Carbon NMR Flashcards
C—:O
Carbon more deshielded because oxygen highly electronegative
Carbon feels more of the external magnetic field
Requires higher frequency to bring it into resonance
Higher chemical shift
Peak appears more downfield and at a higher frequency
what does bring a C into resonance mean
make the carbon aligned with the magnetic field so it vibrates at the same frequency as the magnetic field
Why is the term ‘chemical shift use’
because the ppm values are measured relative to the position of the reference signal (TMS) at 0 ppm
What are the chemical shifts like with different bonds?
Single bond - less of a chemical shift
Double bond - more of a chemical shift (because more electrons so more electron shielding effects)
Anything attached to an electronegative element will have a …
greater chemical shift and have a peak that appears more downfield
less stable –> stable
stable —> less stable
releases energy
absorbs energy
What does a larger chemical shift mean?
there is a bigger differences between the frequencies - eg. C-O has a larger chemical shift than C-C because there is a bigger difference in frequencies of C and O compared to C and C.
How to approach interpreting C-NMR
6 steps
1) look at the RFM
2) Look at the number of peaks to work out number of carbon environments and hence have a rough idea of number of carbons
3) read peaks from left to right: look at the ppm and go to the data booklet to figure out which carbon environment (bond/functional group thing) it best matches in terms of chemical shift
4) once you’ve figured that out write down the general formula of that functional group
5) Then subtract the rfm of anything you know in the general formula (eg. if it is CnH2n+2OH you can subtract rfm of OH which is 17)
6) Then just do trial and error
1) rfm
2) peaks
3) ppm
4) general formula
5) subtract and trial + error
What is CH20 on the far right of the data booklet
formaldehyde
In the data booklet the
O=C-O is the chemical shift for either…
ester carboxylic acid (if there is a H attached to the single bond O)
the group below an ester in the data booklet is called an
amide
What does an arene C-C mean?
The C-C bond must be in an aromatic (cyclical) hydrocarbon
What does just the C-C mean?
The C-C bond must be in an aliphatic (straight or branched) hydrocarbon
If the peaks are thick what does it NOT necessarily mean
that there are multiple carbons in the same environment, sometimes the peaks are thick because that carbon environment is closer to the detector.
If it gives you a spec with eg. 5 peaks and ask you to find 7 different molecules responsible for the spectrum what is it asking?
find 7 molecules (so like big compounds) which are responsible for the WHOLE spec. not like 7 mini things responsible for each peak or something
