Chemical shift based

Fig. 6.—13C-N.m.r. Spectra of A, /3-Limit Dextrin of Rabbit-Liver Glycogen (aqueous solution ambient temperature chemical shifts based on tetramethylsilane) and B, Waxy-barley Amylopectin [in D,0 at 70° chemical shifts (8C) based on external tetra-methylsilane].

Gay and Liang also found large changes in 13C chemical shifts when amines interacted with Bronsted acid sites, and smaller ones when the interaction involved Lewis acid sites. They suggested therefore that fractional coverage of the surface of the amine could be calculated from the magnitude of the 13C chemical shift, based on wo assumptions (1) that chemical shifts are average values from protonated and unprotonated amine molecules and (2) that the shift for a protonated molecule is the same as for the acid solution of the amine. Experiments showed that both assumptions were well founded. It follows that the fraction, /B, of amine molecules bonded to the Bronsted sites can be derived from the equation... 

The resonant frequency is not only a characteristic of the type of nucleus but also varies slightly depending on the position of that atom within a molecule (the chemical environment ). This occurs because the bonding electrons create their own small magnetic field that modifies the external magnetic field in the vicinity of the nucleus. This subtle variation, on the order of one part in a million, is called the chemical shift and provides detailed information about the structure of molecules. Different atoms within a molecule can be identified by their chemical shift, based on molecular symmetry and the predictable effects of nearby electronegative atoms and unsaturated groups. 

CMCs of these polyfluorinated surfactants are of the order of 10 5 m.47 Plots of the observed H chemical shifts versus surfactant concentration of cetyl trimethyl ammonium chloride, cetyl pyridinium chloride, cetyl dimethyl phenyl ammonium chloride, cetyl dimethyl benzyl ammonium chloride, cetyl dimethy 1-2-phenyl ethyl ammonium chloride, and cetyl dimethyl-3-phenyl propyl ammonium chloride, are sigmoidal and were fitted to a model based on the mass action. The H chemical shift-based CMC values are in excellent agreement with those determined by the surface tension method.48 The micellization processes of dodecyl trimethyl ammonium halides (chloride and bromide) studied by calorimetric titration show different behaviors at 298 K. However, these disappear at 313 K, while the results measured by the chemical shift versus surfactant concentration do not show this difference.49 The CMC of 3-aminopropyl triethoxy silane in toluene is ca. 0.47m, measured by H and l3C chemical shifts.50 The CMC of optically active potassium A -n-dodecanoyl alaminate measured by H and l3C chemical shifts is lower (11-15 him) in D20 than that in a mixed solvent of 1,4-dioxane and D20 (19mM).-51 The H chemical shift shows that the CMC of resorcinol-type calix[4] phosphoric esters having four alkyl side-chains, [4]Ar 5P-R-n, is insensitive to the length of the side-chains, n.52 The CMC values of a family of surfactants, the sodium cyclohexyl alkanoates, with different lengths of the alkanoate side-chains, were obtained from 13C chemical-shift measurements. The results show that these amphiphiles have high CMCs (0.12-1.02 m).-53... 

The equilibrium of Schiff bases (48) has been studied in detail because of their interesting properties both in the solid state (Section II.N) and in biological reactions. This can be done as just described for o-hydroxy azo compounds ( 7(N,H) coupling constants and deuterium isotope effects on C and N chemical shifts)Based on /(N,H) it could be concluded that the Schiff bases form a conventional tautomeric equilibrium that can be described by two species. ... 

We did not observe an isotope-dependent chemical shift under our experimental conditions, if a shift is measured for one of the bromine isotopes, it will be the same, within experimental error, for the other isotope. This is consistent with expected isotope effects on chemical shifts. Based upon our preliminary findings, we suggest that the lUPAC chemical shift scale for both bromine isotopes may be converted to an equivalent scale for solids and that the solid-state bromine standard may be either NaBr(s) or KBr(s). If KBr(s) is used,... 

A polar molecule in solution can polarize the surrounding medium giving rise to an electric field, the reaction field (i ) at the solute. In a definitive paper, Buckingham" has developed a theory of the effect of the reaction field on chemical shifts based on the Onsager model. ... 

Answers include the number of signals and (for (he spectrum without hydrogen decoupling) the splitting by directly attached hydrogens (N + 1 rule). Chemical shifts (based on Table 10-6) are very approximate. 

There is substantial overlap between some of these approaches the experimental criteria such as bond lengths determined for instance by X-ray diffracto-metry have served in several classical modeling criteria. Krygowski s harmonic oscillator model and Schleyer s nucleus-independent chemical shifts, based on classical physics principles, can be considered to belong to class (2). A whole recent issue of Chemical Reviews debated problems connected with aromaticity, including local aromaticity (Schleyer 2001). 

A H spin diffusion approach was taken in which a chemical shift-based gradient between the P3HT and PCeiBM was established, and the... 

The Tc chemical shifts based on the ( Tc)(CO)3(NNO) complex conjugated to the antitumor agent 2-(4 -aminophenyl)benzothiazole were reported and the thermal and solvent effects were studied computationally by quantum-chemical methods, using the DFT calculations at the level BPW91/aug-cc-pVTZ for the Tc and BPW91/IGLO-II for the other... 

Measurement of chemical shift based on the magic-angle turning (MAT)... 

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