Undecoupled

Fig.3a,b Homodecoupling experiment on compound 1 (at 200 MHz), a Undecoupled methylene and methyl signals b signals after irradiation of the methyl group... 

Undecoupled spectra are rarely obtained since Jch coupling constants... 

Complete 13C n.m.r. data have been recorded for echinulin, the neoechinulins, the cryptoechinulins,33 and isoechinulins A—C,34 and complete assignments have been made. The (Z) configuration about the 8,9 double-bond in isoechinulin B was deduced from the coupling constant of the proton at C-8 in the proton-undecoupled spectrum, and is the same as that deduced for neochinulin A (40).34... 

The carbon signals of selenolo[2,3-c]thiophene (19) were assigned from the undecoupled spectrum and the values of chemical shifts and coupling constants T(C,H) were compared with C-4 and C-6 substituted derivatives <81IZV1285>. 

Figure 2. Undecoupled 13C NMR Fourier transform spectra of Co(III)-transferrin-CO3. (A) 13CO -labeled transferrin (B) no label (C) after adding H13COf to the preparation used for A. The line at 104 ppm is caused by 13C03 specifically bound to transferrin and is 14 Hz wide. The linewidth of free H13C03 at 96 ppm is 7 Hz (32).

The simulated INEPT 13C spectrum of 2-chlorobutane is shown in Figure 12.14. By comparing this to the corresponding undecoupled 13C spectrum (Figure 8.11), note how each multiplet is both intensified and divided at its center into positive and negative lines that no longer show the same relative intensities as in the undisturbed multiplet. The appearance of these multiplets is quite similar to the multiplet effect CIDNP spectra we saw in Section 11.9, and as was true there, the middle leg of a multiplet with an odd number of lines vanishes. 

FIG. 20. Two-dimensional resolved C spectrum of n-hexane recorded with the technique indicated in Fig. 19. Twenty-two experiments were co-added for each of the 64 values of t, between 0 and 35 ms. Resolution is severely limited by the 64 x 64 data matrix used to represent the two-dimensional Fourier transform. The absolute values of the Fourier coefficients are plotted. The undecoupled one-dimensional spectrum is indicated along the oi, axis, and the proton-decoupled spectrum is shown along the ujj axis. From ref. 157. 

The exploitation of cross-correlation effects in high magnetic fields has introduced a new form of NMR spectroscopy called transverse relaxation-optimised spectroscopy or TROSY. The cross-correlation of the optimised dipole-dipole (DD) and chemical shift anisotropy (CSA) relaxation mechanisms leads to differential transverse relaxation rates for the two components of the l5N- H doublet in undecoupled spectra of l5N-labelled proteins. For one component, DD and CSA relaxation constructively add to produce very efficient relaxation, leading to a broad line, whereas for the other component, the two relaxation mechanisms constructively interfere, leading to a narrow line when the two mechanisms are nearly equal. There is no optimum field where DD and CSA relaxation are equal for all amide bonds, because DD relaxation between the amide protons and other nearby protons differs for each residue.72 Clearly, the overall effectiveness of TROSY is optimized when the non-exchangeable protons in the macromolecule... 

An interesting method, which employs the PFT technique, has been proposed (80) for extracting 15N satellites from the proton spectra of compounds with 15N at natural-abundance in order to obtain the values of 15N- H coupling constants. The method consists of alternatively generating I5N-decoupled and 15N-undecoupled proton free induction decays which are subsequently subtracted from one another. Thus, at least theoretically, the accumulated free induction decay should give, after a Fourier transformation, only the satellite peaks. This technique... 

A recently developed technique of subtracting, 3N-decoupled from 13N-undecoupled proton spectra (81, 221) permits the measurement of larger, 3N-H couplings, mostly /(N-H), from the very weak 13N satellites observed in proton spectra. A number of /(N-H) values for amides have been measured by this method (Table XXXII, note b). 

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