C—H Coupling

You may have noticed another characteristic of NMR spectra—all of the peaks are sin- 

Splitting due to coupling is absent for a different reason, one that has to 

What we gain from broadband decoupling in terms of a simple-looking spectrum comes at the expense of some useful information. For example, being able to see splitting corresponding to one-bond coupling would immediately tell us the number 

---

The C-H coupling constants can also be used to calculate interorbital and internuclear angles. For thiazoie, very precise results have been... 

You will find It revealing to construct a splitting diagram similar to that of Figure 13 20 for the case in which the CIS and trans H—C=C—H coupling con stants are equal Under those circumstances the four line pattern simplifies to a triplet as It should for a proton equally coupled to two vici nal protons... 

The one-bond C- H couplings are dealt with in the next section. 

Aside from C-H coupling constants (often obtained from satellites in NMR spectra) not very much information is available on the NMR spectra of small heterocycles. A recent review (B-79MI50101) devotes only seven pages to the NMR spectra of all three- and four-membered heterocyclics. 

The errors in the APT spectra may arise due to (a) variation in the relaxation times of different carbons, (b) wide variation in / h values, and (c) modulations caused by long range C- H couplings. A procedure known as ESCORT (Error Self Compensation Research by Tao Scrambling) has been developed to yield cleaner subspectra with reduced / cross-talk (Madsen et ai, 1986). This involves replacing the normal APT spectral... 

As stated earlier, since tt]/ = yff2yr and since the gyromagnetic ratio of proton is about fourfold greater than that of carbon, then if C is observed and H is irradiated (expressed as C H ), at the extreme narrowing limit Ti, = 198.8% i.e., the C signal appears with a threefold enhancement of intensity due to the nOe effect. This is a very useful feature. For instance, in noise-decoupled C spectra in which C-H couplings are removed, the C signals appear with enhanced intensities due to nOe effects. 

The structures of all products were determined by either conversion to the corresponding acetates or direct comparison with the corresponding natural p-glucosides. Identification of the -configuration of the anomeric center was easily achieved via the analysis of the C-H/C-H coupling constant. The synthetic (3-D-glucosides (30, 33, 34, 35 and 36) were identical with... 

Figure 15 shows the normal broad-band decoupled and gated decoupled spectra of compound 1 in the latter we can see the multiplets arising from C-H coupling (across one or more bonds) and C-P coupling. The rules for the number of lines in a multiplet and their intensities are the same as for protons, since 13C and 31P are both spin-Vi nuclei. 

In contrast, C-H coupling has been observed from the mixed methyl (hydrido) compounds TpR2PtMeH2 and TpR2PtMe2H.This reactivity and... 

Protonation reactions of the related dimethyl(hydrido)platinum(IV) complex TpMe2PtMe2H (58) leading to rapid methane reductive elimination have also been reported (86). This protonation was shown to occur exclusively at the pyrazole nitrogen, presumably forming a five-coordinate Pt(IV) intermediate. This species should undergo C-H coupling, and while a Pt(II) methane complex is not observed, trapping with... 

Fig. 3. Contour plot of the proton-detected local field (PDLF) spectrum of a static 5CB sample. The traces taken parallel to for each carbon site show distinct doublets that are related to individual C-H couplings. (Reproduced by permission of American Chemical Society.)...

Table 31. Solvent dependence of the P-C-H coupling constant in benzylphosphonium salts...

In addition, the C3-H coupling constant (from a gated decoupling NMR experiment) of 161.8 Hz in 106 compared with 162 Hz in allenyllithium vs 167.5 Hz in methoxyallene and 168 Hz in allene is also in agreement with an allenic structure. However, neither the C-H coupling constant nor the NMR chemical shifts distinguish between the alternatives that 106 has a nonclassical 1,3-bridged structure 108 (M = Li) or an O-coordinated allenic structure (109). Hence the 6Li, -HOESY NMR technique which can be used to detect close proximities (ca < 3.5 A) between XH and 6Li nuclei was applied. The HOESY spectrum of a-lithiomethoxyallene in THF solution (in which 106 is dimeric) is shown... 

---