Structural and spectroscopic properties of piperidinium-4-carboxylic acid hydrogen squarate

• Piperidinium-4-carboxylic acid hydrogen squarate in crystal and vaccum is present.
• Hydrogen squarate anion is surrounded by four piperidinium-4-carboxylic acids.
• Hydrogen squarate anions form β-chains.
• PED’s are used for the assignments of IR bands.
• Resonance signals of axial and equatorial ring protons appear as separate multiplets.
• C-13 CP MAS spectra of the title complex and squaric acid are compared.

The 1:1 complex of protonated piperidine-4-carboxylic acid (P4C+H) with hydrogen squarate anion (HSQ−) was prepared and characterized by a single-crystal X-ray diffraction, B3LYP/6-311++G(d,p) calculations, FTIR and NMR spectroscopies. The crystals are orthorhombic, Pbcn space group. The molecules are linked through the OH⋯O hydrogen bonds of 2.723(1) and 2.574(1) Å formed between P4C+H and HSQ−, and between HSQ− anions linked into a β-chain, respectively. The hydrogen squarate anion is surrounded by four P4C+H cations which are involved in the N+H⋯O hydrogen bonds of the lengths from 2.815(1) to 3.183(2) Å. The potential energy distributions (PED), calculated for the optimized structure of the title complex at B3LYP/6-311++G(d,p) level of theory, were used for the assignments of IR bands in the experimental and calculated spectra. The FTIR spectrum was consistent with the X-ray results. The theoretical and experimental IR spectra have been compared. The NMR spectra in an aqueous solution and in the solid-state have been studied.

Piperidine-4-carboxylic acid forms a crystalline proton-transfer complex with squaric acid both in the solid-state and in the gas phase. The PED contributions are used to assign the IR bands. The experimental and theoretical IR spectra of piperidinium-4-carboxylic acid hydrogen squarate are in good agreement.Figure optionsDownload as PowerPoint slide