Crystal and molecular structure of bis(N-methylpiperidine betaine) hydroiodide

The crystal structure of 2:1 complex of N-methylpiperidine betaine with hydroiodic acid, (MPB)(2)HI, reveals that the betaine units are hemiprotonated and form a dimeric cation through a short and symmetric O...H...O hydrogen bond (R(O...O) = 2.447(1) Angstrom). The piperidinium ring has a chair conformation with the methyl substituent in the axial and the carboxymethyl substituent in the equatorial position. The conformation of the betaine frame is identical as in the analogous (MPB)(2)HBr, even though the acidic proton is attached to the opposite carboxylate 0 atom. This remarkable difference can be viewed as a 180degrees rotation of the -COO.H group or as a proton jump. The framework of the I-/H+ centers is largely unchanged but the betaine units swivel around the H+ sites to bring the alternate carboxylate 0 atoms into the O...H...O bridge, resulting in a pseudo-isomorphous relation between these two crystals. In its altered environment, the F anion rests in an enlarged hydrophobic cavity where it accepts a number of more regularly arranged C-H... I hydrogen bonds. Two conformers (I and II) of the [(MPB)(2)H](+) cation were analyzed by the B3LYP/6-31G(d,p) level of theory. In I the N+ atom is closer to the O...H group, while in II it is closer to the C=O group. The calculated O...O distances are shorter than in the crystal. FTIR spectrum of (MPB)(2)HI shows a broad and intense vOHO absorption in the 1500-400 cm(-1) range, which is similar to that of type A acid salts of carboxylic acids and other 2:1 betaine complexes with mineral acids. (C) 2004 Elsevier B.V. All rights reserved.