Steady state NOE in real life

The first consideration by looking at Eqs. (7.5), (7.6) and (7.7) is that NOE does not contain any structural information within the present scheme. Indeed, both / /(/) and Ji(j) contain the same r/y parameter which cancels out in /(/). In practice, the two-spins scheme is never a good description of any real system. The nucleus I, on which NOE is going to be observed, is always coupled to other nuclei. When dealing with hyperfine coupled nuclei, often the major source of relaxation is not the coupling with J but the coupling with the unpaired electrons. The expression for t)i(j) then is (Appendix VII) 

When the denominator in Eq. (7.10) is dominated by the paramagnetic contribution to nuclear relaxation (p/ = R[m the following simplified equation holds 

11) the r 6 parameter is not canceled out, because cr/(/) depends on rjf but R[m does not. When the nucleus-electron coupling is dipolar in origin, R[m depends on r 6 where r is the distance between the resonating nucleus and the unpaired electron (Eq. (3.19)). This partially holds when R m is not dominating. 

Even if not important in principle, it is important in practice to underline that pi is not a time constant for an exponential process the process would be exponential only in the absence of cross relaxation. In paramagnetic compounds the total relaxation may be dominated by R m and the total time dependence approaches an exponential behavior [6], Therefore, p/ is the measured T,-1 of the nucleus (either from a selective or a non-selective experiment). When R[m is not dominating, after a selective excitation of /, the initial return to equilibrium can be assumed to be exponential and its rate constant, can be taken as pi (see Section 2.2). 

When measuring steady state NOE, the effect measured on saturating / or on saturating J may be different because p and pj are in general different. We say that steady state NOE is not symmetric. It follows that larger NOEs, and then more favorable cases, occur when the signal with larger p is saturated. 

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