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Topic: Explaining the IR stretching frequency of Lactones (cyclic esters)  (Read 664 times)

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Offline workingundergrad1896

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Explaining the IR stretching frequency of Lactones (cyclic esters)
« on: September 15, 2019, 10:38:21 AM »
I'm wondering if anyone has (preferably sourced) explanations for the increased IR stretching frequency of lactones over straight chain esters.

The only published information I can find simply states that an increase in ring strain increases the carbonyl IR stretching frequency. But that (to me) is not an explanation, it's merely just an observation/correlation of two properties. My question is why does an increased ring strain result in an increased IR stretching frequency.

I was taught during lectures that donation of the sigma oxygen lone pair into the carbonyl sigma* results in reduced carbonyl double bond character and thus reduced IR stretch, and this interactions holds esters in the Z-conformation. For cyclic esters, which are held in the E-conformation, this interaction is either interrupted or does not occur for 3/4-membered rings, and thus more double bond character and higher stretching. Fine, makes sense and I understand that.

Something which gives me great doubt over this explanation is that cyclic ketones also show almost identical trends when incorporated into a ring. Ketones do not contain oxygens with sigma lone pairs, and thus there must be another effect at play. Could it simply be the same effect but with hyperconjugation of the C-H sigma orbitals that are alpha to the ketone?

In any case I can't find any decent explanation on this or literature describing it other than ''cuz of ring strain lol''

Any help much appreciated...

Offline Corribus

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Re: Explaining the IR stretching frequency of Lactones (cyclic esters)
« Reply #1 on: September 16, 2019, 10:27:18 AM »
The explanation I was taught was that in strained ring structures, the hybridization of the carbon changes from sp to more p-like (because the geometry approaches 90 degrees). This means the C=O bond contains more s-character as the ring strain is increased, which strengthens/shortens the bond and leads to increased vibrational frequency.

To see if there were any alternative explanations, I first checked Silverstein and Webster (excellent resource, highly recommend to anyone interested in spectroscopic analysis of organic compounds). They put forward the following (p.94):

"In cyclic ketones, the bond angle of the C-(C=O)-C group influences the absorption frequency of the carbonyl group. The C=O stretching undoubtedly is affected by adjacent C-C stretching. In acyclic ketones and in ketones with a six membered ring, the angle is near 120 deg. In strained rings in which the angle is less than 120 deg., interction with the C-C bond stretching increases the energy required to produce C=O stretching and thus increases the stretching frequency. Cyclohexanone absorbs at 1715 cm-1, cyclopentanone absorbs at 1751 cm-1, and cyclobutanone absorbs at 1775 cm-1."

Not particularly informative, unfortunately. So I turned to the interweb and found this paper that might be of interest to you: Galabov and Simov, "The Stretching Vibration of Carbonyl Groups in Cyclic Ketones", Chem Phys Lett. 1970, 5(9), 549.

They discuss the viability of the hybridization explanation as well as an alternative explanation: pure mechanical (kinetic energy) effects, which I interpret to mean that smaller rings physically restrict the amount of motion the various nuclei can possess. The paper concludes that both hybridization and mechanical effects contribute to the shifting vibrational frequencies, but in extremely small rings there must be additional effects at play.

These effects would be expected to influence the vibrational frequencies in other strained ring systems, including alkenes and alkanes. Obviously in lactones you also have the inductive effect of the adjacent oxygen to worry about.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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