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The chemical shift of a proton is an indication of the amount of shielding it receives from the electron density on it or other magnetic anisotropy effects present in the molecule. For the protons of the carboxylic acid functionality, the chemical shift typically exceeds $\pu{10 ppm}$. Baseline estimates given for $\ce {CH, CH2, CH3}$ protons are $ 1.7, 1.3$ and $\pu{0.9 ppm}$ respectively. However, we are often able to observe the chemical shifts of alcohol protons in the range of $\pu{1-2 ppm}$. This is rather counterintuitive since the electronegativity of oxygen is markedly higher than that of carbon and hence we would observe a mucher greater deshielding of the $\ce{OH}$ proton. For alcoholic protons, there is likely to be the presence of hydrogen bonding effects. Is the presence of these $\ce {H}$ bonding effects responsible for the less than expected deshielding?

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Hydrogen bonding leads to deshielding in alcohols. Isolated ethanol in the gas phase has a shift of $\pu{0.55 ppm}$, while the shift is $\pu{0.8 ppm}$ in neat ethanol, so there is a shielding effect due to H-bonding. However, the OH shift is extremely sensitive to self association and/or solvent, resulting typically in greater deshielding. See for instance the table of shifts in reference [1].

According to Hore (in his Oxford NMR book), the charge distribution around the proton is polarized (mainly because the proton is attracted by the H-bond acceptor), effectively deshielding it. In any event, the hydrogen bond acceptor $\ce{O}$ transfers charge to the donor $\ce{O}$. The intervening H apparently gets the hard end of the bargain.

[1]: Fulmer et al, Organometallics 2010, 29, 2176–2179

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  • $\begingroup$ Thanks for the gas phase NMR data. That is highly appreciated. However, the effect of hydrogen bonding seems quite counter-intuitive to me as electron density, in the form of a lone pair, is donated to the hydrogen atom during H bonding. Hence, it is puzzling that the effect is overall deshielding. $\endgroup$ – Tan Yong Boon Feb 16 at 14:16
  • $\begingroup$ Hore's book is not especially illuminating about the source of the deshielding effect in terms of the specific electronic distribution about the proton. You have a valid point that the hydrogen bond acceptor donates charge to form the bond, which would partially reside near the H atom, so this does seem counterintuitive. It might be that the distribution is stretched, ie becomes thinner in the radial direction about the bond. Therefore the average shift is reduced. But I speculate. I only addressed one misconception behind your question. $\endgroup$ – Buck Thorn Feb 16 at 14:52

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