Reversible structural phase transition of pyridinium-4-carboxylic acid perchlorate

Pyridinium-4-carboxylic acid perchlorate (C6H6NO2·ClO4) was synthesized and separated as crystals. Differential scanning calorimetry measurement shows that this compound undergoes a reversible phase transition at about 122 K with a heat hysteresis of 1.8 K. A dielectric anomaly observed at 127 K further confirms the phase transition. The low-temperature (LT; T = 103 K) structure has space group P21/c and cell parameters a = 17.356 (6), b = 13.241 (3), c = 16.161 (7) A, β = 138.055 (17)°. The high-temperature (HT; T = 298 K) structure has space group P21/c and cell parameters a = 5.5046 (11), b = 13.574 (3), c = 11.834 (2) A, β = 99.35 (3)°, but can be re-described using new axes a′ = a, b′ = b, c′ = −2a + c, V′ = V to give the cell a′ = 5.5046 (11), b′ = 13.574 (3), c′ = 17.424 (3) A, β′ = 137.92 (3)° and space group P21/c. The associated coordinate transformation is x′ = x + 2z, y′ = y, z′ = z and the associated reflection index transformation is h′ = h, k′ = k, l′ = l − 2h. The relationship between the two cells is 3a, b, c (HT) approximates a, b, c (LT). The crystal comprises one-dimensional hydrogen-bonded chains of the pyridinium-4-carboxylic acid cations and perchlorate anions. A precise analysis of the main packing and structural differences as well as the changes in the intermolecular interactions between the HT phase and the LT phase reveals that the disorder–order transition of the perchlorate anions may be the driving force of the transition, and the hydrogen-bonding effect may contribute to the transition as a secondary parameter.