¹³C NMR of DIPAC and DIPAB organic ferroelectrics

The diisopropylammonium chloride (C₆H₁₆ClN, DIPAC) and diisopropylammonium bromide (C₆H₁₆BrN, DIPAB) molecular crystals are recently discovered ferroelectrics with sufficiently high spontaneous polarization and Curie temperature. We performed first studies of these crystals by ¹³C NMR. CP MAS spectra were collected within large temperature ranges covering the Curie points. The reconstructive phase transition from the initial orthorhombic P2₁2₁2₁ structure of DIPAB to the monoclinic ferroelectric P2₁ structure leads to an abrupt alteration in the ¹³C spectrum. The ¹³C spectra for DIPAC and DIPAB in the ferroelectric P2₁ phase are quite similar with four lines at lower frequencies, which correspond to the CH₃ groups, and two lines with close chemical shifts, which correspond to two CH groups. The transition into the paraphase leads to gradual reduction of the interline distances in the low-frequency quadruplet and in the doublet. The step-like changes in the interline frequency shifts at this transition indicating its first order. The analysis of the spectrum evolution in the paraphase shows that only a CH group lays in the reflection plane above the P2₁  →  P2₁/m transition, while the second CH group only moves closer to the reflection plane upon further heating.