Crystal and molecular structure of N-methylpiperidine betaine hydrofluoride

Abstract

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=11.656(2), b=9.395(2), c=16.331(3) Å. The piperidine ring adopts a chair conformation with the CH 3 group in an axial and the CH 2COO·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=2.379(1) Å) with the proton most likely to reside in a single-minimum potential well shifted towards the F atom. The F − anion is engaged in three short N +⋯F − 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 CH 2COO·H group. Powder FTIR spectrum shows a broad and intense absorption in the 2500–600 cm −1 region typical for very strong hydrogen bonds.