Shift , chemical of functional groups

The 1 0 NMR chemical shifts of functional groups other than carbonyl ones, including aromatic nitro groups [20], pyridine N-oxides [21,22], phenols [23], and anisoles [24] have also been found to be predictably quite sensitive to electronic effects. Interestingly, the NMR chemical shifts of aryl sulfones [25,26] and sulfoxides [25,27] are essentially insensitive to substituent effects. 

The chemical shift of functional groups is dependent on its ionisation state. Thus, P chemical shifts of phosphates [23], chemical shifts of carboxyl groups [24], and... 

In Tables 21.4-21.6, we showed the calculated chemical shifts of functional groups for the polymer models with the experimental ones for polymers in solution. The calculated results are also in good accordance with experimental values in absolute average deviations of 4.42 ppm. 

Table 9.9 A summary of chemical shifts of functional groups found in lipid molecules ...

NMR spectroscopy is very useful for identifying organic compounds, provided that they can be obtained in a reasonably pure state. Kdnig has published a table of chemical shifts of functional groups found in common surfactants (39). This allows use of proton magnetic resonance to identify components of commercial products, where the range of possible structures is limited. Carminati and coworkers recommend the use of C NMR for the identification of unknown surfactants, both alone and in formulated products. With experience, not only the surfactants, but other components of products can be identified (40). 

The chemical shift is the cornerstone of the chemical applications of NMR. As we noted in Chapter 1, this accidental discovery converted a technique designed initially for probing the structure of the atomic nucleus into one that can provide detailed information about the structure and dynamics of molecules. In this chapter we examine the theoretical underpinning of the chemical shift, explore empirical correlations between chemical shifts and functional groups in organic molecules, and describe simple physical models that can help us to understand and predict chemical shifts. 

A very useful source on various NMR techniques and chemical shifts of functionalized carbon atoms that contains tabulated data on monosaccharides is Reference [23]. Specifically correlating functional groups with chemical shifts of protons and the associated carbon is an important tool for elucidating the stmcture of the monosaccharide moiety of many complex oligosaccharides. Since monosaccharides contain one or more chiral centers, absolute configuration must be known in order to predict the conformational shape of the molecules. The outcome of many stereoselective functionalizations of free and partially functionalized monosaccharides depends on conformation. 

The assignment of structure on the basis of NMR spectra requires knowledge of the relationship between chemical shifts and functional groups. Normally, both proton and carbon spectra are recorded and analyzed. This section considers the relationship between proton resonances and structure. Figure 3-9 summarizes the resonance ranges for common proton functionalities. 

The charge-potential model was used to predict the chemical shifts for functional groups in a number of model compounds containing primarily carbon, fluorine, and hydrogen. The molecular structure examined included aliphatics, olefins, aromatics, and several ketones. 

CH2CH2PPh2 (B) pendant groups. Table 25.2 collects chemical shifts of functionalized polysiloxanes of the type, X-(S)-Y. 

Chemical shifts of methyl groups bonded to functional groups... 

Detailed correlations of chemical shifts with functional groups may be found in articles dealing with particular nuclei. 

For PET/HDPE blends compatibilized by addition of functionalized polyolefins, the crystallization temperature of PET was found to be shifted to temperatures lower than those observed for plain PET and the non-compatibilized blends [99]. Moreover, the crystallinity degree of the polymers depended on the type and concentration of compatibifizer (Fig. 10.25). The crystallinity of both PET and HDPE phases was markedly depressed upon addition of HDPE-g-MA or E-GMA due to the effect of the chemical reactions of functional groups with PET and of the miscibility of the functionalized polyolefins with the HDPE phase. In fact, the crystallization of PET molecules near the interface is affected by the occurrence of interchain grafting reactions, which result in a reduced mobility of the chains and thus depression of crystallization temperature. With decreasing the size of dispersed particles the effect of compatibifizer on the crystallization of PET becomes more pronounced. 

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.)...

Several trends have emerged in the extensive carbon-13 NMR spectroscopy data that have been accumulated for sulfones and sulfoxides. Based on many studies of cyclic systems—particularly five- and six-membered ring sulfur compounds—these trends were shown to generally apply equally to both the cyclic and acyclic systems . Thus (a) oxidation of a sulfide to a sulfone results in a 20-25 ppm downfield chemical shift for sp -hybridized a-carbon atoms and 4-9 ppm upfield shift for / -carbons , and (b) there is very little difference between the chemical shifts of a-carbon atoms of sulfones and sulfoxides despite the difference in the inductive effects of these two functional groups . A difference is observed, however, in the H chemical shift of related cyclic sulfoxides and sulfones . 

The impact of a carboxylic acid function upon the chemical shift of a CF2H group is almost indistinguishable from the impact of a ketone, whereas secondary CF2 groups next to an acid or ester function are slightly deshielded relative to those next to a ketone or aldehyde (Scheme 4.35). 

The impact of combinations of functional groups on CF2 chemical shifts depends on how they are arranged. If they are consecutive, then the closest one largely determines the chemical shift (Scheme 4.40). 

As one can see from the examples given (Scheme 5.30), the primary influence upon the chemical shift of a CF3 group is exerted by that functional group closest to the CF3 group. 

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

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