Figure 2.13

nonequivalent protons will show a large two-bond coupling (13-16 Hz) in addition to any other (vicinal and/or long-range) couplings. The nonequivalent protons of a methylene group are often referred to as "diastereotopic" protons. Formally, if you do not have a mirror plane in the X-C-Y plane or a C2 axis bisecting the H-C-H angle, the two methylene protons are nonequivalent. The methylene protons of a terminal olefin ((R,R/)C=CH2) are a different case; they are equivalent only if the two R groups are identical (C2 axis bisecting the H-C-H angle) or mirror images (mirror plane through C=C and bisecting the H-C-H angle). If they are nonequivalent, the two-bond coupling is small (0-2 Hz).

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