Homonuclear /-resolved experiment

The simplest 2D NMR experiment, which offers enhanced resolution, is the homonuclear /-resolved experiment. ° The experiment is basically a spin-echo sequence, where the echo period is incremented. During this ti period the chemical shift evolution is refocused with a 180° pulse, but homonuclear / couplings are evolved. If we consider a proton H coupled to proton(s) H, the density function of the observed proton magnetization is modulated by these couplings accordingly... 

Fig. 3.27 2D spectrum (tilted) of peracetylated glucose from a homonuclear 2D J-Resolved experiment. 

While the (heteronuclear and homonuclear) residual dipolar couplings have to be eliminated in such J-resolved experiments, they are the main source of information in the WISE experiment described in the next section. For the investigation of viscoelastic materials the experiment thus must be performed under static conditions or under slow MAS. 

HOM2DJ, 2D homonuclear 7-resolved experiment HSC, 2D heteronuclear shift correlation Hz, unit of frequency (cycles per second)... 

Figure 7.7. The homonuclear J-resolved experiment. The 180° pulse at the midpoint of t refocuses proton shifts but not homonuclear couplings, so only these appear in fi.

Figure 7.8. A multiplet from the homonuclear J-resolved experiment showing the characteristic tilting brought about by the presence of proton couplings in both f and f2.

Nuzillard demonstrated that application of LP to the first dimension of two-dimensional homonuclear J-resolved experiments usually displayed in magnitude mode can be made sharper and present a higher S/N ratio. 

The homonuclear version of the /-resolved experiment [17] is most frequently applied in proton spectroscopy, although, again, it is suitable for any abundant nuclide. In principle, the separation of 6 and / should reveal proton multiplets in/j free from overlap and thus available for analysis, and singlets in/2 at the corresponding chemical shifts, such that the/2 projection... 

J Resolved spectroscopy. In the J-resolved experiment, the /i axis is the normal one-dimensional chemical shift frequency axis and /2 represents the frequency displacements of multiplet peaks which result from spin-spin coupling. The effect is to separate chemical shift effects and spin-spin coupling effects into different dimensions, alleviating peak overlap considerably and making it easier to establish J coupling constants of complicated spin systems. Furthermore, by taking a 45° projection of the 2-D spectrum, a spectrum is obtained from which all homonuclear spin-spin coupling is removed. 

In another report by the same group, a high-resolution two-dimensional H double-quantum (DQ) homonuclear recoupling experiments, combined with SAM homonuclear decoupling is presented. The experiment affords highly resolved and clean H DQ-SQ 2D spectra at very-fast MAS rates (35 kHz). It is shown that the new method is well suited to probe H- H... 

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