 In the simplest case where the NOE is relevant, the resonances of two spin- nuclei, I and S, are chemically shifted but not J-coupled.The energy diagram for such a system has four energy levels that depend on the spin-states of I and S corresponding to αα, αβ, βα, and ββ, respectively.The "W"'s are the probabilities per unit time that a transition will occur between the four energy levels, or in other terms the rate at which the corresponding spin flips occur.