Polyiodides of amino acids. Trimeric cation of betaine

A new betaine polyiodide, (BetH···Bet···BetH)₃(I₈)(I₃)₄, was synthesized and structurally characterized. The compound crystallizes in the monoclinic space group P2₁/c, featuring a previously unreported trimeric cation of the type (A⁺···A···A⁺), formed by strong O–H···O hydrogen bonds between betaine moieties. The structure also contains four triiodide anions and an (I₈)²⁻ unit composed of an iodine molecule linked to triiodides via halogen bonds. Vibrational spectra further demonstrate the coexistence of I₂ and I₃⁻ species. Diffuse reflectance measurements reveal a narrow bandgap of 1.67 eV (direct transition) and 1.40 eV (indirect transition). These findings broaden the structural diversity of amino acid salts and highlight the role of polyiodides in their optical properties.