The crystal and molecular structures of indole-2-carboxylic acid (ICA) have been determined using single crystal X-ray diffraction, infrared spectroscopy and theoretical methods.
The crystals are orthorhombic, space group Pna2 1 , with a ΒΌ 30:144Γ°6Γ A Λ, b ΒΌ 6:466Γ°1Γ A Λ, c ΒΌ 3:819Γ°1Γ A Λ, V ΒΌ 744:4Γ°3Γ A Λ3 and z ΒΌ 2: The structure analysis revealed that two chains of ICA molecules form a planar ribbon, held together by intermolecular O -Hβ’ β’ β’O and N-Hβ’ β’ β’O hydrogen bonds. Both the O -H and N -H groups act as the donors, while the O atom of the carboxylic group is the acceptor of two hydrogen bonds. The carboxylic groups of ICA molecules in two chains are oriented perpendicularly to each other, which leads to formation of the zig-zag pattern of intermolecular hydrogen bond. Interestingly, the molecular layers, separated by about 3.819 A Λin a stack, form a herringbone-like arrangement with the adjacent stacks.
Theoretical studies of the four possible conformers of ICA monomer have been performed with ab initio (HF and MP2) and density functional (B3LYP) methods. The calculated bond lengths and angles of the most stable structure are in good agreement with the corresponding experimental results. The infrared spectrum of ICA in the solid state well supports the results from X-ray analysis.