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I was presented today with the following molecule:

2-(cyclopenta-2,4-dien-1-ylidene)-1-methylpyrrolidine

and asked the splitting pattern of $\ce{H_a}$ given that up to $\mathrm{^3J}$ and $\mathrm{^4J}$ coupling is allowed.

I thought that $\ce{H_a}$ couples to $\ce{H_b}$, $\ce{H_c}$ and $\ce{H_d}$ given that

  • there is a $\mathrm{^3J}$ coupling to $\ce{H_b}$ (Clockwise)
  • there is a $\mathrm{^4J}$ coupling to $\ce{H_c}$ (Clockwise)
  • there is a $\mathrm{^4J}$ coupling to $\ce{H_d}$ (Anticlockwise)

Hence the splitting would be ddd.

However, the answer is apparently dd for the splitting of the $\ce{H_a}$ signal.

My thought was that anticlockwise, the $\mathrm{^3J}$ coupling is not possible due to the absence of a π bond in that direction down to $\ce{H_d}$ hence the coupling through the π system would not be possible.

Am I correct or is there another reason for no $\mathrm{^3J}$ coupling to $\ce{H_d}$?

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    $\begingroup$ Please note that using MathJax in the title creates issues; \ce{…} should only be used for chemical compounds and reactions. Also, I added the compound name as image metadata (it's better to fill in something meaningful in place of "Enter image description here"), and I suggest you do the same in your future post as it makes finding your post much easier. $\endgroup$ – andselisk Dec 29 '19 at 15:31
  • $\begingroup$ @andselisk why does it create issues in the title? is mathJax not compatible with other browsers or other mobile devices? $\endgroup$ – vik1245 Dec 29 '19 at 15:35
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    $\begingroup$ See here and there. Besides, in this case there is literally no justification for typing π with MathJax as it can be easily inserted as a Unicode character. $\endgroup$ – andselisk Dec 29 '19 at 15:38

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