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13C NMR Question Reagarding Upfield/Downfield

by chembloke
Tags: 13c, 13c nmr, c13 nmr, nmr
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chembloke
#1
Jan30-12, 03:34 PM
P: 8
So my question pertains to a question about 13C NMR. The figure in question is below, the stars indicate the carbons in question.

http://imageshack.us/photo/my-images/841/updownd.jpg/

In class, the professor said that the carbon on the left would be upfield, while the carbon on the right would be down field. I thought since oxygen was electronegative, it would pull electron density from the adjacent carbon, making it more downfield. It was said in class, however, that the oxygen would be providing electron density to that carbon, making it upfield. So I am a little confused on that part.

As for the carbon on the right, I wasn't sure what the explanation was, but since I am assuming that the oxygen is also pulling electron density from its adjacent carbon? I am not really sure.
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Mike H
#2
Jan31-12, 10:37 AM
P: 485
First thing to note - aromatic carbons in 13C NMR all tend to show up between 100 to 160 ppm. The entire "upfield/downfield" terminology (in addition to being considered archaic by IUPAC) is a bit confusing - it's really "why does this carbon show up at 155 ppm in this compound, versus 130 ppm in this other compound?" in my experience. If you want to refer to something "upfield", just to be safe, you really should be referring to something which typically shows up well out of that range like methyl groups. Conversely, carbonyl groups show up "downfield" of aromatics as a rule.

Given that in both cases you will have a ring current effect (scaled as appropriate for proton vs. carbon), your logic makes sense. While this particular database has been known to be anecdotally problematic -

http://riodb01.ibase.aist.go.jp/sdbs..._frame_top.cgi

- I would recommend searching for and printing out the 13C NMR spectra for anisole and acetophenone (the left and right molecules, respectively, in your figure) and get to your professor for a quick chat. It should be an interesting conversation. :)


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