= Vicinal coupling
<Vicinal coupling> is spin-spin coupling between nuclei separated by three bonds, often hydrogens on neighboring carbons, and is denoted $^3J$. 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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