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Topic: NMR - Effect on Chemical Shift of Nearby Alcohol in Space  (Read 2806 times)

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Offline halcyonmind

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NMR - Effect on Chemical Shift of Nearby Alcohol in Space
« on: September 17, 2013, 09:28:13 PM »
I've got a couple diastereomers that, in the absence of anything other than 1D 1H NMR, I'm trying to distinguish between. There are a few minor differences in their 1H spectra and one enormous one, which I'm hoping will give me a preliminary idea as to which is which. This one proton leaps from ~1 to ~2 ppm from one isomer to the other. I know which proton it is, and I know that the only difference in its local environment is that in one isomer a hydroxyl group will be reasonably close to it in space, whilst in the other isomer the alcohol is far away and the only thing close in space is another C-H proton (on the same carbon as the hydroxyl group).

I know that my shifting proton is part of a methylene (CH2) group in a saturated ring, so it's not forming any sort of hydrogen bond with the hydroxyl oxygen. With that in mind, would you expect proximity through space to a hydroxyl group to shield or deshield the shifting proton? In my mind, I'd expect the opposite to what would be the case if we were considering through-bond interactions. Through bonds, I'd expect deshielding as electron density is withdrawn by the electronegative oxygen. But in space, I'd expect it to have a shielding effect as the electron cloud around the oxygen shields other nuclei close by in space.

Can anyone shoot a hole in this hypothesis? If it is correct, it would conflict with quite a bit of other data I have for other members of the series, which suggest the opposite in terms of identifying the two diastereomers. You thoughts please!

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