Geminal coupling is spin-spin coupling between two nuclei separated by two chemical bonds, commonly two hydrogens attached to the same carbon. It is denoted to distinguish it from a three-bond vicinal coupling.
Equivalent hydrogens do not split one another into separately observable multiplets in a simple first-order spectrum, even if a geminal interaction exists. Nonequivalent methylene hydrogens can show it clearly. The magnitude and sign depend on molecular structure and electronic environment, so the coupling is structural evidence rather than one fixed universal constant.
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