

The polymers included in this study are bisphenol-A polycarbonate (Bis-A PC), polyesteicarbonates (PEC), whose structures are defined in Figure 1, phenolphthalein bisphenol-A copolycarbonate, and polysulfone. All samples were compression molded and physically aged as described elsewhere, with the exception of those for the Fourier transform infrared (FTIR) measurements and solid state variable temperature nuclear magnetic resonance (VTNMR)... 

PisanieUo DL, Helm L, Meier P, Merbach AE. High pressure NMR kinetics. 19. Variable pressure and temperature nuclear magnetic resonance and visible spectrophoto-metric studies of lanthanide ions in dimethylformamide solvation and solvent exchange dynamics. J Hm Chem Soc. 1983 105 4528-4536. 

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