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Why does [18]annulene show only one peak in its proton NMR at $\pu{100^\circ C}$? Does any structural change happen such that all the hydrogens are alike?

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    $\begingroup$ I’m guessing the key point to solving this is the temperature. $\endgroup$ – Jan Oct 3 '17 at 12:54
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    $\begingroup$ Actually, this question is more interesting than it seems. I would prefer if this were not closed. Some references: J. Chem. Theory Comput. 2015, 11, 5083; also J. Am. Chem. Soc. 2000, 122, 722; also Pure Appl. Chem. 1971, 25, 573. $\endgroup$ – orthocresol Oct 3 '17 at 13:00
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    $\begingroup$ The answer obviously has to be yes since drawing the simple structure yields two sets of equivalent protons.... $\endgroup$ – Zhe Oct 3 '17 at 13:08
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    $\begingroup$ Chem. Commun. (London), 1966,0, 904-905 refers to an earlier work (which I haven't found) describing the temperature dependence of the NMR spectrum of [18]annulene due to rapid proton interchange at high temperature. $\endgroup$ – AndyW Oct 3 '17 at 13:39
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    $\begingroup$ @Martin-マーチン Yup, and I think that's why this problem is interesting like orthocresol says. :) $\endgroup$ – Zhe Oct 3 '17 at 15:07
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Very roughly, we would expect aromaticity to do two relevant things:

  1. It tends to make the ring rigidly planar, but...
  2. Its energetic effect decreases with increasing ring size.

With $18$ atoms in the ring the energetic effect is decreased to the point where the ring isn't really all that rigid. It becomes fluxional on warming.

This is one of many examples of aromaticity not always having all the effects we're accustomed to seeing in benzene rings.

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  • $\begingroup$ So as temperature increases, the molecule becomes more destabilized? $\endgroup$ – Prashant Govind Oct 5 '17 at 3:24
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    $\begingroup$ In a sense all molecules become less stable when heated, they vibrate more from their most stable confirmations. In [18]-annular one such vibration mode scrambles the hydrogens. $\endgroup$ – Oscar Lanzi Oct 5 '17 at 12:14
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The aromaticity of [18]-annulene1 was confirmed by room temperature 1H NMR (60 MHz) in 1964.2 [τ values have been converted to δ values; δ = 10 - τ] When the spectrum was recorded at room temperature the 12 deshielded, exterior hydrogens appeared at δ 8.9 while the 6-interior hydrogens were highly shielded (δ -1.8). Evidence for flexiblity in the ring at room temperature was confirmed by observing a further downfield shift of the 12 hydrogens (δ 9.28) and an upfield shift of the 6 hydrogen signal (δ -2.99) when the spectrum was recorded at -60oC. Recording the spectrum at 110oC caused the two signals to coalesce to a singlet peak at δ 5.45. This value is close to the weighted average of the resolved chemical shifts. Ostensibly, the coalesence is due to the interchange of external and internal hydrogens. When the sample is cooled, the resolved spectra are restored.
enter image description here 1) Sondheimer, F., Wolovsky, R., Amiel, Y. J. Am. Chem. Soc., 1962, 84, 274.

2) Gaoni, Y., Melera, A., Sondheimer, F., Wolovsky, R. Proc. Chem. Soc., 1964, 397.

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