Vicinal coupling is spin-spin coupling between nuclei separated by three bonds, often hydrogens on neighboring carbons, and is denoted . Its dependence on the intervening dihedral angle is described approximately by a Karplus relation.
This makes a measured coupling useful for estimating molecular conformations. The relationship also depends on substituents and bonding, and different angles can give similar coupling magnitudes. A single coupling therefore rarely determines a conformation uniquely; several measurements and stereochemical constraints are normally combined.
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