16.5 : 核奥弗豪塞尔效应 (NOE)

Irradiating a spin-active nucleus can alter the signal intensity of a neighboring nucleus that need not be chemically bonded or J-coupled to it.

This through-space phenomenon called nuclear Overhauser enhancement, or NOE,  decreases with the internuclear distance and is usually not seen beyond 4 angstroms.

Here, dipole–dipole interactions between neighboring spin-active nuclei result in the transfer of nuclear spin polarization.

These interactions can cause an increase or decrease in the signal intensity resulting in a positive or negative NOE, respectively.

NOE can be applied for improving signal intensity, revealing spatial proximity, and assigning stereochemistry.

For example, this dioxane derivative can exist in the cis and trans conformations.

Irradiating the methyl protons increases the signal intensity of the nearby proton only in the cis conformer, confirming the stereochemistry.