>>11119636The physics:
All protons are little magnets, and they all oscillate at a specific frequency. In a strong magnetic field, that frequency becomes 300-500MHz or more.
As you would expect, the little magnets in a magnetic field all align with the magnetic field. But due to "thermal agitation" (see 1951 Varian patent, line 53), half of them align with the field, and half go against the field. Half point up, half point down.
Since the magnets are all aligned with the field, their oscillations have very low amplitude. The 90 degree pulse rotates all the little magnets so they are perpendicular to the field -- think like pulling back a large pendulum -- and then are allowed to "swing" freely, at a high amplitude, back into alignment. The frequency of this high amplitude can be measured.
The chemistry:
Electrons also behave like little magnets, but we aren't terribly concerned about those. All you need to know is, they "shield" the magnetic hydrogen nucleus from the external magnetic field, so that the magnetic field the nucleus "sees" is slightly weaker than what you've applied. Since the precession frequency depends on the external field strength, their frequency decreases. The frequency shifts "downfield."
Different chemical environments correspond to different electron densities around the hydrogen nucleus. Therefore, different degrees of shielding give different frequency shifts. These shifts are fairly unique, and can provide valuable data about the functional group around that proton.
Splitting is a result of the fact that half of the spins align with, and half align against the field. The up and down spins of neighboring protons interfere with the observed proton, creating one signal which is slightly higher freq. and another slightly lower freq. than average. These patterns can help puzzle-piece together large organic molecules.
