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$O hydrogen bond of 2.446(2) A ° . The piperidine ring has a chair conformation with the methyl group in the axial and the –CH2COO substituent in the equatorial position. The Me–NC–CH2–COO unit has a gauche conformation. The investigated complex is isomorphous with (MPB)2H$Br and pseudoisomorphous with (MPB)2H$I. The short hydrogen bond is described by a symmetrical double-minimum potential. The halide anions form weak C–H/XK hydrogen bonds with the NC–CH2 and NC–CH3 groups. A linear relation exists between the C–H/XK distance and the halide ionic radius. Eight conformers of the [(MPB)2H]C cation and three conformers of the anion formed by (1-methyl-cyclohexyl)acetate units, [(MCHA)2H]K, were analyzed by the B3LYP/6-31G(d,p) level of theory in order to investigate the effect of the NC/O electrostatic interactions on the conformation of MPB and on the short O$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. q 2005 Elsevier B.V. All rights reserved.