Comparison of chemical shifts

Comparison of Chemical Shifts of Methyl Substituted and Unsubstituted... 

C NMR spectra are recorded for a low molecular weight atactic PP dissolved in a variety of solvents over a broad temperature range [293 - 393 K). Comparison of chemical shifts calculated via the y effect method with the observed resonances, whose relative chemical shifts are solvent independent, permits their assignment to most of the methyl heptad, methylene hexad, and methine pentad stereosequences. Agreement between observed and calculated chemical shifts requires y effects, he., upfield chemical shifts produced by a gauche arrangement of carbon atoms separated by three bonds, of ca. - 5 ppm for the methyl and methine carbons and ca. - 4 ppm for the methylene carbons. 

Comparison of Chemical Shifts of Pyrrolic" and Vinylic Protons of 1- and 2-Naphthylpyrroles (14,16) and their Vinyl Derivatives (15,17) (82KGS1351)... 

The most studied nucleus, particularly for in-vivo studies has been 31P. Naturally abundant 31P occurs in metabolites that move freely in the tissue. The most abundant of these are inorganic phosphate sugar phosphate nucleotide and nucleoside di- and triphosphates. The assignments of the signals have been discussed by Roberts (1987). The chemical shift of P in phosphates is sensitive to pH over the physiological range of 5.8 to 7.8. Estimations of intracellular pH can thus be made by comparison of chemical shifts in vivo with those in vitro (Roberts et at., 1981). Use of this method has shown that in oil palm cell suspension cultures, there is loss of pH control with sudden increases in the external pH (Fox and Ratcliffe, 1990). 

As has been shown, several solvents produce specific effects on chemical shifts, and care should be taken when assigning lines or structures on the basis of comparison of chemical shifts obtained in different solvents or when using literature data with no experimental details given. 

There are intentions to invoke NMR in assessing aromaticity by comparisons of chemical shift data associating a downfield shift with the presence of an aromatic ring current. The validity of... 

The ability to identify metabolites and biotransformed products in biological fluids during separation is a considerable challenge. The potential application of CE-NMR and CEC-NMR for analysis of metabolites in biofluids has been demonstrated [51,52], For example, CE-NMR has successfully analyzed the major metabolites of paracetamol in human urine [51]. Two major metabolites, paracetamol glucuronide and paracetamol sulfate conjugates, as well as endogenous material (hippurate) have been characterized. Comparison of chemical shifts has confirmed the presence of these compounds. The estimated amount that can be detected in this study with a S/N of 3 is —10 ng. 

A complete NMR characterization1 was performed on this purified sample, as well as on the parent free base and related sulfone degradant for comparison. Methanol was chosen as the solvent, since solubility in methanol was demonstrated in the LC method development. The parent was also soluble in methanol, which allowed for a straightforward comparison of chemical shifts. Enough sample was isolated (30 mg) to acquire a carbon spectrum, which was particularly useful to detect the quaternary carbon resonances in the proposed oxidation region. Compared to the parent carbon spectrum, significant downfield chemical shifts were observed for both quaternary carbons neighboring the sulfur entity. In addition, upheld chemical shifts were observed for the aromatic methines immediately next to these quaternaries. The NMR data therefore confirmed oxidation had occurred at the sulfur site, and hence the sample was identified as the sulfoxide of the parent structure. 

Once spectra have been obtained, it is necessary to obtain valid assignments for each resonance. As a first step, comparison of chemical shifts of spectral lines with shifts of known metabolites obtained in vitro under physiological conditions (see Fig. 10) permits tentative assignments. These tentative assignments can be checked by conventional analysis of tissue extracts, to see if the amount observed in the extract corresponds to the amount indicated by the in vivo spectrum. Another approach is to obtain spectra of extracts as a function of pH, assigning on the basis of pK [17]. These approaches become less reliable as spectra become more complex (as is often the case for in vivo (see, e.g., Ref. 18) and H-NMR [19] (Fig. 16)), or broad, overlapping lines are encountered (e.g., the monophosphate ester region of P-NMR spectra). 

Fig. 7.24 Comparison of chemical shift values for and p -biidging phosphanidenes...

Models have been proposed to correlate chemical shift data with absolute configuration [30]. In 1973, Mosher observed that derivatization of enantiomerically pure esters and amides with both enantiomers of MTPA and comparison of chemical shifts produced some interesting trends. With reference to the top structures in Figure 2.5 (L2 is smaller than L3), two experimental trends were observed ... 

Figure 7.3. Graphical comparison of chemical shifts of C-H hydrogens versus the C chemical shift of the attached carbon. [From Proton-Carbon Chemical Shift Correlations, by R. S. Macomber, Journal of Chemical Education, 68, 284-285 (1991). Reprinted with permission.]...

Solvent. For all practical purposes, NMR spectra are recorded in solution, although pure ( neat ) liquids and even gases can, in principle, also be examined. The solvents must meet certain requirements (Sec. 12.1) and a compromise must often be employed between using concentrated solutions (for high sensitivity) and dilute solutions (for measuring chemical shifts uninfluenced by solute-solute interactions). Besides the commonly used carbon tetrachloride, deuterochloroform, and D2O, a range of deuterated solvents (dimethyl sulfoxide, benzene, pyridine, acetone, dioxane) is commercially available. It must be emphasized that direct comparison of chemical shifts obtained in different solvents is invalid, as solvent-induced changes of up to... 

Comparison of Chemical Shifts of Cl, C4 and C6 Carbons of Ramie In the Solid State with Those of Low Molecular Weight Cellulose In Solution... 

Steric and conformational effects on shieldings have been investigated for a number of methyl-substituted cyclohexanones which are presumed to exist primarily in chair conformations with equatorial methyl groups. From a comparison of chemical shifts with those of cyclohexanone, a series of substituent parameters was derived for sp -hybridized carbons. Thus for 2-methylcyclohexanone a-, p-, y-, and 5-methyl induced shifts (p.p.m.) are 3.4 9.1 0.2 (C-4), —0.1 (C-6) and 0.9 p.p.m., while for 3-methylcyclohexanone the values are 7.0 8.0 (C-2), 8.3 (C-4) —1.8 and -0.9. The corresponding data from methylcyclohexane are 6.0, 9.0, 0.1, and —0.2. From these values the shieldings for cis-2,6-, cis-3,5-, and trans-2,5-dimethylcyclo-... 

The comparison of chemical shifts for N-CH - groups in tertiary and quaternary aminohydroxysiloxanes, shows a significant shift in low field for quaternary compounds. 

In conclusion, measuring aromaticity by comparison of chemical shifts as representations of ring current is possible, provided the examples are chosen carefully. The protons to be compared must be located in similar regions of the magnetic field the molecules must have similar planarity and not be subjected to strong anisotropic or charge effects. 

In Chapter 3 Howarth et al. describe an extension of the approach employed above to actually derive the conformational descriptions (RIS models) of polymers by comparison of chemical shifts calculated via the y-gauche effect method with their observed CNMR spectra, followed by iterative adjustment of the RIS models until they yield calculated Cs in agreement with observed values. 

Although we have dealt exclusively with the analysis of polymer microstructures and conformations by comparison of chemical shifts calculated via... 

Table 7 Comparison of chemical shifts of perfluoroheteroaromatic systems with the corresponding hydrocarbons analogues (shifts relative to liq. NH3)...

---