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

Pyridinium-4-carboxylic acid perchlorate (C6H6NO2 center dot 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 P2(1)/c and cell parameters a = 17.356 (6), b = 13.241 (3), c = 16.161 (7) A, beta = 138.055 (17)degrees. The high-temperature (HT; T = 298 K) structure has space group P2(1)/c and cell parameters a = 5.5046 (11), b = 13.574 (3), c = 11.834 (2) A, beta = 99.35 (3)degrees, 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, beta' = 137.92 (3)degrees and space group P2(1)/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.