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This is H NMR spectrum of cis-stilbene. There are some traces of trans stilbene. I have to determine how many trans-stilbene is present, the exact percentage and I have problem with that.

I guess that it has to do with integration of peaks but I'm still not sure is trans-cis isomer ratio 1:10 or am I wrong?

cis stilbene H NMR

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  • $\begingroup$ Step 1: identify which peaks correspond to cis and which to trans. $\endgroup$ – matt_black Feb 8 '18 at 11:13
  • $\begingroup$ Small peaks at 7,35 ppm and 7,51 ppm correspond to trans isomer and peaks at 7,27-7,15 ppm and 6,59 ppm correspond to cis isomer. $\endgroup$ – Marina Feb 8 '18 at 11:20
  • $\begingroup$ For accurate integration you also need to set the recycle delay to a longer value than usual (e.g. ~20 s), although I appreciate this may not be possible in your case. $\endgroup$ – orthocresol Feb 8 '18 at 12:22
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You have to focus on the $\ce{^1H}$ of the double bond. They are singlets because both molecules are completely symmetrical. As per the tables, the chemical shift of the cis isomer is $5.25+1.38-0.07=6.56$ ppm. On the other hand, the chemical shift of the trans isomer is $5.25+1.38+0.36=6.99$ ppm.

These theoretical values correspond to the peaks of 6.60 and 7.11 ppm. Since their integrals are of 1 and 1.1571 respectively, the proportion of trans is of:

$\frac{0.1571}{0.1571+1}100=14\%$

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  • $\begingroup$ Instead of using tables, a better way is to simply look up the data: cis-stilbene comes in at 6.572 and trans-stilbene at 7.15. $\endgroup$ – orthocresol Feb 8 '18 at 12:19
  • $\begingroup$ @orthocresol Very correct, but the question looks like the OP is learning to interpret NMR, so I thought it was better to do it with the tables $\endgroup$ – Raoul Kessels Feb 8 '18 at 12:23
  • $\begingroup$ Thank you very much! Yes, I'm still learning to interpret NMR. :) $\endgroup$ – Marina Feb 8 '18 at 14:04

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