I understand that some peaks may coalesce upon an increase in temperature due to an increase in free rotation, this does occur with some peaks in my data. Why might you see a peak split into two other peaks? I have thought it may be to do with an equilibrium shift in a rapid conversion between species but really I would expect that to show as one peak shrinking and another one growing. The observed effect is one peak clearly splitting into two. Any ideas why this could happen?

Edit: The sample is a diamagnetic complex with aromatic ligands with t-Bu groups in the meta and para positions. It is hard to assign peaks as I believe there may be an equilibrium between different conformations in solution. The peak that splits is in the alkyl region of the NMR (around 1.5 ppm) and probably corresponds to some part of the t-Bu groups. The 2 peaks produced do appear to get sharper with temperature and the two peaks are the same size, however the difference in chemical shift (between the 2 peaks) at higher temperature is much greater than the linewidth at the lower temperature (where it is one peak), so I'm not sure if this could be J-coupling being resolved.

If it was the case of 2 conformers being present, would you not expect the single peak to reduce in intensity as another separate peak appears and increases in temperature? Rather than a splitting into 2 new peaks?

enter image description here

  • 3
    $\begingroup$ Is the peak (are the peaks) getting sharper as you increase T? Are the peaks symmetrical (same intensity)? You might be narrowing the lines, allowing you to resolve a J coupling. $\endgroup$
    – Buck Thorn
    Mar 20 '19 at 21:46
  • 4
    $\begingroup$ May you add an example where you observe such splitting of one into two / multiple peaks while increasing the sample temperature? $\endgroup$
    – Buttonwood
    Mar 20 '19 at 21:57
  • $\begingroup$ What kind of sample are you measuring? $\endgroup$
    – Karl
    Mar 20 '19 at 22:08
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    $\begingroup$ That's pretty cool looking data. I don't know that this question needed to be put on hold. It helps that you provided the data. Would be nice with some additional info regarding T and particularly field strength (scale is ppm but what is it in Hz) etc, but it's quite clear my hunch was right, except that the coupling would be little big :), so actually prob not a J splitting. $\endgroup$
    – Buck Thorn
    Mar 21 '19 at 16:44
  • 1
    $\begingroup$ I'd quite like to see the structure. This would really help eliminate some of the possibilities. $\endgroup$
    – matt_black
    Mar 22 '19 at 12:58

Since I do not know which compound it is, my guess is that you have two different $\ce{H^1}$ that have almost the same chemical shift at room temperature. The chemical shifts change with temperature as for instance in sucrose:

enter image description here

Here are represented mainly the $\ce{H^1}$ of the $\ce{-OH}$ but the $\ce{C-H}$ (the peak of 1) shifts also.

In your case, one is almost fixed, while the other one moves to higher $\delta$ and, therefore, they split into two.

More info on the sucrose spectrum here.


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