Chemical shifts, carbon, table

Figure 7. Correlation of corrected proton chemical shifts with carbon-13 chemical shifts. See Table 1.

Comparison of the chemical shift data (Table 1) reveals that the peak positions of a and 6 carbons of 20-crown-4 are quite different from the corresponding carbons of THF or the polymeric PTME. Small but distinct chemical shift differences were also found for macrocyclic oligomers of other ring sizes. 

Several groups have examined the 13C spectra of the aporphine alkaloids (6, 7, 38-40) examples are found in Fig. 10. The chemical shift assignments (Table X) for the aliphatic carbon atoms of rings B and C were made by comparison to laudanosine (28) (6) and to the reduced proaporphines (41). In the case of caaverine (49) a comparison with (+)-lirmidine (50) revealed the P deshielding effect of the TV-methyl group on C-5 and C-6a and the shielding of C-7 by a y effect. 

The chemical shift assignments (Table XII) for the majority of the carbon atoms were made by comparison with the appropriately substituted simple isoquinolines such as 2 and 4. Since the chemical shifts of the aliphatic... 

Before we conclude our discussion, we note that the INADEQUATE spectrum of caryophyllene oxide contains an uncommon phenomenon worth exploring. Carbons 6 and 7 of caryophyllene oxide nearly overlap in the 13C spectrum with each other and with the C-15 methyl we list their chemical shifts from Table 5.1 as 29.5 and 29.2 ppm. Because they are bonded to one another in caryophyllene oxide, they should show correlation in the INADEQUATE spectrum, but, instead of an AX system, we have an AB system with ZW/ being much less than ten. For this special case, we no longer expect two doublets whose midpoint lies on the diagonal instead, we predict that an AB multiple (see Chapter 3) should fall on the diagonal line itself. This prediction is borne out in Figure 5.19 where we find a cross peak directly below C-6 and C-7, and this cross peak intersects the diagonal line. 

Just as chemical shifts in NMR are measured relative to the protons of tetramethyl-silane, chemical shifts in NMR are measured relative to the carbons of tetra-methylsilane as the zero point of the chemical-shift scale. Table 13.3 lists typical chemical-shift ranges for some representative types of carbon atoms. 

The shift for C3, the /carbon, seems quite at odds with the expected effect of an electronegative substituent. This anomaly points up the need to consult detailed correlation tables for C chemical shifts. Such tables appear in Appendix 7 and are discussed in the next section. 

NMR spectra of 3 were recorded with a Broker AC 250 instrument at room temperature and at low temperatures (Fig. 3). The spectrum at the lowest temperature demonstrates the presence of two conformers. Coalescence occurs around 124 K. The assignment of the signals to the various carbon atoms of 3 and to equatorial and axial conformers is based on calculated chemical shift values (Table 2). This assignment is also in agreement with data for 1. The ratio of the two conformers was obtained by analysis of the spectrum at 110 K. 

This technique was used to distinguish between the two aromatic methine carbon atoms, C(4) and C(6), in sterigmatocystin (32) (Pachler et aL, 1976a). Olf-resonance proton decoupling was of no avail, owing to the small difference in the C(4) and C(6) proton chemical shifts (see Table I). The C(4) resonance appeared as a doublet of triplets J(CH) 165 Hz, J(CH) 7 Hz]. 

The chemical shifts of in natural abundance have been measured for thiazole and many derivatives (257,258). They are given in Tables 1-37 and T38. These chemical shifts are strongly dependent on the nature of the substituent CNDO/2 calculations have shown (184) that they correlate well with the ((t+tt) net charge of the atom considered. As a consequence, the order of the resonance signals is the same for protons and for carbon atoms. 

TABLE 1-38, CARBON CHEMICAL SHIFTS OF SUBSTITUE-NTS IN THIAZOLE AND SOME OF ITS DERIVATn/ES" (257)... 

Table 13 1 collects chemical shift information for protons of various types The beginning and major portion of the table concerns protons bonded to carbon Within each type methyl (CH3) protons are more shielded than methylene (CH2) protons and meth ylene protons are more shielded than methme (CH) protons These differences are small as the following two examples illustrate... 

Section 13 15 C signals are more widely separated from one another than proton sig nals and C NMR spectra are relatively easy to interpret Table 13 3 gives chemical shift values for carbon in various environments... 

Table 7.50 Estimation of Chemical Shifts of Alkane Carbons 7.102...

Table 7.52 Estimation of Chemical Shifts of Carbon Attached to a Double Bond 7.103...

Table 7.53 Carbon-13 Chemical Shifts in Substituted Benzenes 7.104...

Table 7.55 Carbon-13 Chemical Shifts of Carbonyl Group 7.106...

Table 7.60 Carbon-13 Chemical Shifts of Deuterated Solvents 7.110...

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