Assignment of quaternary carbons via two‐bond heteronuclear selective NOE difference, a useful structure elucidation aid

Very low power irradiation of suitable narrow peaks in the proton frequency range generated NOE enhancement in neighbouring quaternary carbons, positioned two bonds from the irradiated protons. Subsequent FID acquisition under normal broad band decoupling yielded the NOE‐containing FID, which, after subtraction of the unperturbed broad band decoupled FID (devoid of NOE), gave the final NOE difference FID to which only the carbons undergoing NOE enhancement had contributed. Fourier transformation of the latter gave the NOE difference spectrum, usually a one‐ or two‐peak spectrum. This technique proved particularly useful for polysubstituted aromatic rings.