CH Multiplicities

If the CH balance given by the CH multiplicities differs from the number of H atoms in the molecular formula, then the additional H atoms are bonded to heteroatoms. The NMR spectra in Fig. 2.5 show, for example, for isopinocampheol (2), CxoHigO, a quaternary C atom (C), four CH units (C4H4), two CH2 units (C2H4) and three CHj groups (C3//9). In the CH balance, CioHn, one H is missing when compared with the molecular formula, CxqH//) to conclude, the compound contains one OH group. 

The multiplicities of signals due to Jqh coupling (splitting occurs due to CH coupling across one bond) indicates the bonding mode of the C atoms, whether quaternary (R4C, singlet S), tertiary doublet D), secondary (R2CH2, triplet T) or primary (RCHj, quartet Q). 

2C + 3CH + 2CH2 + 3CH3 = C2 +C3H3 +C2H4 + CiHg = CioHm 

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Figure 2.4. CH multiplicities of a-pinene (1) [hexadeuterioacetone, 25 C, 50 MHz], (a) h broadband decoupled NMR spectrum (b) DEPT subspectrum of CH (c) DEPT subspectrum of all C atoms which are bonded to H (CH and CH3 positive, CH2 negative) (d) an expansion of a section of (c). Signals from two quaternary C atoms, three CH units, two CH2 units and three CH3 units can be seen...

Figure 2.4 illustrates the usefulness of CH multiplicities for the purpose of structure elucidation. The addition of all C, CH, CH and C//j units leads to a part formula CJiy,... 

C NMR spectra 14 were recorded of cw-l,2-dimethylcyclohexane at the temperatures given the DEPT experiment at 223 K was also recorded in order to distinguish the CH multiplicities (CH and CHs positive, C// negative). Which assignments of resonances and what thermodynamic data can be deduced from these spectra ... 

Part structure A is recognised to be a 2,5-disubstituted cyclohexa-1,3-diene on the basis of its chemical shift values. The ethyl group is one substituent, the other is a carboxy function judging by the chemical shift value of 8c = 174.1. The CH multiplicities which follow from the DEPT subspectra, 2C, 4CH, 5CH2 and CHj, lead to the CH part formula C2 + C4H4 + CsHw + CH3 = C12///7. Comparison with the given molecular formula, Ci2/7j 03, indicates an OH group. Since... 

Here it is possible to consider how the starting materials may react and to check the result with the help of the spectra. Another approach would start by tabulating the C shifts, CH multiplicities... 

The correlation signals of the INADEQUATE experiment directly build up the ring skeleton A of the compound. Elere characteristic C shifts (5c = 123.1, 137.6 148.9, 109.1) establish the existence and position of two double bonds and of one tetrahedral C-0 single bond (5c = 70.5). DEPT spectra for the analysis of the CH multiplicities become unnecessary, because the INADEQUATE plot itself gives the number of CC bonds that radiate from each C atom. 

The CH COLOC diagram shows correlation signals for the methyl protons which are particularly clear (Table 51.3). Interpretation of these completes the assignments shown in formula D by reference to those CH multiplicities which have already been established (Table 51.1). 

The practical use and the advantage of proton off-resonance decoupling - less multiplet overcrowding and more signal noise relative to coupled spectra - is illustrated in Fig. 2.47. for a triterpene derivative in comparison to modern and more accurate methods for determination of CH multiplicity. An unequivocal assignment of the number of directly attached hydrogens may be possible for all carbons. 

Fig. 2.42. I3C NMR spectra of D-camphor in tetradeuteriomelhanol at 15.08 MHz (a). /-modulation of aliphatic carbon signals depending on the decoupling delay z, a verification of Fig. 2.41 (b) proton broadband decoupled spectrum (c-e). /-modulated spin-echo experiments with z = 4, 6, and 8 ms for CH multiplicity analysis (f-g) spectra with off-resonance (0 and gated decoupling of protons (g) for comparison.

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