NMR spectroscopy cations

The aluminium species in activated zeolites have been studied using NMR spectroscopy. Aluminium and iron in the framework of zeolites have been analysed quantitatively by Al NMR spectroscopy. The templating of high-silica zeolites by THF has been studied by C NMR spectroscopy. O NMR spectroscopy has been used to study the atomic multipoles in Fl-form aluminosilicate frameworks. A comparison of fast amplitude modulation shifted-echo pulse sequences for O 3Q- and 5Q-MAS NMR experiments has been carried out for oxygen sites in zeolites. By using a combination of the Monte-Carlo technique with Si NMR spectroscopy, cation sites in zeolites have been located. " The growth of zeolites on kaolin microspheres has been investigated by NMR spectroscopy. F1 MAS spin echo NMR spectroscopy,... 

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

Compound 5 was analyzed by NMR spectroscopy to gain information relative to conformation and complexation preferences. When complexation with potassium cations was attempted, the N—CHj signals were affected more than others. When the cation present was Ag , the protons adjacent to sulfur were more strongly affected. This observation may indicate the relative binding sites for soft versus hard cations in this system. ... 

The mechanism was then reexamined 25 years later in 1997 by Kappe. Kappe used H and C spectroscopy to support the argument that the key intermediate in the Biginelli reaction was iminium species 16. In the event, 5 reacted with 3a to form an intermediate hemiaminal 17 which subsequently dehydrated to deliver 16. Iminium cation 16 then reacted with 6 to give 14, which underwent facile cyclodehydration to give 15. Kappe also noted that in the absence of 6, bisureide 8 was afforded as a consequence of nueleophilic attack of 16 by urea (3a). This discovery confirmed the conclusion of Folkers and Johnson in 1933. As far as the proposal from 25 years earlier by Sweet and Fissekis, Kappe saw no evidenee by H and NMR spectroscopy that a carbenium ion was a required species in the Biginelli reaetion. When benzaldehyde (5) and ethyl... 

As the alkaloid was extracted with hexane, acetone, and ethanol, subjected to column chromatography, acidified (AcOH) and then neutralized (NaOH), the cationic form was formulated as a hydroxide salt. However, only two OH groups were detectable on H NMR spectroscopy. Only slight differences were found in the UV spectra taken in methanol [kmax (loge) = 218 (4.68), 302 (4.39), 394 (4.08) nm] and methanol+NaOH [T-max (loge) = 228 (4.66), 310 (4.39) nm]. Three tautomeric forms can be formulated which are shown in Scheme 42. Two of them possess the isoquinolium-7-olate moiety. The H NMR data are presented in Table IV. They indeed unambiguously resemble the cationic species 112. 

Relatively little has been reported regarding the determination of the purity of the halide salts other than by standard spectroscopic measurements and microanalysis. This is largely because the halide salts are rarely used as solvents themselves, but are generally simply a source of the desired cation. Also, the only impurities likely to be present in any significant quantity are unreacted starting materials and residual reaction solvents. Thus, for most applications it is sufficient to ensure that they are free of these by use of FF NMR spectroscopy. 

In this context it is important to note that the detection of this land of alkali cation impurity in ionic liquids is not easy with traditional methods for reaction monitoring in ionic liquid synthesis (such as conventional NMR spectroscopy). More specialized procedures are required to quantify the amount of alkali ions in the ionic liquid or the quantitative ratio of organic cation to anion. Quantitative ion chromatography is probably the most powerful tool for this kind of quality analysis. 

Raman and multinuclear NMR spectroscopy have been used to study the reaction product of pentafluoropyridine with XeF+AsF6 in anhydrous HF. Identified in the mixture was a fluoro(pentafluoropyridine)xenon(II) cation with the Xe bonded to the ring N (88CC257). 

Conclusive proof as to the survival of Si-H groups in HSi-PIB after cationic polymerization was obtained by H1 NMR analysis. To this end sufficiently low molecular weight samples were prepared (Mn = 600) and examined by H1 NMR spectroscopy. A representative spectrum is shown in Fig. 32. 

Kennedy and co-workers10 studied the kinetics of the reaction between Me3Al and t-butyl halides using methyl halide solvents as a model for initiation and termination in cationic polymerization. Neopentane was generated rapidly, without side reactions and rates were determined by NMR spectroscopy. The major conclusions were ... 

The ratio of the different isomeric products was found to vary with time, temperature, and initial concentration. This suggested that some kind of equilibration was occurring between isomers. I3C NMR spectroscopy of a reaction mixture showed, upon cooling, the reversible formation of a pair of signals in the anomeric region. These signals were ascribed to the anomeric carbon atoms of fructofuranosyl fluorides (10), which were presumed to be in equilibrium with the reactive fructofuranosyl cation, 11. 

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

The study of the MEEP/PEO-(LiX)n and MEEP/PPO-(LiX)n complexes, (X= BF4, CIO4) by Li NMR spectroscopy, DSC and X-ray diffraction showed that they were multiphase with amorphous MEEP and crystalhne PEO phases. Strong cation-anion association effects were also observed [612]. 

Because this diketene acetal is so susceptible to cationic polymerization, acids cannot be used to catalyze its condensation with diols because the competing cationic polymerization of the diketene acetal double bonds leads to a crosslinked product. Linear polymers can, however, be prepared by using iodine in pyridine (11). Polymer structure was verified by 13c nmR spectroscopy as shown in Fig. 

The brown colour of a chloroform solution of the salt [24 2 ] suggests the formation of the covalent hydrocarbon [24-2] by cation-anion combination (33). The structure of [24-2] was determined by and nmr spectroscopy in CDCI3 (Okamoto et al., 1988, 1990). 

Only PAE, a novel acetyl esterase, could remove acetyl from beet pectin, to a maximum of 30%. This was shown to be one specific acetyl group in theJiomogalacturonan chain of pectin (smooth region) by NMR spectroscopy. PAE activity was influenced by buffer salts and the addition of bivalent cations. PAE worked cooperatively with pectolytic enzymes. 

Earlier studies using thermal denaturation analysis and spectrophotomet-ric titration with TxA T and CxC-C" containing DNA triplexes showed that coralyne binds strongly to these triplexes by intercalation and does not exhibit a significant sequence-selectivity [222]. In a later study by Morau Allen et al. [217], employing DNase footprinting, thermal denaturation analysis, UV-visible spectrophotometric titrations, circular dichroism and NMR spectroscopy, showed that coralyne is fully intercalated into TxA T triplex DNA whereas in C GxC triplex, it is partially intercalated due to electrostatic repulsion between the cationic alkaloid and the protonated cytosine [217]. Kepler et al. [223] demonstrated that coralyne intercalated to parallel triplex DNA but did not intercalate to antiparallel triplex DNA. Recently Hud and coworkers [219,224] demonstrated that duplex poly(dA) poly(dT) is trans-... 

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