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More Complex Spin-Coupling: The ABC System
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Many terminal alkenes exhibit splitting patterns in which there is no element of symmetry to the coupling constants and a complex ABC pattern is observed. For many terminal alkenes, this pattern can be predicted and described in terms of the coupling constants for the cis, trans and geminal protons. The coupling constants for these protons are generally 10, 15 and 
2 Hz, respectively, as shown below.
The proton on carbon a is split by c with J = 15 Hz into a doublet (n + 1 peaks). These peak are further split by b with J = 10 Hz into a doublet of doublets, as shown below.
The proton on carbon b is split by a with J = 10 Hz into a doublet (n + 1 peaks). These peak are further split by c with J = 2 Hz into a doublet of tiny doublets. The proton on carbon c is likewise split by a with J = 15 Hz into a doublet, and these peak are again split by b with J = 2 Hz into another doublet of tiny doublets.
For many terminal alkenes, a predictable ABC pattern is observed. If, however, the chemical shifts of a, b and c are similar so that 
J, second-order coupling occurs and the spectrum becomes non-trivial. Higher resolution spectrometers (i.e., 600 MHz) are again useful at resolving fine structure in the spectrum such as this.
H NMR Correlation Table