Homonuclear splitting pattern

Unless a molecule is artificially enriched by synthesis, the probability of finding two atoms in the same molecule is low. The probability of finding two atoms adjacent to each other in the same molecule is even lower. Therefore, we rarely observe homonuclear (carbon-carbon) spin-spin splitting patterns where the interaction occurs between two atoms. However, the spins of protons attached directly to atoms do interact with the spin of carbon and cause the carbon signal to... 

In general, proton-decoupled Si, Sn, and Pb spectra are simple, and consist of single lines for each type of chemical environment for the nucleus in question in the relevant sample. Satellite resonances can be more complicated, as mentioned above, but they can usually be readily distinguished from the main peaks, even for Sn. When or P is present, first-order splitting patterns are normal. Proton-couoled spectra are not often studied, so that second-order Si, Sn, or Pb spectra have rarely been treated (see, however, 25). Enrichment with silicon-29 can lead to second-order features for symmetrical species, and an example (26) is shown in Figure 2. In such cases homonuclear decoupling is a desirable technique (22,24). 

Rather than using HMQC, it is preferable to use HSQC-based pulse sequences. In the former, cross-peak patterns are split by /pp homonuclear couplings in both the /i and/2 dimensions (Vpp = 100-300 Hz, 7pp = 5-15 Hz, " /pp = 10-90 Hz, and /pF = 2-30 Hz, [16,17]). These complex cross-peak patterns greatly diminish signal intensities and lead to broad cross-peak patterns which become difficult to resolve from one another. In HSQC-based experiments, the splittings from 7pp couplings are eliminated from the /i dimension leading to sharper and more intense cross-peak patterns. 

For the determination of homonuclear or hetero-nuclear three-bond coupling constants, three fundamentally different approaches are used (1) direct measurements of splittings caused by /-couplings, (2) the so-called E.COSY-signal patterns in which the splittings can be measured as shift differences of signals and (3) special experiments which lead to modulation of signal intensities via / (Table 3). A frequent requirement for the latter is a N-enriched sample, e.g. for the /-modulated ( N, iH)-COSY experiment or the HNHA experiment. Whereas labelled compounds are routinely used for proteins and nucleic acids they are expensive for peptides and therefore rarely used. 

---