Peak areas in nmr

Palmitic acid, 354 Paramagnetism. 17 Pararosaniline, 428 Pauli exclusion principle, 14 Peak areas in nmr, 241 Peak base in nmr, 248 Peak-splitting in nmr. 242 ... 

Compound 22 can be conveniently prepared in multigram quantities and has been found to be useful for assessing the enantiomeric purity of 1,2-glycols. Because the ketal carbon represents a new chiral center, the formation of four diastereomers is possible. However, the diastereomeric pair 23a and 23b (or 23c and 23d) shows 1 1 peak height in 13C NMR or equal peak areas in HPLC the diastereomer composition measured by the ratio of 23a to 23b or 23c to 23d reflects the enantiomer composition of the original 1,2-glycol. 

Problem 13.14 At room temperature, the fluorine nmr spectrum of CF2BrCBr2CN (3,3-difluoro-2,2,3-tribromopropanenitrile) shows a single sharp peak. As the temperature is lowered this peak broadens and, at -98 is split into two doublets (equal spacing) and a singlet. The combined area of the doublets is considerably larger than—more than twice as large as—the area of the singlet. Interpret each spectrum, and account for the relative peak areas in the low-temperature spectrum. 

Problem 2.14 Determination of the individual peak areas in the a-methyl region of the NMR spectra of poly(methyl methacrylate) shown in Fig. 2.17 yielded the following values [18] ... 

For CW spectra and FT spectra of H nuclei the area under each absorption peak is proportional to the number of H nuclei present in it this is not true for nuclei however. Thus knowledge of the ratio of the peak areas in a pmr spectrum helps in the identification of the chemical structure responsible for each of the peaks. Because of this, all nmr spectrome-... 

The residual vinyl groups of all labelled samples were analysed quantitatively from peak areas in the NMR spectra by two methods. First, the area of the 137 ppm vinyl peak was compared with the area of all of the aromatic carbon signals in the spectrum due to styrene and divinylbenzene carbons in natural abundance. Second, the area of the 137 ppm peak was compared with the area of all of the aliphatic carbon signals in the spectrum, which includes signals from polymerised labelled carbons of the DVB and from all other aliphatic carbons at natural abundance. It was assumed that all carbon atoms in the sample are equally detectable in each NMR spectrum (Table 9.6). All of the labelled polystyrene networks were also analysed by bromination of residual vinyl groups. 

Aerdts and co-workers [141] conclude that the MMA-centred triads and the styrene-centred triads can be directly calculated from the a-CHj and Cj peak areas in the carbon NMR spectra. This is contrary to the proton NMR spectra of the SMMA copolymers, where the peak splitting of the methoxy protons is so complicated that the peak areas cannot be translated directly to sequences. It is only possible to... 

Percec et al7 used Method 1 to determine of polyether sulfones (PESs) such as 34. Their polymers did not yield H ID-NMR spectra with resolved resonances from the chain end. They incorporated a 3,5-dinitrobenzoyl (DNB) tag at the polymer chain ends by producing the ester derivative, 35, from reaction of the polymer with 3,5-dinitrobenzoyl chloride. Incorporation of nitro groups into aromatic rings shifts the ortho and para protons far downfield compared to other typical aromatic proton resonances such as those in the PESs. They calculated DP from ratio of polyether sulfone methyl (Apes) to 3,5-dinitrobenzoyl (Adnb) peak areas in the H NMR spectrum, corrected for the number of protons in each group, as shown in eqn [46]. Note that the sulfone and bisphenol A groups together were treated as the repeat unit in this analysis. M can then be calculated using eqn [36], with a correction factor to account for the MW of the chain-end structures. This method assumes that all chain ends react with the DNB tag an assumption that is not always valid. 

The simplest use of an NMR spectnim, as with many other branches of spectroscopy, is for quantitative analysis. Furthennore, in NMR all nuclei of a given type have the same transition probability, so that their resonances may be readily compared. The area underneath each isolated peak in an NMR spectnim is proportional to the number of nuclei giving rise to that peak alone. It may be measured to 1% accuracy by digital integration of the NMR spectnim, followed by comparison with the area of a peak from an added standard. 

Problem 13.18 How many peaks would you expect in the H NMR spectrum of 1,4-dimethyl-benzene (pom-xylene, or p-xylene) What ratio of peak areas would you expect on integration of the spectrum Refer to Table 13.3 for approximate chemical shifts, and sketch what the spectrum would look like. (Remember from Section 2.4 that aromatic rings have two resonance forms.)... 

The Md NMR spectrum of /)-bromotoluene, shown in Figure 15.15, displays many of the features just discussed. The aromatic protons appear as two doublets at 7.02 and 7.45 8, and the benzylic methyl protons absorb as a sharp singlet at 2.29 8. Integration of the spectrum shows the expected 2 2 3 ratio of peak areas. 

Integration (Section 13.10) A technique for measuring the area under an NMR peak to determine the relative number of each kind of proton in a molecule. Integrated peak areas are superimposed over the spectrum as a "stair-step" line, with the height of each step proportional to the area underneath the peak. 

An example of determination of dimethyl sulphoxide (in solutions and ointments) was given by Kram and Turczan124. They recorded the NMR spectra at 60 MHz of the sulphoxide in methanol + water or chloroform and integrated the peaks the amount of sulphoxide was calculated from the ratio of its peak area to that of methanol. 

FIGURE 7.8 Quantitative 13C-NMR (peak areas proportional to concentration) of dithionite-reduced I ()a-13C-labeled WV-15 (structure in Scheme 7.6). 

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