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Hey everyone,

I've started practicing reading NMR again after having not done so for years, and I'm getting caught up on what must be a very simple question. For the 1H spectrum of 2-methoxybenzaldehyde, why do two of the aromatic protons exhibit identical chemical shifts (6.94 ppm)? As far as I can tell, their environments should not be equivalent. Values from the spectra may be found here: http://www.sciencesoft.net/C8H8O2/

Thanks in advance.

asked Jun 13 '13 at 14:16

John%20Xavier's gravatar image

John Xavier

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Hi John,

As Kirk says the protons are not really equivalent - if you look at the spectrum from aldrich:


and zoom in close to that peak, you can just about see that you have a triplet and a doublet, with the rightmost peak of the triplet very close to the leftmost peak of the doublet. Whilst looking at this I came across this from Ray Abraham at Modgraph:


which indicates that the two most similar protons are 3 and 5 (where position 1 is the aldehyde, 2 is the methoxy etc), whereas your source has 3 and 4 as similar. I don't think 4 can have the same shift as 3 - the aldehyde is strongly ortho, para (pi) electron withdrawing, so would deshield proton 4, the methoxy is sigma-withdrawing (hence high shift of carbon 2) and ortho,para (pi) donating, so will have limited effect at proton 4. Thus you expect proton 6 to have the highest shift (ortho-withdrawing effect of aldehyde, no dotating effect from methoxy), then 4 (para-withdrawing effect of aldehyde, no donating effect from methoxy), 3 has sigma-withdrawing and pi-donating effects, and 5 not much of anything, so not possible to guess at those. A quick PERCH calculation agrees with this assignment.

For aromatics changing from CDCl3 to benzene-d6 can have a aprticularly pronounced effect on the shifts.


answered Jun 17 '13 at 12:06

Pete%20Gierth's gravatar image

Pete Gierth

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The four aromatic protons in this molecule are, in principle, non-equivalent. However, sometimes non-equivalent protons just happen to have very similar chemical shifts. The spectrum shown on the "sciencesoft" site is fairly low resolution. A higher resolution spectrum would probably show them as non-equivalent. Changing the solvent is another good trick when dealing with cases of accidental near equivalence of chemical shifts.


answered Jun 17 '13 at 07:41

Kirk%20Marat's gravatar image

Kirk Marat

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