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I have synthesized (1S, 2S)-1,2-diphenylethene-1,2-diol, and I'm wondering why the $\ce{OH}$-peaks in the IR-spectrum looks the way it does. I would expect a broad signal (because the solution wasn't diluted, and because free $\ce{OH}$-stretch happens at $\pu{3580-3700 cm^{-1}}$). So why these two separate, distinct, signals at $\pu{3394}$ and $\pu{ 3499 cm^{-1}}$?

(the IR-spectrum)

enter image description here

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  • $\begingroup$ Could you describe the preparation method (solvent? water?) and IR technique (i.e., thin film on salt plates?)? $\endgroup$ – James Gaidis Mar 13 '18 at 13:38
  • $\begingroup$ Crystalline product dissolved in diethyl ether (didn't measure the amount of product dissolved). I'm not really sure what type of IR-instrument i was using, but I placed some of the solution on the instrument and let the ether evaporate, then i ran the test. $\endgroup$ – Ola Nordmann Mar 13 '18 at 17:06
  • $\begingroup$ Would be see any vicinal coupling effects? Or are only geminal ones seen in IR? $\endgroup$ – Eashaan Godbole Mar 14 '18 at 17:50
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Off top of my head, I would think this is due intramolecular hydrogen bonding (a 5 membered ring), giving rise to a sharp peak at $\pu{3499.14 cm^-1}$, and intermolecular bonding, giving a slightly broader peak at $\pu{3394.45 cm^-1}$. But I don't have any references, this is just a hypothesis.

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1,2-Ethanediol, 1,2-diphenyl (also known as hydrobenzoin) can exist in three crystalline forms. One is S,S (formerly known as d or -), and its twin, R,R (formerly known as l or +), with a mp of 148-149 C; second is the meso form R,S (same as S,R), mp 139-140; and a third form, easily indicated as dl (or S,S:R,R), mp 122-123. It would be a eutectic.

Tartaric acid has the same kind of enantiomeric situation, but the racemic form (dl) is the highest melting form and the meso form is the lowest.

The formation of the low-melting dl form of diphenylethanediol suggests an intermolecular bonding which would likely be thru the hydroxide groups. So intramolecular bonding could give rise to one type of hydrogen bond while intermolecular bonding could lead to another type, with a different stretching frequency.

This is merely an expansion of the answer by ralk912.

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