IR Spec Question

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The infrared absorbtion peak for a carbonyl group is very sharp while that for an OH group is quite broad. Explain the origin of this difference.


Any help would be great.

 

[crazy250]

O-H group can absorb a broader range of frequencies due to a more polar bond.

 

[babinski bob]

actually it has NOTHING to do w/ the polarity and everything to do with the fact that the OH bond is a hydrogen bond donor and acceptor. Hence, it can absorb a very wide range of frequencies depending on the orientation (distance) between it's H-bonding partner.

 

[Rendar5]

it's not polarity. It is due to hydrogen bonding. That cuases a wide variety of differences in the distances and strengths of O-H bonds, both within the same molecule, and the O-H bond between two separate molecules.

C=O peaks are fun and bouncy, but don't really interact with much, so it's goign to have pretty much the same energy regardless of position relative to other molecules.

 

[crazy250]

Originally posted by crazy250
O-H group can absorb a broader range of frequencies due to a more polar bond.

My bad. I'm ******ed.

 

[R_C_Hutchinson]

interesting side note that has helped me on many spectroscopy problems:
if you DO get a non-broad OH peak, it usually means a highly nonpolar, high molecular weight molecule. i remember we once got some spectra and had to figure out a large nonpolar alcohol (dodecanol i think, could be totally misspelled) and that was huge because nobody could assign the OH peak because it looked like a normal, non broad absorbance

 

[Brickhouse]

Hydrogen bonding occurs due to the polarity of the O-H bond. You cannot say that something has to do with H-bonding but not with polarity - they are intimately associated.

 

[babinski bob]

you can get sharp OH peaks in tertiary alcohols or alcohols that are sterically hindered.

 

[Rendar5]

Originally posted by Brickhouse
Hydrogen bonding occurs due to the polarity of the O-H bond. You cannot say that something has to do with H-bonding but not with polarity - they are intimately associated.

While it is true that there is a close association between polarity and hydrogen bonding, not all highly polar species can hydrogen bond (as someone mentioned, tertiary alcohols would have a hard time doing so). The stretched out O-H peak is not an effect of polarity, as polarity in itself cannot cause a wide range of energies for a bond. Hydrogen bonding can.