Chapter 12 - Structure Determination: Mass Spectrometry And Infrared Spectroscopy Page 29
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44Carboxylic Acids
The carboxyl group is associated with two characteristic infrared stretching
absorptions which change markedly with hydrogen bonding.
Carboxylic acids exist predominantly as hydrogen bonded dimers in condensed
phases.
The O-H stretching absorption for such dimers is very strong and broad,
-1
extending from 2500 to 3300 cm
.
This absorption overlaps the sharper C-H stretching peaks, which may be seen
-1
extending beyond the O-H envelope at 2990, 2950 and 2870 cm
.
The smaller peaks protruding near 2655 and 2560 are characteristic of the
dimer.
-1
The carbonyl stretching frequency of the dimer is found near 1710 cm
, but is
-1
increased by 25 cm
or more in the monomeric state.
29
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