13.5 : カルボン酸の IR および UV-Vis 分光法

Carboxylic acids are easily detected via IR spectroscopy by identifying two characteristic absorptions formed from C=O and O–H stretching of the –COOH group.

In general, the –C=O group of a monomeric carboxylic acid absorbs at 1760 cm⁻¹. However, dimerization due to hydrogen bonding lowers the stretching frequency to ∼1710 cm⁻¹.

In conjugated systems, as resonance delocalization weakens the C–O double bond, the stretching frequency is further lowered to 1690 cm⁻¹.

Additionally, the O–H bond stretching of the –COOH group generates a broad band at ∼2500–3500 cm⁻¹, masking the C–H stretching peaks.

In UV–Vis spectroscopy, carboxylic acids absorb UV light in the range of 200–215 nm, which corresponds to an n→π∗ transition. It is a weak band with a molar absorptivity of <100.

However, the presence of an extensive conjugation shifts the absorption maxima to a higher wavelength with increased molar absorptivity of 25,000.