Vibrational spectra and molecular conformation of ciliatine

Abstract

Vibrational spectra of ciliatine (2-aminoethylphosphonic acid) have been measured in the solid state and in aqueous solutions. Four ionic states of ciliatine have been identified in aqueous solution on the basis of the characteristic PO stretching vibrations of the PO 3H 2, PO 3H − and PO 2− 3 groups, and the NH + 3 and NH 2 deformation vibrations. Normal coordinate treatment has been carried out on the rotational isomers of ciliatine with trans and gauche conformations about the NCCP axis. Spectral changes from aqueous solution to the solid state have revealed that the trans and gauche conformers coexist in aqueous solution, but only the trans conformer persists in the solid state. The CP stretching vibration is sensitive to the conformation about the NCCP axis, giving Raman bands at about 780 cm −1 for the trans conformation and at about 710 cm −1 for the gauche conformation. The temperature dependence of these Raman bands has shown that the trans conformer is always more stable than the gauche in aqueous solutions of different pH values. This indicates that the electrostatic attraction between the NH + 3 and PO 3H − (PO 2− 3) groups is not an important factor for the conformational stabilization of the NCCP axis. Crystals of α- and β-ciliatine are composed of a zwitterionic species, NH + 3CH 2CH 2PO 3H −, with the trans conformation about the NCCP axis. The intermolecular hydrogen bonds OH⋯O and NH⋯O are different between the two crystals. The existence of extraordinarily strong intermolecular OH⋯O hydrogen bonds in crystals of β-ciliatine has been confirmed by the PO stretching Raman band.