POLARIZED INFRARED AND RAMAN-SPECTRA OF DIGLYCINE NITRATE SINGLE-CRYSTAL

Polarized infrared (4000-ca. 350 cm-1) and Raman (4000-10 cm-1) spectra of diglycine nitrate (DGN) single crystal were measured at room temperature; polarized infrared spectra were also measured at several low temperatures (230, 180, 15 K). In the paraelectric phase, the glycine ion pairs were found to be joined by a symmetrical (C(i)) O...H...O hydrogen bond and the nitrate ions exhibited high symmetry (most likely D3h) as a result of effectively free rotation. The ferroelectric phase transition occurs because of inhibition of the rotation of the nitrate ions, with consequent lowering of their symmetry to C1, as a result of increased N-H...O hydrogen bonding interaction with the +NH3 groups of the neighbouring glycine ions. The symmetry of the glycine ion pair also falls to C1 in the ferroelectric phase, but the proton motion in the O...H...O bond does not play a significant role in the phase transition.