Structure of complex of N-methylpiperidine betaine with p-hydroxybenzoic acid studied by X-ray, FT-IR and DFT methods

Crystal structure of the complex of N-methylpiperidine betaine (N-carboxymethyl-N-methylpiperidinium inner salt, MPB) with p-hydroxybenzoic acid (HBA) has been determined by X-ray diffraction. The crystals are triclinic, space group Pī, with a = 6.1156(5), b = 10.6869(10), c = 12.0320(10) Å, α = 109.55(1)°, β = 95.25(1)°, γ = 99.22(1)°, Z = 2, R = 0.034. Two molecules of p-hydroxybenzoic acid and two molecules of N-methylpiperidine betaine are linked together forming a centrosymmetric dimer, (MPB·HBA)2, by four O–H···O hydrogen bonds of lengths 2.622(1) and 2.617(1) Å, between the carboxylic and hydroxy groups of HBA and both oxygen atoms of the carboxylate group of MPB, respectively. The piperidine ring has a chair conformation with the CH2COO− substituent in the axial position and the CH3 group in the equatorial one. Two parallel aromatic rings in (MPB·HBA)2 are distanced by 3.457 Å. In the crystals the complexes form “islands” related to the neighboring complexes by the inversion centers, weak C–H···O bonds and van der Waals forces. A broad band in the 3100–2400 cm−1 region and two bands attributed to the νCO (1689 cm−1) and νasCOO (1607 cm−1) vibrations in the FT-IR spectrum confirm the structure of the title complex. The two structures of MPB·HBA, denoted as A and B, have been optimized by the B3LYP/6-31G(d,p) method. In A, MPB forms a O–H···O hydrogen bond (2.562 Å) with the carboxylic group of HBA shorter than in the crystals, while in B it interacts with the phenolic group of HBA by a longer O–H···O hydrogen bond (2.661 Å) than in the crystals. Complex A is slightly more stable than B (0.15 kcal/mol).