title: Reversible phase transition of 2-carboxypyridinium perchlorate-pyridinium-2-carboxylate (1/1).
abstract: The title salt, C6H6NO2(+)·ClO4(-)·C6H5NO2, was crystallized from an aqueous solution of equimolar quantities of perchloric acid and pyridine-2-carboxylic acid. Differential scanning calorimetry (DSC) measurements show that the compound undergoes a reversible phase transition at about 261.7 K, with a wide heat hysteresis of 21.9 K. The lower-temperature polymorph (denoted LT; T = 223 K) crystallizes in the space group C2/c, while the higher-temperature polymorph (denoted RT; T = 296 K) crystallizes in the space group P2/c. The relationship between these two phases can be described as: 2a(RT) = a(LT); 2b(RT) = b(LT); c(RT) = c(LT). The crystal structure contains an infinite zigzag hydrogen-bonded chain network of 2-carboxypyridinium cations. The most distinct difference between the higher (RT) and lower (LT) temperature phases is the change in dihedral angle between the planes of the carboxylic acid group and the pyridinium ring, which leads to the formation of different ten-membered hydrogen-bonded rings. In the RT phase, both the perchlorate anions and the hydrogen-bonded H atom within the carboxylic acid group are disordered. The disordered H atom is located on a twofold rotation axis. In the LT phase, the asymmetric unit is composed of two 2-carboxypyridinium cations, half an ordered perchlorate anion with ideal tetrahedral geometry and a disordered perchlorate anion. The phase transition is attributable to the order-disorder transition of half of the perchlorate anions.
authors.affiliation: School of Chemistry and Chemical Engineering, Faculty of Light Industry, Hubei University of Technology, Wuhan, Hubei 430068, People's Republic of China.
authors.forename: Bi-Qin
authors.initials: BQ
authors.lastname: Wang
authors.affiliation: Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan, Hubei 430068, People's Republic of China.
authors.forename: Hai-Biao
authors.initials: HB
authors.lastname: Yan
authors.affiliation: School of Chemistry and Chemical Engineering, Faculty of Light Industry, Hubei University of Technology, Wuhan, Hubei 430068, People's Republic of China.
authors.forename: Zheng-Qing
authors.initials: ZQ
authors.lastname: Huang
authors.affiliation: Synergistic Innovation Center of Catalysis Materials of Hubei Province, Wuhan, Hubei 430068, People's Republic of China.
authors.forename: Yun-Hua
authors.initials: YH
authors.lastname: Zhang
authors.affiliation: Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan, Hubei 430068, People's Republic of China.
authors.forename: Jing
authors.initials: J
authors.lastname: Sun
keywords: 2-carboxypyridinium; crystal structure; differential scanning calorimetry; pyridinium-2-carboxylate; reversible phase transition