Heteronuclei coupling

D spectra are in principle possible for heteronuclei coupled by either dipolar or scalar interactions. However, the magnetic moments of heteronuclei are sizably smaller than that of the proton, and since cross relaxation depends on the square of the magnetic moment it appears that this is a serious limitation for the observation of NOESY or ROESY cross peaks. However, as already discussed, in scalar-coupled systems the relevant coherences build up with sin(nJ/jt). Since Jjj in directly bound 13C- H and l5N- H moieties is of the order of 102 Hz, as opposed to about 10 Hz between proton pairs, it is conceivable that scalar correlation experiments are successful. Heterocorrelated spectra have the advantage of allowing one to detect signals of protons attached to carbons or nitrogens when they are within a crowded envelope. 

Coupling constants are routinely used to determine the side-chain conformation of amino acids in peptides and proteins. Whereas proteins nowadays are almost exclusively studied as C- and N-labeled isotopomers, peptides usually have these isotopes in natural abundance, i.e. the magnetically active heteronuclei are highly diluted. Most amino acids contain a methylene group at the ji-position for which the X angle is determined by the conformation of the Ca—Cp bond. Two vicinal Jhh coupling constants can be measured Ha to and H to Usually... 

Kurz, M., Schmieder, P., Kessler, H. HETLOC, an efficient method for determining heteronuclear long-range couplings with heteronuclei in natural abundance. Angew. Chem. Int. Ed. 1991, 30,1329-1331. 

The detection of heteronuclear long-range spin-spin interactions and the measurement of the corresponding coupling constants serve to assign the signals of heteronuclei, especially of quaternary centers and yield most important structural informations, i.e., connectivities between molecular frag-... 

Heteronudear shift correlation spectroscopy is a 2D technique that can be used to determine the connectivity of H and C nuclei (or other heteronuclei), formally bonded together through one or more chemical bonds. The corresponding experiments make use of either the large or the smaller long-range "Jv, couplings for polarization transfer. 

Couplings between carbon and many heteronuclei, e.g. the JCF data shown in Table 3.14, do not depend simply on carbon s character as is the case for JcH and Jcc. Nevertheless, electron withdrawal at the coupling carbon often increases Jcx. This trend is obvious when the JCF values of mono- di-, and trifluoroethanol, or those of mono- and trifluoroacetic acid are compared (Table 3.14). 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

Spin-coupling interactions are not limited to hydrogens. Any magnetic (/ 0) nuclei within three bonds can split a signal. Let us divide our discussion into heteronuclei with I =, and then all others. 

Similar to the HSQC experiment, multiple quantum coherences can be used to correlate protons with Q-coupled heteronuclei. The information content of the Heteronuclear Multiple Quantum Correlation (HMQC) experiment (56) is equivalent to the HSQC, but the sensitivity can be improved in certain cases. Additionally, by proper tuning of delays and phase cycling, it can be transformed into the heteronuclear multiple bond correlation experiment (57-59), which results in correlations between J- and J-coupled nuclei. 

Planar zero-quantum coupling tensors are characteristic for most heteronuclear Hartmann-Hahn experiments (see Section XI). Here the effective coupling constant between two heteronuclei i and j is scaled by s, <1/2 [see Eq. (115)]. Planar /-coupling tensors (with... 

The main problem for the proton detection of low natural abundance heteronuclei turns out to be the suppression of the undesired proton resonances from H nuclei not coupled to the magnetically active heteronucleus of interest. Basically, the central intense resonance is of course much more difficult to suppress than the H- Sn (and H-" Sn) satellites. However, the suppression of undesired signals is far less problematic in H-" Sn than in HMQC... 

---