14N NQR and proton NMR study of ferroelectric phase transition and proton exchange in organic ferroelectric (H2-TPPZ)(Hca)2

Abstract

The complete ¹⁴N nuclear quadrupole resonance spectrum has been measured in ferroelectric (H₂-TPPZ)(Hca)₂ using nuclear quadrupole double resonance. The quadrupole coupling tensors are assigned to various nitrogen positions in the crystal structure. Two types of asymmetric N–H⁺⋯N hydrogen bonds are observed in the ferroelectric phase. A slow dynamics influencing the ¹⁴N NQR spectrum and relaxation has been observed in the paraelectric phase. The analysis of the ¹⁴N NQR spectra in the paraelectric phase shows that above T_c each hydrogen bond exchanges between the two types observed in the ferroelectric phase. The change of the type of hydrogen bond is associated with the transfer of protons within the bond.