Nuclear Overhauser effect/Advanced

The Noe enhancement is quantitatively defined as

$\eta ={\frac {S_{z}-S_{z,equil}}{S_{z,equil}}}$

In the steady state, when the resonance frequency of spin I is irradiated and the intensity of spin S is monitored, the equations for cross relaxation shown above indicate that

$\eta ={\frac {<S_{z}>-<S_{z,equil}>}{<S_{z,equil}>}}={\frac {\sigma }{\rho _{S}}}{\frac {\gamma _{I}}{\gamma _{S}}}$

This indicates that considerable enhancement in the intensity of the S signal can be obtained by irradiation at the frequency of the I spin, provided that ${\frac {\gamma _{I}}{\gamma _{S}}}>1$, because ${\frac {\sigma }{\rho _{S}}}\rightarrow 1/2$ when $w\tau _{c}<<1$. However, when $w\tau _{c}>>1$, ${\frac {\sigma }{\rho _{S}}}\rightarrow -1$ and negative Noe enhancements are obtained.
The sign of $\eta$ changes from positive to negative when $w\tau _{c}$ is close to one and under such conditions the Noe effect may not be observable. This happens for rigid molecules with relative molecular mass about 500 at room temperature e.g. many hexapeptides.