Structure of the 2:1 complex of 1-piperidineacetic acid and p-hydroxybenzoic acid studied by X-ray, FTIR, 1H, 13C NMR, and DFT methods

The crystals of the 2:1 complex of piperidineacetic acid with p-hydroxybenzoic acid, (PAA)(2).HBA, are monoclinic, space group C2 and Z = 4, a = 29.666(4), b = 6.1208(10), c = 14.200(2) angstrom, beta = 117.755(16)degrees, R = 0.035. The crystals of the complex contain molecules of three types: 1-piperidiniumacetate (zwitterion, ZPAA), protonated piperidineacetic acid (HPAA), and p-hydroxybenzoate anion (AHBA). Two piperidiniumacetate moieties, ZPAA, and HPAA, form a non-symmetric cyclic dimer through two N+-H center dot center dot center dot O-C hydrogen bonds of the lengths of 2.773(3) and 2.820(3) angstrom. The COOH group of the cation HPAA is engaged in the O-H center dot center dot center dot O hydrogen bond of 2.519(2) angstrom with the carboxylate group of p-hydroxybenzoate anion (AHBA), while the COO- group of the zwitterion ZPAA interacts with the OH group of p-hydroxybenzoate anion (AHBA) of the neighboring complex molecule through the O center dot center dot center dot H-O hydrogen bond of 2.589(2) angstrom, which links molecules into infinite chains. The structures of two complexes, denoted as A and B, have been optimized by the B3LYP/6-31G(d,p) method. In both complexes two molecules of ZPAA form a cyclic dimer. In A one molecule of ZPAA interacts with the carboxylic group of HBA via the O center dot center dot center dot H-O hydrogen bond of 2.582 angstrom, without the proton transfer, while in B with the phenolic group of HBA via the O center dot center dot center dot H-O hydrogen bond of 2.629 angstrom. Complex B is more stable than A by 1.55 kcal/mol. The FTIR spectrum shows a broad band in the 3120-2070 cm(-1) region assigned to the nu NH vibrations and a broad absorption in the 1750-800 cm(-1) region corresponding to the short O-H center dot center dot center dot O hydrogen bonds. Three bands: one attributed to the nu C = O and two ones to the nu asCOO vibrations at 1675, 1634, and 1606 cm-1, respectively, are observed in the second-derivative spectrum, which is in agreement with the X-ray results. The H-1 and C-13 NMR spectra have been analyzed to elucidate the complex structure in solution. (C) 2007 Elsevier B.V. All rights reserved.