Undecoupled spectra

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

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... 

NMR spectra were obtained on a Brucker WP-80 spectrometer operating at 20.1 MHz. Undecoupled spectra were obtained by the gated-decoupling technique. Infrared spectra were taken on a Perkin-Elmer 283 spectrometer. 

The sensitivity limitations inherent in the observation of proton decoupled N signals become more significant if undecoupled spectra are desired. The method of selective population inversion (M 31i), which is capable of noticecibly enhancing the line intensities of coupled spectra, therefore seems very attractive. In this method a tt pulse is applied selectively to a given component (or an appropriate sub-multiplet) of the satellite proton spectrum pertaining to the molecules which contain the N isotope. Just after this selective inversion, a non selective... 

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>. 

An example of a NMR spectrum is shown in Figure 5.1. It shows the NMR spectrum of butan-l-ol, CH3-CH2 CH2 CH2-OH. The figure shows two traces. The upper is known as the decoupled spectrum, and shows one line for each carbon atom in a different environment. In this case, there are four lines, since all the four carbon atoms are in different environments, and hence have different chemical shifts. The lower trace shows the undecoupled spectrum, in which each of these peaks is split into a multiplet by the attached hydrogen atoms. 

An example of such spectra is shown in Fig. 16 (n-hexane). The traces plotted for various values of co how that parallel to the coj-axis, the full multiplet structure is retained whereas in the coj direction the completely decoupled spectrum results. The undecoupled spectrum is to be considered as a projection of the spectra for various values of onto the coj-axis. This rather involved technique is less sensitive than ordinary FTNMR. It has been used mainly in C-NMR spectroscopy. A number of applications as well as the solution to problems connected with this method have been reported -307>... 

H N multiplets and the corresponding decoupled proton lines, which are inverted with respect to the undecoupled spectrum, are simultaneously observed. Alternatively, with a moderately strong CW field,... 

JcH, Jch) ar observed, with the result that an undecoupled spectrum can be fairly complicated due to extensive overlap of split signals. spectra are therefore usually recorded using proton decoupling techniques. This is usually represented as the nucleus outside the brackets... 

Figure 3.27. Undecoupled spectrum of dioxane (A) with gated decoupling to retain NOE (B) without gated decoupling.

Figure I Spectra of i-methylpentaiw (25.15 MHz). Twenty scans were accumulated for the undecoupled spectrum (a) and the.off-resonance decoupled spectrum (b). The proton-noise-decoupled spectrum (c) is only a single scan (Reproduced by permission of L. F. Johnson).

Fig. 8. 3ip-NMR spectra of calf thymus DNA. (a) Solid DNA, cross-polarized but undecoupled spectrum at 61 MHz. (b) Solid DNA, proton-decoupled at 61 MHz. (c) Solid DNA, magic-angle spinning and proton-decoupled at 61 MHz. (d) DNA in solution at 30°C, 145 MHz (8.5 T). (e) DNA in solution at 30°C, 61 MHz (3.5 T). (f) DNA in solution at 30°C, proton-decoupled at 61 MHz (3.5 T). From Opella et al. (1981). Copyright 1981 American Chemical Society.

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. 

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