Explain the following observations, in words, concerning the NOE in a two-spin system.
(a) In a transient NOE experiment, and in the initial rate, the NOE enhancement depends only on the cross-relaxation rate constant.
(b) At longer times, the NOE enhancement in this transient experiment depends on the self-relaxation rate constants of both spins as-well as on the cross-relaxation rate constant.
(c) In a steady-state NOE experiment, the NOE enhancement of spin two, observed when spin one is saturated, depends on the ratio of the cross-relaxation rate constant to the self-relaxation rate constant of spin two; the self-relaxation rate constant of spin one does not affect the size of the enhancement.