Splitting across a double bond in ¹H NMR

Why do the Hs in (*E*)-1-bromo-2-chloroethene split into doublets instead of staying as a singlet. Wouldn't they be isolated from each other since the double bond does not rotate?


![(E)-1-bromo-2-chloroethene](https://i.stack.imgur.com/l6PUB.png)




The hydrogens are not 'isolated' - they are still vicinal protons (3-bonds away) and so the J-coupling between them is strong. The strength of a splitting interaction is primarily determined by the number of bonds in between the hydrogens involved. 


The angle between them also plays a role but to a lesser extent. The [Karplus equation](http://en.wikipedia.org/wiki/Karplus_equation) describes the three-bond J-coupling between two protons as a function of their dihedral angle. 


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The hydrogens are not identical, one H is connected to a carbon directly connected to a bromine and the other to a chlorine. Since chlorine is more electronegative, the H on the carbon with Cl will be more unshielded.


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