Question

Spectroscopy concept: spin-spin splitting

What does it mean "This magnetic coupling causes the proton to absorb slightly downfield when the external field is REINFORCED and slightly upfield when the external field is opposed.

Answer 1

The proton is having I=1/2 is spin active. When it is placed in external magnetic field(Ho) its starts spinning around its nuclear axis and generates another magnetic field(H). The generated magnetic field may opposses (means the magnetic lines of proton alligns opposite to the external magnetic lines) the applied external magnetic field then its magnetic field decreases(Ho-H') and hence shows upfield shift.

If the magnetic field of proton reinforce (means the magnetic lines of proton alligns along the external magnetic lines) the applied external magnetic field then its magnetic field increases(Ho+H') and hence shows downfield shift.