[skytrent]

Hey everyone, I was just trying to rationalize the C13 NMR shifts for carbonyl groups and acid derivatives.  Why would a ketone shift more down field than an aldehyde? Logically, I would think that the electron donating R groups of a ketone would create a greater electron density around that carbonyl carbon than in an aldehyde, therefore the aldehyde carbonyl carbon would be more electrophilic, and more electron deshielded so shouldn't the aldehyde have the greater shift value (more down field)?  Also acetamide (172.6 ppm) is shifted more down field than acetyl chloride (170.3), but by the same logic, the carbon in the acyl chloride will be much more electron poor and as a result deshielded.  Why does this occur only for C13 shifts.  For proton NMR and IR stretching comparisons,these explanations make sense and validate the data.  If anyone can weigh in that would be great.

[Babcock_Hall]

I am not an expert in C-13 shifts.  However, sp3 carbons usually move downfield in the order primary, secondary, tertiary.  Therefore, it is not entirely surprising that ketones are downfield of aldehydes.

[cheese (MSW)]

Like many before you: you’re trying to put a square peg in a round hole!
The effective magnetic field at a spin½ nucleus is given by Beff  = (1- σ)Bo where Bo is the applied magnetic field and σ is the shielding parameter.  The σ term however contains several terms (e.g., σsolv,  σ(ring current)) but the terms of most concern are σdia and σpara.  σdia concerns the spherical e⁻ density around the nucleus: an increase in the  e⁻ density results in increased shielding and the resonance is shifted upfield (easy to comprehend).  For H this is the most important term because H bonds via the spherically symmetric 1s AO.  For ALL other spin½ nuclei however the paramagnetic term, σpara, dominates (the term has nothing to do with unp  e⁻).  It concerns the asymmetric circulation of e⁻ density about the nucleus.  (If your C-13 shifts are consistent with simple ± e⁻ donating arguments it is a coincidence!)  The paramagnetic term contains a 1/ΔE term where ΔE is the H*OMO-LUMO gap (would I kid you?), and p “imbalance” terms (for TM cmpds throw in d orbital imbalance as well).  The σpara has the opposite sign to σdia and so a large σpara causes the resonance to shift downfield (by huge amounts: think nothing of 200 ppm vs ~10 ppm for σdia) .
For your cmpds you may therefore get further by relating the shifts to the n→π* H*OMO-LUMO gap: the smaller the ΔE the further the shift downfield.  But to do a reasonable job of rationalizing the shifts you have to do MO calcs.
Let me get you to do some research: google:  NMR paramagnetic term chemical shifts.  (I'll also see what it brings up.)

[qw098]

I gave you a mole snack Cheese; great answer!

[cheese (MSW)]

www.oci.uzh.ch/group.pages/zerbe/NMR.pdf
This person (pages 54-55) gets it right!!
qw098: Thank you for my mole snack but you shouldn’t encourage me as
it will go to my head!