The influence of halide anions on the molecular structure of bis(N-methylpiperidine betaine) monocation with short hydrogen bond

The crystal structure of the 2:1 (basic) salt of N-methylpiperidine betaine (zwitterionic molecule) with hydrochloric acid (MPB)2H·Cl, reveals that the N-methylpiperidine betaine units are hemiprotonated and form a dimeric cation through a short O·H·O hydrogen bond of 2.446(2) Å. The piperidine ring has a chair conformation with the methyl group in the axial and the -CH2COO substituent in the equatorial position. The Me-N+-CH2-COO unit has a gauche conformation. The investigated complex is isomorphous with (MPB)2H·Br and pseudo-isomorphous with (MPB)2H·I. The short hydrogen bond is described by a symmetrical double-minimum potential. The halide anions form weak C-H⋯X− hydrogen bonds with the N+-CH2 and N+-CH3 groups. A linear relation exists between the C-H⋯X− distance and the halide ionic radius. Eight conformers of the [(MPB)2H]+ cation and three conformers of the anion formed by (1-methyl-cyclohexyl)acetate units, [(MCHA)2H]−, were analyzed by the B3LYP/6-31G(d,p) level of theory in order to investigate the effect of the N+⋯O electrostatic interactions on the conformation of MPB and on the short O·H·O hydrogen bond. The effect of the counter-ions on the FTIR and NMR spectra of the (MBP)2H·X complexes is also discussed.