Cross-Polarization Origin of the Nuclear Overhauser Effect

The seleetion rales, derived from quantum meehanics, require that the only allowed transitions involve a change of only one spin at a time (these are called single-quantum transitions). The allowed transitions, proton exeitations (labeled H) and carbon excitations (labeled C), are shown. Notice that both proton transitions and both carbon transitions have the same energy (remember that we are ignoring sphtting due to J interactions). 

Because the four spin states have different energies, they also have different populations. Beeause the spin states N3 and N2 have very similar energies, we can assume that their populations are approximately equal. Now use the symbol B to represent the equihbrium populations of these two spin states. The population of spin state Ni, however, will be higher (by an amount S), and the population of spin state N4 will be reduced (also by an amount S). The intensities of the NMR hnes will be proportional to the difference in populations between the energy levels where transitions are occurring. If we compare the populations of each energy level, we can see that the intensities of the two carbon lines (X) will be equal. 

FIGURE 4.6 Spin energy level diagram for an AX System. 

Assuming that the populations of the energy levels are at equilibrium, the carbon signals will have intensities  

Consider now what happens when we irradiate the proton transitions during the broad-band decoupling procedure. The irradiation of the protons causes the proton transitions to become saturated. In other words, the probability of an upward and a downward transition for these nuclei (the proton transitions shown in Fig. 4.6) now becomes equal. The population of level N4 becomes equal to the population of level N2, and the population of level N3 is now equal to the population of level Ni. The populations of the spin states can now be represented by the following expressions. 

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6 Cross-Polarization Origin of the Nnclear Overhauser Effect 187 

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CROSS-POLARIZATION ORIGIN OF THE NUCLEAR OVERHAUSER EFFECT... 

Under continuous uv irradiation, the observed steady-state polarization (whether by cw or by FT spectrometers) may be substantially modified by various nuclear relaxation processes. For example, Closs and Czeropski (35,36) have demonstrated that CIDNP can be transferred from a group of polarized nuclei to another group not originally polarized. Both the dipolar and the scalar relaxation mechanisms (of the nuclear Overhauser effects) can be operative. The extremely interesting case of intramolecular dipolar nuclear cross relaxation reported by Closs and Czeropski (35) involves the thermal reaction of... 

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