Both NMR and EPR describe the response of magnetic spin to external field. When collecting data, how do you know you're looking at nucleus spin flip or electron spin flip? In other words, since every sample has both protons and electrons, and all have magnetic spin, how do you separate between the protons' response and the electrons' response to the external perturbation?
Quantum Mechanics – What’s the Difference Between NMR and EPR?
quantum mechanicsquantum-spinresonancespectroscopy
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The interaction between nucleons in a nucleus is very strong so the energy changes associated with flipping a spin are very high. The first few energy levels of a carbon nucleus can be found in this document. The energy spacing between the ground state $J=0$ and the first excited state $J=2$ is 4.44 MeV. This isn't much by the standards of modern accelerators, but it is far, far too big for the transition to be caused by an external magnetic field.
So as far as NMR is concerned the carbon atom behaves as an elementary particle of spin zero with no internal excitations.
Some insight into this question may be gained from considering calculations of the spin orbit splitting $\Delta$ by F. Herman et al., Phys. Rev. Letts. $\textbf{11}$, 541 (1963), which I have plotted below. There we see a saw-tooth dependence of $\Delta$ with atomic number $Z$, with $\Delta = 0$ for a single valence electron in an outer shell (alkali metals) to a maximum for an electron in a full shell (inert gases).
This suggests that it is only the core electrons that screen the nucleus, with little or no screening of a valence electron by other electrons in the outer shell. For a given row of the periodic table, the number of screening core electrons will stay the same as $Z$ increases, so the local electric field seen by the electron will indeed increase with the size of the atom and hence give rise to a stronger spin-orbit splitting.
Best Answer
The electron magnetic moment is about 660 times larger than that of the proton, and the proton's magnetic moment is the largest of all the nuclei. Although most electrons occur in pairs, unpaired electrons, as they occur in radicals, give rise to electron paramagnetic resonance (EPR) signals.
Signal frequencies in magnetic resonance are, to a very good approximation, proportional to the magnetic moment (unless the external magnetic field becomes very weak or in the case of large quadrupolar splittings).
In a typical nuclear magnetic resonance (NMR) experiment one would thus observe either the proton or the $^{13}$C carbon NMR signal (much like listening to different FM radios). For a 10 Tesla magnet, these would have frequencies of approximately 400 MHz and 100 MHz, respectively. It is possible to excite proton or carbon NMR simultaneously, but this requires two channels, tuned to the respective (radio) frequencies.
On the other hand, an electron spin would precess at a (microwave) frequency approaching 300 GHz, requiring different excitation and detection pathways (waveguides rather than coaxial cables, and cavity resonators rather than $LC$-resonators).
However, the presence of free electron spins may manifest itself in the NMR detection via reducing the relaxation time $T_1$, a phenomenon known as paramagnetic relaxation enhancement (PRE).