IR of amino acids

Leucine

Amino acids exist as zwitterions, so they can be considered both carboxylate salts and amine salts.

COO^- stretch: Frequency is lowered compared to parent acid, since resonance gives the carbonyl much more single-bond character. Strong asymmetric at 1600 and strong symmetric at 1400.

N-H⁺ stretch: Broad stretch at 3300-2600. Ammonium ions absorb to the left of this range, while tertiary amine salts absorb to the right. A broad peak often appears near 2100.

N-H⁺ bend: Occurs at 1610 to 1500. Tertiary amines absorb only weakly.