Crystal and molecular structure of 1-piperidiniumacetic acid chloride studied by X-ray, B3LYP and PM3 calculations

The 1:1 complex of 1-piperidineacetic acid (PAA) with HCl has been studied by X-ray diffraction and by B3LYP/6-31G(d,p) and PM3 calculations. The crystals are monoclinic, space group P21=c; a ¼ 13:268ð1Þ; b ¼ 6:349ð1Þ; c ¼ 11:083ð1Þ A°, b ¼ 99:79ð1Þ8; Z ¼ 4; R ¼ 0:0372: The carboxylate group of PAA is protonated and forms the O–H· · ·Cl2 hydrogen bond with chlorine anion (2.964(1) A ° ). The chlorine anion is also engaged in the Nþ–H· · ·Cl2 hydrogen bond (3.062(1) A ° ) and C–H· · ·Cl2 contacts (3.552(2) and 3.784(2) A ° ), consolidating the layers forming their hydrophilic internal areas. On both sides of the layers there are hydrophobic piperidinium residues having only van der Waals contacts with the next sandwich-like layers. In the optimised by the B3LYP/6-31G(d,p) calculations two protonated molecules of PAA are joined by the O–H· · ·Cl2 and Nþ–H· · ·Cl2 hydrogen bonds forming the cyclic dimer. In the structure predicted by the PM3 method the neutral PAA molecule is engaged in the N· · ·H–Cl hydrogen bond, two such molecules are connected by one O–H· · ·Cl hydrogen bond forming an open dimer. The molecular structures of complexes of simple amino acids with mineral acids are discussed. q 2004 Elsevier B.V. All rights reserved.