Why Does Electron Spin Play No Role in NMR?

[blue2script]

Hi all!
I only have a short question concerning nuclear magnetic resonance: the basic principle is that we apply an external magnetic field, the protons (the core of H^1) is split into two energy levels (depending on the alignment of its spin) and we can apply an external high-frequency pulse to do the usual stuff.

Now, there are also electrons in hydrogen. Normally the spin of the proton and the spin of the electron should couple. However, in the normal basic description of NMR the electron plays no role at all. Why is that?

Thank you very much in advance!
Blue2script

 

[ZapperZ]

Hi all!
I only have a short question concerning nuclear magnetic resonance: the basic principle is that we apply an external magnetic field, the protons (the core of H^1) is split into two energy levels (depending on the alignment of its spin) and we can apply an external high-frequency pulse to do the usual stuff.

Now, there are also electrons in hydrogen. Normally the spin of the proton and the spin of the electron should couple. However, in the normal basic description of NMR the electron plays no role at all. Why is that?

Thank you very much in advance!
Blue2script

That's why there is this "resonance" part in the name. You "tune" into the exact frequency that corresponds to the nuclear transition. The electronic transition would probably be quite far off the bandwidth of what you are detecting.

BTW, there is also the NMR equivalent for electron, called electron paramagnetic resonance, or electron spin resonance. In that case, you tune out the nuclear resonance and just zero in on the frequency corresponding to the electrons.

Zz.

 

[Bob_for_short]

That's why there is this "resonance" part in the name. You "tune" into the exact frequency that corresponds to the nuclear transition. The electronic transition would probably be quite far off the bandwidth of what you are detecting.

Indeed, the electron magnetic moment is quite different due to lighter mass: µ ∝ 1/m.
The resonance is different for different nuclei. This is how one distinguishes (measures) the atomic content of the sample in question.

 

[Bob_for_short]

Now, there are also electrons in hydrogen. Normally the spin of the proton and the spin of the electron should couple. However, in the normal basic description of NMR the electron plays no role at all. Why is that?

Coupling spins is important in the ground state energy calculations. But in presence of an external field we have additional interaction Hamiltonians for each spin. We may affect the atom by acting on a particular atomic particle (subsystem). Of course the atomic state changes but too little in case of nucleus spin flipping. That is important in a sense of non-destructive intervention (interaction).

 

[Bob S]

There is also the resonance of the proton magnetic moment in the magnetic moment of the atomic 1s electron, known either as the 21-cm line or the 1420-MHz line.
Bob S

 

[Bob_for_short]

Right, but it is an atomic resonance, a whole system resonance (a proper frequency, no external field involved).

 

[DrDu]

An important point, which has not been mentioned till now, is that NMR is usually only performed in diamagnetic media, where all electronic spins are coupled into a singlet.
By definition, the magnetic momentum of a singlet is vanishing so that it won't couple to the magnetic moment of the nucleus.
The energy necessary to break up an electronic singlet state is typically that of an electronic bond, so that it is not possible with radio frequency radiation.