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 Brønsted 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.