4

My reasoning for the somewhat implausible "$\mathrm{sp^2}$" character of the hydrogen between these two oxygens on the enol structure of the right comes from recognizing that if it were, then we would have 6 $\mathrm{sp^2}$ atoms with 6 π-electrons, so maybe that extra bit of instability from having a higher orbital on hydrogen could be sort of taken care of by being stable like an aromatic compound and forming a hydrogen bond.

So how could I go about figuring out if this is true, either with MO theory or something else?

dicarbonyl

(source: ucdavis.edu)

andselisk
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  • So are you asking whether the hydrogen $2p$ orbitals are involved in bonding? – bon Sep 03 '15 at 16:41
  • Yes, I think so, or if they are occupied ever, for instance would I expect this compound to experience a chemical shift when put into a magnetic field similar to an aromatic compound. –  Sep 03 '15 at 16:46
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    Not a chance. 2p are just too far away. Look at the energy levels diagram. – Ivan Neretin Sep 03 '15 at 16:52
  • As @IvanNeretin points out, the hydrogen is s-hybridized like normal. This $\ce{O..H..O}$ structure is just another example of a 3-center-2-electron bond involving hydrogen. Diborane ($\ce{B2H6}$) is another example. See here, and links therein, for more information on such bonds involving hydrogen. – ron Sep 03 '15 at 16:58
  • In reference to the possibility of downfield NMR shifts, the C-13 and H-1 spectra from SDBS show no signs of aromaticity. – bon Sep 03 '15 at 17:03
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    Wait a minute. Diborane is electron-deficient, but this thing is not. See, we had a normal 2-electron O-H bond, then another O went in with its lone pair (2 more electrons). If anything, this should be a 3-center-4-electron construction. – Ivan Neretin Sep 03 '15 at 17:17
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    @IvanNeretin Yes, I agree, I should have written "3-center-4-electron" in my earlier comment. Thanks for catching that. – ron Sep 03 '15 at 20:19
  • Yes, it has 3c-4e character, but it is limited, as here's asymmetric h-bond - one of contributors much more dominating – Mithoron Sep 03 '15 at 20:53

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