13C NMR Question Reagarding Upfield/Downfield

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In summary, the conversation is about the interpretation of 13C NMR spectra. The question is about the location of the carbons on the left and right sides of the figure provided. There is confusion on whether the oxygen pulls electron density from the adjacent carbon, making it more downfield, or if it provides electron density, making it upfield. The discussion also includes the terminology of "upfield" and "downfield" and the suggestion to compare the spectra of anisole and acetophenone for further clarification.
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chembloke
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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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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/cgi-bin/direct_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. :)
 

FAQ: 13C NMR Question Reagarding Upfield/Downfield

What is the difference between upfield and downfield in 13C NMR?

In 13C NMR, upfield refers to chemical shifts to the left of the spectrum, while downfield refers to chemical shifts to the right. This is based on the reference compound, tetramethylsilane (TMS), which is assigned a chemical shift of 0 ppm. Upfield shifts indicate that the carbon atom is shielded from the magnetic field, while downfield shifts indicate that the carbon atom is deshielded.

How do upfield and downfield shifts relate to electronegativity?

Upfield shifts are generally associated with more electronegative atoms or functional groups, such as oxygen or nitrogen, which pull electron density away from the carbon atom and shield it from the magnetic field. Downfield shifts are typically seen with less electronegative atoms or functional groups, such as alkyl or aromatic groups, which donate electron density and deshield the carbon atom.

Can upfield and downfield shifts be used to determine the chemical structure of a compound?

Yes, upfield and downfield shifts can provide valuable information about the functional groups and molecular environment of a carbon atom, which can aid in determining the chemical structure of a compound. However, they should be used in conjunction with other spectroscopic techniques and chemical knowledge for a more accurate determination.

How can the intensity of peaks in a 13C NMR spectrum be used to distinguish between upfield and downfield shifts?

In general, upfield shifts tend to have higher intensities compared to downfield shifts. This is because upfield shifts are usually associated with more electronegative or polar functional groups, which contribute to a larger peak due to their higher chemical shift range. Downfield shifts, on the other hand, tend to have lower intensities due to their association with less polar functional groups.

Are upfield and downfield shifts affected by the solvent used in 13C NMR?

Yes, the solvent used in 13C NMR can have an effect on the chemical shifts of a compound. This is because solvents can interact with the sample molecule, altering its electronic environment and therefore, its chemical shifts. In general, more polar solvents will cause downfield shifts, while less polar solvents will cause upfield shifts.

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