Homonuclear -resolved spectroscopy

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 

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Homonuclear teclmiques such as J-resolved spectroscopy also exist for rotatmg all multiplets tlirough 90°, to resolve overlaps and also give a ID spectrum from which all homonuclear couplings have been removed [26]. 

The pulse sequence used in homonuclear 2D y-resolved spectroscopy is shown in Fig. 5.18. Let us consider a proton, A, coupled to another proton, X. The 90° pulse bends the magnetization of proton A to the y -axis. During the first half of the evolution period, the two vectors (faster... 

Figure 5.18 (A) Pulse sequence for homonuclear 2D y-resolved spectroscopy. (B) Effect of 90° H and 180° H pulses on an H doublet. (C) In the absence of coupling, the vectors are refocused by the 180° H pulse after t. This serves to remove any field inhomogeneities or chemical shift differences.

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

MREV-8 (or another homonuclear pulse sequence) must be set up very carefully. The H frequency offset must be optimized using adamantane and the pulse sequence for ID J-resolved spectroscopy in solids. 

The pulse sequence used in homonuclear 2D /-resolved spectroscopy is shown in Fig. 5.18. Let us consider a proton. A, coupled to another proton, X. The 90° pulse bends the magnetization of proton A to the y -axis. During the first half of the evolution period, the two vectors Hf (faster vector) and Hv (slower vector) of proton A precess in the x y -plane with angular velocities of 0/2 + (J/2) and 0/2 - iJ/2), respectively. The 180° H pulse results in (a) the vectors adopting mirror image positions across the x -axis so that comes to lie ahead of Hf when viewed in a clockwise fashion. However, since the 180° H pulse was not selective, it also affects... 

Figure 5.18 (A) Pulse sequence for homonuclear 2D /-resolved spectroscopy.

Homonuclear NMR Structural elucidation becomes much more difficult as spectral complexity increases. Under these circumstances, proton NMR spectroscopy has benefited considerably from the use of 2D NMR techniques. For example, homonuclear correlation spectroscopy (COSY) identifies spin-coupled pairs of nuclei as well as spin-coupled networks of nuclei in a molecule, even without prior structural information. The 2D /-resolved spectroscopy method permits even highly overlapping resonances to be resolved into readily interpretable multiplets. This enables chemical shift assignments to be made in a very straightforward manner. Both of... 

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. 

Figure 5.47. (A) Pulse sequence used in homonuclear 2D J-resolved spectroscopy. (B) Eflect of the pulse sequence on a doublet the component vectors of the doublet are represented as and Hg. The 180° pulse rotates both vectors by 180° across the x -axis (c ) simultaneously an exchange of their Larmor frequencies takes place so that the faster vector Hg lies ahead of the slower vector and they continue to diverge during the subsequent ti/2 period. At various values of ti the...

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