Crystal structure and vibrational spectrum of N-methylpiperidine betaine hexafluorosilicate

Abstract

The crystal structure of N-methylpiperidine betaine hexafluorosilicate has been determined by X-ray diffraction at 100K. The complex consists of two protonated betaines and one SiF62− anion. In both cations the piperidinium ring assumes a chair conformation, but the CH2COOH substituent is axial in one cation and equatorial in the other. Two O–H⋯F hydrogen bonds are present in the crystal, R(O⋯F)=2.565(1) and 2.617(1)Å. The FTIR spectrum shows the νOH modes in the 3300–2300cm−1 region and two νCO bands, at 1749 and 1726cm−1. An assignment of the observed bands due to vibrations of the SiF62− ion is proposed. In the crystal, the hydroxy protons in both COOH groups are in syn conformation, while in an PM3-optimized model (II) they are anti.