Crystal and molecular structure of N-methylpiperidine betaine hydrofluoride

A 1:1 complex between N-methylpiperidine betaine and hydrofluoric acid, MPB·HF, has been characterized by single crystal X-ray analysis, FTIR spectroscopy, and DFT calculations. The crystals are orthorhombic, space group Pbca, with a ¼ 11.656(2), b ¼ 9.395(2), c ¼ 16.331(3) A° . The piperidine ring adopts a chair conformation with the CH3 group in an axial and the CH2COO·H group in an equatorial position. In the crystal, the HF molecule is engaged in a very short O· · ·H· · ·F hydrogen bond (O· · ·F ¼ 2.379(1) A ° ) with the proton most likely to reside in a single-minimum potential well shifted towards the F atom. The F2 anion is engaged in three short Nþ· · ·F2 intermolecular electrostatic interactions and in several C–H· · ·F contacts. Four conformers of MPB·HF (two axial and two equatorial) have been examined by the B3LYP/6-31G(d,p) method. Unlike in the crystal structure, the isolated molecule is most stable with axial CH2COO·H group. Powder FTIR spectrum shows a broad and intense absorption in the 2500–600 cm21 region typical for very strong hydrogen bonds. q 2004 Elsevier B.V. All rights reserved