Discussion
What is the typical range of a three bond proton to proton coupling in a freely rotating chain?
Answer: A
A vicinal coupling averaged over free rotation lands at 6 to 8 Hz, which is why ethyl groups show that spacing so consistently. Rigid systems depart from it widely: a trans coupling across a double bond reaches 12 to 18 Hz and a cis one 6 to 12, while a dihedral angle near ninety degrees gives almost zero. Long range four bond couplings are the ones in the 0 to 1 range, and the larger figures quoted belong to couplings involving fluorine or phosphorus. The expected value is what makes a departure from it informative.
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