Shift , chemical concentration

The 119Sn chemical shift of dimethyltin dichloride in carbon tetrachloride and other non-polar solvents remains practically invariant to large changes in concentration. It has a value of ca. +140 ppm. This indicates the ease with which the molecules are able to dissociate into discrete tetrahedral species in solution as a result of the very weak inter-molecular Sn... Cl bonds which exist in crystalline dimethyltin dichloride. (55) On the other hand, a chemical shift-concentration study of trimethyltin formate in deuterochloroform solution (56) has revealed a dramatic change in chemical shift from +2-5 ppm for a 3 M solution to + 152 ppm on dilution to 0-05 m in the same solvent. This has been attributed to self-association of monomeric tetrahedral trimethyltin formate molecules, [3]. As the concentration is increased, five-coordinate oligomeric or polymeric species, [4], could be formed. These are known to exist in the solid state. (57)... 

For non-fluorinated surfactants, H and l3C nuclei are commonly used. Transformation of the shapes of sodium dodecylsulfate (SDS) micelles in aqueous solution was found by following the changes of the H chemical shifts of CH3 and a-CH2 of SDS. A break at 8.0 mM from both the chemical shift versus SDS concentration curves corresponds to the first CMC, and breaks at 84 mM and 70 him for protons of the methyl and a-CH2 groups, respectively, correspond to the second CMC of SDS.46 The CMCs of a series of polyfluorinated cationic surfactants, C F2n +,CONH(CH2)2N(CH3)3Br, and their hydrocarbon analogs, were deduced by the change in proton chemical shift-concentration curves. The... 

The fluorine NMR chemical shifts of terminally fluorinated cationic surfactants 12,12,12-trifluorododecyltrimethylammonium bromide and 10,10,10-tri-fluorodecyltrimetylammonium bromide are similar to those of the corresponding anionic and nonionic surfactants with a terminal trifluoromethyl group [32]. When the counterion is fluoride, instead of bromide, the chemical shift of the fluoride counterion is concentration dependent. The trifluoromethyl chemical shifts were interpreted utilizing a double-equilibrium model. The aggregation number was estimated to be 25 in dilute solutions from the curvature of the chemical shift concentration plots near the first cmc. Above the second cmc, the aggregation number was assumed to be 60, considered the spatial limit for 12-carbon surfactant chains in a spherical micelle. 

AIO4 tetraliedra directly linked to an SiO. tetraliedron can be detennined from Si NMR since different chemical shifts are observed for tire corresponding Si nuclei. In tire absence of large concentrations of silanol defects, which... 

The chemical shifts of O—H and N—H protons are temperature and concentration dependent... 

The chemical shift of the hydroxyl proton is variable with a range of 8 0 5-5 depending on the solvent the temperature at which the spectrum is recorded and the concentration of the solution The alcohol proton shifts to lower field m more concen trated solutions... 

The chemical shift of the hydroxyl proton signal is variable depending on solvent temperature and concentration Its precise position is not particularly significant m struc ture determination Because the signals due to hydroxyl protons are not usually split by other protons m the molecule and are often rather broad they are often fairly easy to... 

Section 15 14 The hydroxyl group of an alcohol has its O—H and C—O stretching vibrations at 3200-3650 and 1025-1200 cm respectively The chemical shift of the proton of an O—H group is variable (8 1-5) and depends on concentration temperature and solvent Oxygen deshields both the proton and the carbon of an H—C—O unit Typical... 

NMR The H NMR signals for the hydroxyl protons of phenols are often broad and their chemical shift like their acidity lies between alcohols and carboxylic acids The range is 8 4-12 with the exact chemical shift depending on the concentration the solvent and the temperature The phenolic proton m the H NMR spectrum shown for p cresol for example appears at 8 5 1 (Figure 24 4)... 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an oletinic proton. Thus the olefin stmcture (1) for PX is also supported by nmr. 

If a sample contains groups that can take up or lose a proton, (N//, COO//), then one must expect the pH and the concentration to affect the chemical shift when the experiment is carried out in an acidic or alkaline medium to facilitate dissolution. The pH may affect the chemical shift of more distant, nonpolar groups, as shown by the amino acid alanine (38) in neutral (betaine form 38a) or alkaline solution (anion 38b). The dependence of shift on pH follows the path of titration curves it is possible to read off the pK value of the equilibrium from the point of inflection... 

Give a clear indicaUon of solvent, concentration, and temperature. These parameters have a much greater effect on chemical shifts and coupling constants for fluorine than for protons. 

The rehability of these analytical methods may be questionable when chemical shift differences of derivatives are of the same magnitude as variations encountered from solvent, concentration, and temperature influences. Reported fluorine chemical shift ranges for tnfluoroacetylated alcohols (1 ppm), p-fluorobenzoylated sterols (1 ppm), and p-fluorobenzoylated ammo acids (0.5 ppm) are quite narrow, and correct interpretation of the fluonne NMR spectra of these denvatized mixmres requires strict adherence to standardized sampling procedure and NMR parameters. 

Fig. 3-4. (A) Changes in chemical shift of protons of cyclophane -CH - groups between bipyridinium and phenyl in H NMR spectra of 3 as a function of (R)-DOPA concentration (a) 0, (b) 0.111, and (c) 0.272 mol (B) Change in chemical shift plotted against the analytical concentration of (R)- and (5)-DOPA. The solid line is calculated for 1 1 host - guest complexation. (Reprinted with permission from ref. [79]. Copyright 1998, American Chemical Society.)...

For illite and kaolinite with decreasing solution concentration (Figure 5) there are two important changes. The relative intensity for inner sphere complexes increases, and the chemical shifts become substantially less positive or more negative due to the reduced Cs/water ratio, especially for the outer sphere complexes. Washing with DI water removes most of the Cs in outer sphere complexes and causes spectral changes parallel to those caused by decreasing solution concentration (data not shown). 

P]0 is the total molar concentration of the protein and [L]0 is the total molar concentration of ligand. X is the molar concentration of the bound species determined according to the chemical shift change ... 

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