Ring inversion in 4-hydroxy-1-methylpiperidine betaine studied by X-ray, FTIR, 13C CP MAS NMR and DFT calculations

The structure of the 4-hydroxy-1-methylpiperidine betaine has been determined by X-ray diffraction. The crystals belong to the monoclinic system, space group Pc and unit cell parameters a = 8.972(2), b = 9.747(2), c = 9.562(2) Å and β = 96.10(3)o. Two molecules are present in the asymmetric unit. They are in two antipodal chair conformations, in the first (A) with the methyl and hydroxyl groups in the equatorial positions and the CH2COO substituent in the axial one, and in the second (B) with the methyl and hydroxyl groups in the axial positions and the CH2COO substituent in the equatorial one. The molecules A and B form two separated zigzag chains, parallel to the z-axis. In each chain, the molecules are linked by the O–H···OOC hydrogen bonds of 2.723(3) and 2.704(3) Å, in A and B, respectively. The optimized structure of the A conformer, at the B3LYP/6-31G(d,p) approach, is by 2.09 kcal/mol more stable than B. Two resonance signals attributed to the CH3 group in 13C CP MAS NMR spectrum confirm the conformational equilibrium in the solid-state. Two bands at 1631 and 1618 cm−1 in the solid-state FTIR spectrum suggest the presence of two inequivalent COO groups. The calculated FTIR spectra of conformers A and B differ in the frequencies of the O–H and C–H modes.