If we perturb one spin, e.g., I, i.e., change its populations of the a and B state(e. g, by saturating the resonance= creating equal population of both states ), then relaxation will force I' back to the equilibrium BOLTZMANN distribution. With the W, mechanism, spin I' will just relax without effecting spin 12. However, the other two mechanism will effect F2 B一个个个一个二阝 E B一个个个一个B W a4个 个个个个a a个个 个个个个个α With the I polarization going back from saturation to the BOLTZMANN equilibrium, the Wo mechanism will cause the neighbouring(so far unperturbed) spin to deviate from its BOLtZmann equilibrium towards a decrease in a, B population difference. After a 90 pulse, this will result in a decrease in signal intensity for 12-a"negative NOE effect" On the other hand, the W2 mechanism will cause the population difference of the undisturbed spin 12 to increase, corresponding to an increase in signal intensity a"positive NOE effect These effects can be directly observed in a very simple experiment, the id difference Noe sequence AQ CW One spin is selectively saturated by a long, low-power CW(continuous wave)irradiation. As soon as the spin deviates from its BOLTZMANN population distribution, it starts with TI relaxation. Via the Wo or w2 mechanisms it causes changes in the population distribution of neighbouring spins. After a 90o pulse, these show up as an increase or decrease in signal intensity93 If we perturb one spin, e.g., I1 , i.e., change its populations of the a and b state (e.g., by saturating the resonance = creating equal population of both states), then relaxation will force I1 back to the equilibrium BOLTZMANN distribution. With the W1 mechanism, spin I1 will just relax without effecting spin I2 . However, the other two mechanism will effect I2 : With the I1 polarization going back from saturation to the BOLTZMANN equilibrium, the W0 mechanism will cause the neighbouring (so far unperturbed) spin to deviate from its BOLTZMANN equilibrium towards a decrease in a,b population difference. After a 90° pulse, this will result in a decrease in signal intensity for I2 — a "negative NOE effect". On the other hand, the W2 mechanism will cause the population difference of the undisturbed spin I2 to increase, corresponding to an increase in signal intensity: a "positive NOE effect". These effects can be directly observed in a very simple experiment, the 1D difference NOE sequence: One spin is selectively saturated by a long, low-power CW (continuous wave) irradiation. As soon as the spin deviates from its BOLTZMANN population distribution, it starts with T1 relaxation. Via the W0 or W2 mechanisms it causes changes in the population distribution of neighbouring spins. After a 90° pulse, these show up as an increase or decrease in signal intensity