ABSTRACT
Interactions between two different spin systems come in two kinds: dipole-dipole interactions and exchange interactions. Dipolar interactions are purely magnetic in nature, are significant over long distances, and are relatively weak. Exchange interactions are electrostatic in nature, are important only over short distances, and can be orders of magnitude stronger than dipolar interactions. Dipolar interactions are operative “through space” (including vacuum); exchange interactions work “through bond,” that is, directly between two atoms (through a single bond) or indirectly via intermediate atoms (through multiple bonds).
