Shift , chemical temperature

Ion Chemical Shift Temperature, °C Ion Chemical Shift Temperature, °C... 

Table 13 Chemical shift temperature dependencies of some Yb(II) compounds...

Table 3. Chemical shift temperature dependence and relaxation times in TmDOTP (adapted from refs 37 and 38)...

Chang FC, Swenson RP(1999)The midpoint potentials for the oxidized-semiquinone couple for Gly57 mutants of the Clostridium beijerinckii flavodoxin correlate with changes in the hydrogen-bonding interaction with the proton on N(5) of the reduced flavin mononucleotide cofactor as measured by NMR chemical shift temperature dependencies. Biochemistry 38 7168 7176... 

Labelling with alone can be sufficient to overcome spectral overlap for proteins of up to 20 kDa and, for these proteins, virtually complete resolution can often be achieved for the backbone amide groups in 2D-iH-i HSQC experiments. These experiments are very robust and can be used to determine amide proton exchange rates or chemical shift temperature coefficients. For high protein concentrations H-i HSQC data sets can be acquired rapidly typically within 10 min for a 2 mM sample or 2-3 hr for a 0.2 mM protein sample. Consequently, this experiment has become the mainstay of NMR approaches to monitor the binding of ligands to i -labelled proteins through titration experiments. 

Figure B2.4.1 illustrates this type of behaviour. If there is no rotation about the bond joining the N, N -dimethyl group to the ring, the proton NMR signals of the two methyl groups will have different chemical shifts. If the rotation were very fast, then the two methyl enviromnents would be exchanged very quickly and only a single, average, methyl peak would appear in the proton NMR spectrum. Between these two extremes, spectra like those in figure B2.4.1 are observed. At low temperature, when the rate is slow, two...

Figure B2.4.3. Proton NMR spectrum of the aldehyde proton in N-labelled fonnainide. This proton has couplings of 1.76 Hz and 13.55 Hz to the two amino protons, and a couplmg of 15.0 Hz to the nucleus. The outer lines in die spectrum remain sharp, since they represent the sum of the couplings, which is unaffected by the exchange. The iimer lines of the multiplet broaden and coalesce, as in figure B2.4.1. The other peaks in the 303 K spectrum are due to the NH2 protons, whose chemical shifts are even more temperature dependent than that of the aldehyde proton.

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. 

The mean chemical shifts of A- unsubstituted pyrazoles have been used to determine the tautomeric equilibrium constant, but the method often leads to erroneous conclusions (76AHC(Sl)l) unless the equilibrium has been slowed down sufficiently to observe the signals of individual tautomers (Section 4.04.1.5.1). When acetone is used as solvent it is necessary to bear in mind the possibility (depending on the acidity of the pyrazole and the temperature) of observing the signals of the 1 1 adduct (55) whose formation is thermodynamically favoured by lowering the solution temperature (79MI40407). A similar phenomenon is observed when SO2 is used as solvent. 

The temperature-dependent NMR spectrum of the ion can be analyzed to show that there is a barrier (8.4 kcal/mol) for the ring flip that interchanges the two hydrogens of the methylene group. The C-NMR chemical shift is also compatible with the homoaromatic structure. MO calculations are successful in reproducing the structural and spectroscopic characteristics of the cation and are consistent with a homoaromatic structure. ... 

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. 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

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