Conformational analysis of 1-piperidineacetic acid by X-ray, FTIR and ab initio calculations

The 1-piperidineacetic acid was synthesized as monohydrate and its structure has been determined by X-ray diffraction methods. The crystals are orthorhombic, space group P2(1)2(1)2(1), a = 6.7693(7) b = 10.816(1), c = 11.452(1) Angstrom, Z = 41 R = 0.037. The acid molecule appears in the zwitterionic form with two equivalent carboxylic oxygen atoms. The water molecules link carboxylic groups into infinite chains parallel to the z axis, by O-H...O hydrogen bonds of the lengths 2.85(2) and 175(2) Angstrom. The N+(1)-H proton forms bifurcated hydrogen bond intramolecular with O(1) and intermolecular with O(2') of the length 2.795(2) and 2.775(3) Angstrom, respectively. Five of the most stable conformers of I-piperidineacetic acid and four of its monohydrate were analyzed by B3LYP/6-31G(d,p) calculations. For anhydrous acid, NPA1 conformer with intramolecular N...H-O hydrogen bond is the most stable. The structure of conformer NPA1 is similar to that of the most stable conformer of N,N-dimethylglycine. The zwitterionic form, ZPA1, is stabilized by the electrostatic interaction between the Positively charged N+H and negatively charged oxygen atoms of COO- group. ZPA1 is less stable than NPA1 and the energy difference between them is 23.8 kcal/mol. In the case of monohydrate the difference is only 4.6 kcal/mol. This indicates that addition of water molecule increases the stability of the zwitterionic form of 1-piperidine acetic acid, ZPAW3. (C) 2002 Published by Elsevier Science B.V.