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CH1012
COUPLING CONSTANT J
The spacing between the peaks in a multiplet is termed the coupling constant (J) and this value in Hz indicates the size of the H-H coupling.    eg. 1,1-dichloroethane 
d 5.9 (quart., J=7Hz, 1H, CH); 2.0 (doub., J=7Hz, 3H, CH3)
Jab   =  Jba
When there are many
couplings complex
multiplets arise  eg.
vinyl acetate. Hc signal ®
J
J