[electrogeek]

Hi everyone,

I was looking at the Hydrogen NMR spectrum for dibenzalacetone in the attachment, and I don't know how to assign the doublets to the molecule.

I know that the multiplets in the region 7.35 - 7.65 ppm relate to the different protons contained on the phenyl groups of the compound.

I thought that the doublet with the highest chemical shift (about 7.72 ppm) would relate to the hydrogen closest to the carbonyl group, due to the electronegativity of the oxygen deshielding the adjacent protons on the carbons next to this functional group (and therefore the other doublet refers to the other H nearer the phenyl groups). But when I look online at labelled NMR spectra for the same compound, they say that the doublet around 7.08 ppm relates to the hydrogen's closest to the carbonyl group (as if they're shielded).

I can't work out the reason why this would be the case - can someone explain why this to me?

[Babcock_Hall]

One, besides electronegativity there are other effects that cause deshielding.  Two, I am not sure how you are determining "closeness" in this context.

[electrogeek]

What are the other effects at play (1st year at Uni - a small explanation would be appreciated or a link to a website about the factors)? Do the other factors explain why I got the peaks assigned wrong in the first place?

By "closeness" I mean how far along the carbon chain from the carbonyl group the hydrogen atoms are in question. So the hydrogen closest to the carbonyl group is on the carbon atom between the carbonyl group and the double bond, and the further hydrogen is on the carbon between the phenyl group and the double bond.

I'm only considering one half of the molecule because the environments would repeat themselves for the other half.

[Babcock_Hall]

What factor causes the hydrogen atoms bonded to carbon atoms of a phenyl ring to be between 7.35 and 7.65 in this molecule?  It cannot be electronegativity.  I would also keep in mind that whether or not something is shielded depends on what one is using for comparison.  Both hydrogen atoms are more downfield than the hydrogen atoms of trans-2-butene, for example.

[electrogeek]

Do they become de-shielded due to an induced ring current generating a magnetic field which is in the same direction as the magnetic field...

So does this induced magnetic field also affect the electron density around the hydrogen on the carbon between the phenyl group and the double bond, meaning it becomes more de-shielded?

Also, how much of an effect does anisotropy have on the chemical shift of the hydrogen on the carbon between the carbonyl group and the double bond?

I know that hydrogen bonding wouldn't affect the molecule - so that wouldn't affect the chemical shifts...

[Babcock_Hall]

The ring current effect applies to any double bond or any aromatic ring.  The effect gets weaker with distance, and it is anisotropic.  I would not say that it has anything to do with redistributing electron density.  To get an idea of its magnitude, compare the chemical shift of a methyl group in propane with the methyl group in toluene.

[electrogeek]

Thanks for the help

If it applies to any double bond, why is it a smaller effect on the double bond in the carbonyl than in the benzene ring - I'd guess it's to do with the number of electrons being shared or delocalised (more delocalised electrons in Benzene results in a stronger induced magnetic field than what the carbonyl can provide)?

[Babcock_Hall]

I am not sure of all of the factors that go into producing this effect.  The effect for the aromatic ring for benzene derivatives is a little larger than the effect for an isolated double bond.  The hydrogen atom of an aldehyde is quite far downfield, but I don't know quantitatively how much of the effect is the ring current from the carbon-oxygen double bond.

[electrogeek]

Oh okay - thanks for the help. I really appreciate it.

[Babcock_Hall]

In researching this topic this morning, I was reminded of something that I almost forgotten.  The standard explanations involving ring currents have come under question in the last twenty years or so.  Empirically, the downfield positions of these protons is obviously not open to question, but the reasons behind the downfield shift may not be what standard textbooks say. 
Chaitanya S. Wannere and Paul von Rague ́ Schleyer, ORGANIC LETTERS 2003 Vol. 5, No. 5 605-608.

[electrogeek]

I'll have a look at the paper - thanks for the title. I think they want us to talk about ring currents for now because we've done that in lectures. I guess they'll tell us more about it in later years...

Thanks anyway!