Coupling-correlated spectra

The X,"Y correlation techniques described so far do not allow to determine the number of magnetically equivalent detected "X nuclei. Such information is frequently of importance for structure elucidation for example, it would permit to determine the number of phosphine ligands in a metal complex from a P-detected phosphorus-metal shift correlation. A remedy to this problem has been found by recording fully coupled correlation spectra in which the number of X spins can be derived from the multiplet structure in FI. This is easily achieved in both HMQC and HSQC pulse schemes by omitting the refocusing 180° pulse during ti, and various correlations have been performed in this For an interpretation of the results,... 

Fig. 28 C, H correlation spectrum for compound 1 (set for long-range coupling, 400 MHz, 5% in CDC13, measurement time 18 min, inverse detection)... 

The technique can be applied to both spinning and static samples, taking advantage of the fact that chemical shifts are proportional to the Larmor frequency while second-order quadrupole couplings are inversely proportional to the Larmor frequency (9). Thus, the spectrum of the CT can be seen as the projection along a specific angle a of a 2D chemical shift/second-order quadrupolar correlation spectrum. The angle a. for a spectrum acquired at u>o is defined as... 

Fig. 15. The stereoisomeric thiourea-platinum complexes shown in (a) display large 4/ptjC couplings for the a-carbon atom in that branch of the X.X-dialkyl substituent which is in a favourable W configuration with respect to the metal atom (thick bonds). The assignment of the (fortuitously overlapping) H and 195Pt NMR signals of all three stereoisomers was feasible by means of indirect detection of these couplings from a XH detected H/(13C)/195Pt correlation spectrum which is shown in (b) together with the one-dimensional and 195Pt NMR projections. Reproduced from Ref. 44 by permission of John Wiley Sons.

The partial long range C-H chemical shift correlation spectrum presented in figure 3 shows signals from the aliphatic side chains of the trimeric compound. This 2-D NMR experiment provides information about the H-C connection 2 to 3 bonds away from the carbon (coupling constants less than 20 Hz). For example, Ha shows... 

An important point about quaternary carbons requires comment. Until now, we have had no direct correlations for carbons without protons, nor have we been able to see through heteroatoms such as oxygen, nitrogen, sulfur, etc. Both the two- and three-bond coupling correlations of HMBC provide us with both types of critical information. For example, C-4 of caryophyllene oxide at 59.1 ppm has no attached protons, and so far it has only appeared in the l3C spectrum of the compound, and we know that it is quaternary... 

D shift-correlated spectrum. When the Ft axis corresponds to a coupling constant scale (e.g., H- H or H- - C), we generate a 2D 7-resolved spectrum. 

Figure 27. Lower, three-dimensional presentation of the spectral region, 0.5-6.0 ppm of a 360 MHz spin-echo-correlated H-NMR spectrum of BPTI. The chemical shift, 5, on the horizontal axis corresponds to that in conventional, one-dimensional spectra. Ad on the vertical axis stands for the difference frequencies between correlated nuclei. Cross-peaks between J-coupled nuclei are at 0.5 Ad. The solvent singlet resonance is at 4.35 ppm. Upper, contour plot of the same spin-echo-correlated spectrum of the inhibitor.

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