Crystal and molecular structure of N-methylpiperidine betaine hydrochloride

A 1:1 complex between N-methylpiperidine betaine and hydrochloric acid, MPBH.Cl, has been characterized by single crystal X-ray analysis, FTIR spectroscopy, and DFT calculations. The crystals are monoclinic, space group P2(1)/n, with a = 6.0644(3), b = 13.0220(6), c = 12.7653(7) Angstrom, beta = 101.925(5)degrees. The piperidine ring adopts a chair conformation with the -CH2COOH group in an axial and the-CH3 group in an equatorial position. In the crystal, the Cl-anion is engaged in a mediumstrong hydrogen bond with the COOH group (O-(HCl-)-Cl-... = 2.9503(7) A), in several C-H... Cl- contacts and, additionally, in three N+...Cl- intermolecular interactions. Four conformations (axial and equatorial, both protonated and unprotonated) of MPBHCl were examined by the B3LYP/6-31G(d,p) method. The calculated structure of MPBH.Cl(ax) is very similar to that in the crystal, except the N(1)-C(8)-C(9)-O(1) and N(1)-C(8)-C(9)-O(2) units, which are planar in the crystal but nonplanar in the isolated molecule. Powder FTIR spectra of MPBH-Cl and its deuterated analogue (MPBD.Cl) were measured and assignments of the observed bands to vibrations of the hydrogen bond and to internal vibrations are proposed. (C) 2003 Elsevier Science B.V. All rights reserved.