Determination by NMR

These methods are now obsolete in comparison with spectroscopic methods. Werbel has shown that the structures of these isomers are easily determined by NMR (125) (see also Table VI-5). Furthermore. 2-imino-4-thiazoline derivatives are characterized by their stretching C=N vibration at 1580 cm , absent in their 2-aminothiazole isomers, and by the stretching NH vibration that appears in the range of 3250 to 3310 cm for the former and between 3250 to 3340 cm" for the latter (131). Ultraviolet spectroscopy also differentiates these isomers (200). They can be separated by boiling in ethanol the thiazoline isomer is usually far less soluble in this solvent (131),... 

Glass-forming systems other than siUca have been examined. The fraction of three- and four-coordinated boron in borate glasses can be determined by nmr (29). Both nmr and x-ray diffraction (30) results led to the suggestion that the boroxyl ring is the stmctural unit of vitreous The... 

The conversion of aromatic monomers relative to C-5—C-6 linear diolefins and olefins in cationic polymerizations may not be proportional to the feedblend composition, resulting in higher resin aromaticity as determined by nmr and ir measurements (43). This can be attributed to the differing reactivity ratios of aromatic and aHphatic monomers under specific Lewis acid catalysis. Intentional blocking of hydrocarbon resins into aromatic and aHphatic regions may be accomplished by sequential cationic polymerization employing multiple reactors and standard polymerization conditions (45). 

Figure 3 Model building by Modeller [31], First, spatial restraints in the form of atomic distances and dihedral angles are extracted from the template stmcture(s). The alignment is used to determine equivalent residues between the target and the template. The restraints are combined into an objective function. Finally, the model for the target is optimized until a model that best satisfies the spatial restraints is obtained. This procedure is technically similar to the one used in structure determination by NMR.

In order to examine whether this sequence gave a fold similar to the template, the corresponding peptide was synthesized and its structure experimentally determined by NMR methods. The result is shown in Figure 17.15 and compared to the design target whose main chain conformation is identical to that of the Zif 268 template. The folds are remarkably similar even though there are some differences in the loop region between the two p strands. The core of the molecule, which comprises seven hydrophobic side chains, is well-ordered whereas the termini are disordered. The root mean square deviation of the main chain atoms are 2.0 A for residues 3 to 26 and 1.0 A for residues 8 to 26. 

The situation is different for other examples—for example, the peptide hormone glucagon and a small peptide, metallothionein, which binds seven cadmium or zinc atoms. Here large discrepancies were found between the structures determined by x-ray diffraction and NMR methods. The differences in the case of glucagon can be attributed to genuine conformational variability under different experimental conditions, whereas the disagreement in the metallothionein case was later shown to be due to an incorrectly determined x-ray structure. A re-examination of the x-ray data of metallothionein gave a structure very similar to that determined by NMR. 

Again, the chloride is almost exclusively the exo isomer. The distribution of deuterium in the product was determined by NMR spectroscopy. The fact that 1 and 2 are formed in unequal amoimts excludes the symmetrical bridged ion as the only intermediate. The excess of 1 over 2 indicates fliat some syn addition occurs by ion-pair collapse before the bridged ion achieves symmetry with respect to the chloride ion. If the amount of 2 is taken as an indication of the extent of bridged-ion involvement, one would conclude that 82% of the reaction proceeds through this intermediate, which must give equal amoimts of 1 and 2. 

The magnitude of the preference for the formation of the less substituted enamine from unsymmetrical ketones as expressed by the general rule given above is not entirely clear. House and Schellenbaum 48) have reported that 2-methylcyclohexanone and pyrrolidine produce a product mixture of tetra- and trisubstituted enamines in a ratio of 15 85. The estimate of this ratio was made from NMR data. In contrast Stork and co-workers (9) report the formation of 100% trisubstituted enamine as determined by NMR spectroscopy. 

Different methods have been devised to represent proteins. A structure for porcine pancreatic procolipase is reported in the Protein Databank, as determined by NMR spectroscopy. Many such structures are reported without the hydrogen atoms, since their positions often cannot be determined experimentally. Most MM packages will add hydrogens. Figure 1.18 gives the hydrogen-free procolipase structure in line representation. 

Other authors have used coupling constants instead of (or simultaneously with) chemical shifts. In some cases they have been determined by NMR spectroscopy, in other cases, labeled compounds and or NMR spectroscopies have provided these couplings. These couplings have been used for determining tautomeric composition (see the discussion by Begtrup in 87MI371). Most examples involved and... 

Both PS-A and PS-B have a tendency to hydrate like panal, and they also form adducts with methylamine. The adducts, PS-A/MA and PS-B/MA, are prepared by incubating PS-A or PS-B in 75 % methanol containing an excess amount of methylamine hydrochloride plus some sodium acetate to neutralize the HC1, at 45°C for 30 min. The adducts can be purified by HPLC on a PRP-1 column (80% acetonitrile containing 0.05% acetic acid). Their chemical structures have been determined by NMR and mass spectrometry as shown in Fig. 9.8 (p. 288). Both adducts are colorless and show an absorption maximum at 218 nm. 

Tacticity is most often determined by NMR analysis and usually by looking at the signals associated with the -CXY- group (refer Figure 4.3). The analysis then provides the triad concentrations (mm, mr and rr) and the value of m or P(m) is given by cq. 10. 

The conformational preference of the monosulfoxides of 1,2-, 1,3- and 1,4-dithianes (179-181) were determined by NMR experiments which included variable-temperature studies, double irradiation, solvent effects and the influence of lanthanide shift reagents167. For 179 and 181, the axial conformers were the dominant species in CD3OD, but for 180, the equatorial conformer was in excess. 

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 ... 

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. °°... 

Since the reactivity of the sulfanes, dissolved in ether or chloroform, towards such reagents is higher than that of Ss a separate determination of these components in mixtures is possible [80]. However, nowadays the sulfanes can be more easily determined by NMR spectroscopy and the dissolved sulfur by Raman spectroscopy (see above). 

Details are given of the successful construction of a novel reversible system of network polymers between bifunctional monomers by utilising the equilibrium polymerisation system of a spiro orthoester. Molecular structures were determined by NMR and IR spectroscopy. 9 refs. 

Fig. 2.12 The j8-peptide 3,4-helical structure. (A) Stereo-view along the helix axis of the left-handed 3,4-helix formed by, 8 -peptide 66 in solution as determined by NMR in CD3OH (adapted from [103, 154]). Side-chains have been omitted for clarity. (B) Top view.

Fig. 2.47 The (P)-2.5-helical structure of N.N -linked oligoureas as determined by NMR meaurements in pyridine-c/5. (A) Stereo-view along the helix axis of a low energy conformer of nonamer 178 generated by restrained molecular dynamics calculations. (Adapted from [274]). The helix is characterized by (i) a rigid +)-SYnclinal arrangement around the C(a)-...

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. 

Wield was determined by NMR spectroscopy. BQ = benzoquinone. Experiments were run with 1 equiv. of D2O. 

The DB of the hyperbranched acrylates obtained by SCV(C)P of the acrylate-type inimer 1 can be determined by NMR spectroscopy [23, 31]. Figure 2a shows the respective NMR spectrum of a hyperbranched polymer obtained... 

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