6 The noe effect c Gerd Gemmecker, 1999 Nuclear OVERHAUSER Effect(NOE): resonance line intensity changes caused by dipolare cross- relaxation from neighbouring spins with perturbed energy level populations To understand the nature of the NOE, we have to look at a two-spin system I' and 12. Since NOE does not involves coherences, but merely polarization, i.e. population differences between the a and p states, we can use the energy level diagram here W W1() W1(I) W1() The possible transitions for this two-spin system can be classified into three groups W, transitions involving a spin flip of only one of the two spins(either I or 1), corresponding merely to T relaxation of the spin a Wotransition involving a simultaneous spin flip a-B for one spin and a>p for the other one (i.e, in summa a zero-quantum transition) a W2 transition involving a simultaneous spin flip of both spins in the same direction corresponding to a net double-quantum transition We are just contemplating spin state transitions here caused by relaxation, which do not involve a coherent process(like, e.g., Ix/Iv coherences, which require a phase coherent transition between two states that is generated by an r f. pulse)!92 6 The NOE Effect © Gerd Gemmecker, 1999 Nuclear OVERHAUSER Effect (NOE): resonance line intensity changes caused by dipolare cross￾relaxation from neighbouring spins with perturbed energy level populations. To understand the nature of the NOE, we have to look at a two-spin system I1 and I2 . Since NOE does not involves coherences, but merely polarization, i.e. population differences between the a and b states, we can use the energy level diagram here: The possible transitions for this two-spin system can be classified into three groups: - W1 transitions involving a spin flip of only one of the two spins (either I1 or I2 ), corresponding merely to T1 relaxation of the spin. - a W0 transition involving a simultaneous spin flip a®b for one spin and a®b for the other one (i.e., in summa a zero-quantum transition). - a W2 transition involving a simultaneous spin flip of both spins in the same direction, corresponding to a net double-quantum transition. We are just contemplating spin state transitions here caused by relaxation, which do not involve a coherent process (like, e.g., Ix / Iy coherences, which require a phase coherent transition between two states that is generated by an r.f. pulse)!