Structural phase transitions induced by molecular motions within an (anilinium)(l-tartrate) ionic molecular crystal

Abstract

Ionic molecular crystals of (anilinium+)(L-tartrate−) were prepared in C2H5OH–H2O via Bronsted acid–base reaction between aniline and L-tartaric acid. Strong intermolecular O–H⋯O− hydrogen-bonding interactions between the terminal –COOH and –COO− groups of the L-tartrate− monoanion (L-HTart−) are essential in forming the one-dimensional hydrogen-bonding (L-HTart−)∞ chains. Furthermore, N–H+⋯O− hydrogen-bonding interactions between L-HTart− and the ammonium moiety of anilinium (Ani+) form a π-stacking arrangement of the Ani+ cations that are orthogonal to the (L-HTart−)∞ chains. DSC analysis revealed a two-step behavior with reversible phase transitions at ∼220 and ∼270 K, in which three types of crystal phases (I, T < 220 K; II, 220 < T < 270 K; III, 270 K < T) were identified. Phase I possesses the same acentric space group (P1), while its unit cell volume is half of that of phases II and III, in which the π-stacking periodicity of the Ani+ cations of phases II and III are double that of phase I. Although the conformation of the hydroxy (–OH) groups of L-HTart− of phase II (T = 250 K) was identical to that of phase I (T = 100 K), the rotation of the C–OH bond (activation energy of ∼70 kJ mol−1) was observed for phase III. The thermal fluctuation of the polar –OH group in phase III was dominated by a single minimum-type potential energy curve, as evaluated from the calculations. The temperature- and frequency-dependent dielectric constants suggested a low-frequency thermal fluctuation at temperatures above ∼250 K.