NOEs in a multispin system

The previous section considered the NOE for the hypothetical case of a two-spin system in which the spins relax exclusively via mutual dipole-dipole relaxation. In progressing to consider more realistic multispin systems, two key issues will be 

NOEs onto quadrupolar nuclei are very rarely observed (the exception being Li as mentioned in Table 8.2). 

This statement must still be treated with some caution, however, as illustrated in Fig. 8.12 in which 7b A is considered for a three-spin system. A, B and C, where the B-C distance is varied (and ignoring any direct AC interaction). When C is distant from B it 

A further important feature emerges if we consider the results from saturating B and studying the effect on A, j/aIB (Fig. 8.13) and compare these with those of Eig. 8.12. With C distant from B, the relaxation of A is essentially completely dominated by B, so it experiences almost the maximum enhancement. As C approaches B, the enhancement on A is reduced only a little since its relaxation is still dominated by the much closer spin B. Thus, in general, steady-state NOEs between spins are not symmetrical, that is, /a B 7b A, because the neighbours surrounding A are unlikely to match those surrounding B in both number and proximity. 

The influences of the various factors described above are further illustrated by reference to the four-spin system of Fig. 8.17, which lays testament to the need to consider all neighbouring spin interactions to correctly interpret steady-state NOE data. Clearly, the NOE enhancements between B and C differ dramatically despite these effects arising over 

The previous section considered the NOE for the hypothetical case of a two-spin system in which the spins relax exclusively via mutual dipole-dipole relaxation. In progressing to consider more realistic multispin systems two key issues will be addressed how the presence of other spins affects the magnitudes of steady-state NOEs and how these reintroduce distance dependence to the NOE. These considerations lead to the conclusion that steady-state NOE measurements must be used in a comparative way to provide structural data, and that they do not generally provide estimates of intemuclear distances per se. 

This statement must still be treated with some caution, however, as illustrated in Fig. 8.12 in which t)b A) is considered for a three-spin system. A, B and C, where the B-C distance is varied (and ignoring any direct A-C interaction). When C is distant from B it has little influence on its relaxation, allowing the A-B cross-relaxation to dominate, producing close to the maximum NOE. When both A and C are equidistant from B they play an equal role in relaxing B and the NOE is thus half the maximum possible value. As C becomes very much closer to B than is A, it now dominates B-spin relaxation and A-B 

The magnitudes of negative three-spin enhancements are usually rather small since they rely on the build-up of a sizeable NOE on a neighbouring spin suitable for relaying. Similarly, they also tend to be slow to develop and show 

Complications arising from the transfer of saturation from one resonance to another by means of chemical exchange (Fig. 8.19) also differ in the 

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