Question

in terms of proton spin what causes an NMR signal?

Answer 1

Proton has two spin states α {+1/2) and β(-1/2) When magnetic field is applied an energy difference is created between the two energy levels. Then the radio-frequency equal to the energy difference between the two states is applied and gives a signal. Depending upon the environment of a particular proton a secondary magnetic field is created because of proton spin which can be in the same direction or in opposite direction to the applied magnetic field. Thus we get different signals for different protons.