Carboxylic acids, hydrogen bonds, bond vibrations

The carboxylic acid OH bands in the region of 3300-2500 cm-l are weak and broad, indicating that hydrogen bonding with the carbonyl may be present.(3) The intense peak at about 1715 cm l is probably due to the C=0 stretching of the carboxylic acid. The peak at 1620 cm l may be due to the C=0 stretch of the carbonyl at position 4 or the C=C stretch of C-2 and C-3, conjugated with the carbonyl, or a combination of these two vibrations. 

O—H Stretching Vibrations In the liquid or solid state, and in CC14 solution at concentrations much over 0.01 M, carboxylic acids exist as dimers due to strong hydrogen bonding. 

Other structures with strong hydrogen bonding, such as /3-diketones, also absorb in the 3300-2500-cm-1 region, but the absorption is usually less intense. Also, the C=0 stretching vibrations of structures such as /3-diketones are shifted to lower frequencies than those observed for carboxylic acids. 

The most obvious feature in the infrared spectrum of a carboxylic acid is the intense carbonyl stretching absorption. In a saturated acid, this vibration occurs around 1710 cm-1, often broadened by hydrogen bonding involving the carbonyl group. In conjugated acids, the carbonyl stretching frequency is lowered to about 1690 cm-1. 

Cyclic dimers of carboxylic acids represent important model systems forming two coupled intermolecular hydrogen bonds [Fig. 15.1 (d), inset of Fig. 15.4]. The linear vibrational spectra of carboxylic acid dimers have been studied in detail, both in the gas and the liquid phase, and a substantial theoretical effort has been undertaken to understand the line shape of their 0-H and/or O-D stretching bands. In contrast, there have been only a few experiments on the nonlinear vibrational... 

---