IMPORTANT IR CHROMOPHORES

This difference between hydrogen bonded and free OH frequencies is clearly related to the weakening of the 0-H bond as a consequence of hydrogen bonding. 

Even though the ranges for individual types often overlap, it may be possible to make a definite decision from information derived from other regions of the IR spectrum. Thus esters also exhibit strong C-0 stretching absorption between 1200 and 1300 cm- while carboxylic acids exhibit 0-H stretching absorption generally near 3000 cm-.  

The characteristic shift toward lower frequency associated with the introduction of a, p-unsaturation can be rationalised by considering the Valence Bond description of an enone  

The additional structure C, which cannot be drawn for an unconjugated carbonyl derivative, implies that the carbonyl band in an enone has more single bond character and is therefore weaker. The involvement of a carbonyl group in hydrogen bonding reduces the frequency of the carbonyl stretching vibration by about 10 cm-. This can be rationalised in a manner analogous to that proposed above for free and H-bonded 0-H vibrations. 

Aryl aldehydes or ketones, a, P-unsaturated aldehydes or ketones 

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To a first approximation, the absorption frequencies due to the important IR chromophores are the same in solid and liquid states. 

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