Bis( N-methylpiperidine betaine) hydrobromide: crystal structure and hydrogen bonding

Abstract

Bis( N-methylpiperidine betaine) hydrobromide, (MPB) 2HBr, was synthesised and its crystal structure has been solved by X-ray diffraction at 100 K and refined to R=0.020. The crystals are monoclinic, space group C2/ c, a=22.885(2), b=7.4498(5), c=10.6896(7) Å, β=92.89(1)°, V=1820.1(2) Å 3, Z=4. The piperidine ring adopts a chair conformation with the N +–CH 2COO·H group in an equatorial and the N +–CH 3 group in an axial position. A pair of betaine molecules are bridged by a proton to form a dimeric cation featuring a very short centrosymmetric hydrogen bond (O⋯O distance 2.445(2) Å). FTIR spectra of (MPB) 2HBr and (MPB) 2HCl show very similar broad absorption in the 1000–500 cm −1 region typical for very short hydrogen bonds. In the second and fourth derivative spectra, the carbonyl band appears as a single band. PM3 calculations for the dimeric cation reproduced a hydrogen bond that is much shorter than in the crystal and slightly asymmetric, while B3LYP predicts O⋯O distance that is comparable to that in the crystal, and corresponding to a symmetrical or asymmetrical hydrogen bond, depending on the conformation of the carboxymethyl substituent (equatorial–equatorial or axial–axial, respectively).