Correlation charts carbon-13 chemical shifts

Figure 22 shows a chemical-shift correlation chart for C, illustrating the 5c ranges for functional groups frequently found in polymers [2,16] all shifts are cited relative to the usual reference standard, tetramethylsilane (TMS), at 0 ppm. Aliphatic carbons bonded to silicon (C - Si) also resonate near this point. C-C aliphatic carbons appear at somewhat higher 8c s, between 5 and 50 ppm. As a rule of thumb, 5c increases as the number of protons decreases ... 

An important parameter derived from carbon-13 spectra is the chanical shift. The correlation chart in Figure 6.1 shows typical chanical shifts, listed in parts per million (ppm) from tetramethylsilane (TMS) the carbons of the methyl groups of TMS (not the hydrogens) are used for reference. Approximate chemical shift ranges for selected types of carbon are also given in Table 6.1. Notice... 

The 13C chemical shift is diagnostic of different functional groups. An abbreviated shift scale is shown in Figure 9. This shift scale is useful for spectral interpretation and is reminiscent of similar structure-spectral correlation charts used in infrared (IR) spectroscopy. Yet NMR provides much more detailed structural information than IR. It has been known since the 1960s that the shifts of alkanes are approximately additive with increasing number of substituents. Similar empirical additivity rules have been found for alkyl carbons with different functional groups and other additivity rules have been proposed for olefinic, aromatic, and carbonyl carbons. These linear additivity rules are a distinct feature of NMR, and they simplify spectral interpretation. 

Over 100 SbCls affinity values are collected in Table 2.2. The sample of Lewis bases contains mainly oxygen bases and, among these, mainly carbonyl compounds. Only nine nitrogen bases (seven nitriles, one pyridine, one amine) have been studied. The scale lacks second-row bases, carbon bases and halogen bases. We have not reported the values estimated indirectly from correlations between DN and solvent basicity-dependent properties, such as nuclear magnetic resonance chemical shifts of the Na nucleus [27] or of the chloroform proton [28], These values would be valid only if there was no separation into families of bases in the correlation chart, which is rarely found. As shown in this book, family-dependent correlations between basicity-dependent properties are generally the norm. 

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