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Identifying Meta coupling in a 1H NMR Spectrum

In a substituted benzene ring, aromatic protons that are in the meta position can exhibit coupling to each other. This is referred to as meta or 4J coupling. The coupling pattern is typically a doublet with a coupling constant of ~2 Hz.

On the contrary, a spectrum without any meta coupling indicates a lack of protons in the meta position. Although a 2D COSY experiment can produce the same result, one can save time by looking for this information in a 1H NMR spectrum first.

Illustrated below is a portion of a 1H NMR spectrum for a substituted benzene ring. Proton A is a doublet with a 2.3 Hz coupling and proton B is also a doublet with 2.2 Hz coupling. Proton A and B are coupled to each other due to the similar coupling constant (+/- 0.2 Hz). Another indication of coupling is the slight tilt of the multiplets to each other.

Metacoupling_apr72008

TIP: Be careful on the extent of line broadening applied to the FID, too much and the meta coupling information can be lost.