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CSJ Journals

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2018, Vol.91, No.4

669-677

Insight into Structural Demand for Cold Crystallization of a Small Molecule. A Case Study for Schiff Base Compounds that Exhibit Prototropic Tautomerization

Katsunori Iwase ,*1Yasuhiro Toyama ,1Isao Yoshikawa ,2Yasuhisa Yamamura ,3Kazuya Saito ,3 and Hirohiko Houjou *2
1DENSO CORPORATION, 500-1 Minamiyama, Komenoki-cho, Nisshin, Aichi 470-0111, Japan
2Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
3Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
https://doi.org/10.1246/bcsj.20170386

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Abstract

To elucidate the relationship between prototropic tautomerism and the cold crystallization phenomenon, o-hydroxy aryl Schiff base compounds derived from three different toluidine isomers were examined. Comparing their thermal behavior, we confirmed that cold crystallization occurred to give a different crystalline phase compared to the initial state. To elucidate the mechanisms of the cold crystallization of the materials, their thermal, structural, and energetic properties were investigated. The DFT calculation of the molecules in their OH- and NH-forms suggested that the NH-form has higher molecular flexibility, which may prevent normal crystallization upon cooling, compared to the OH-form. Spectroscopic analyses implied that the molecules are present as a mixture of the OH- and NH-forms, and that they efficiently crystallize when a preferable OH/NH ratio is achieved. For this molecular system, we concluded that the equilibrium between NH- and OH-forms controls the supercooling of the liquid as well as nucleation and crystal growth.

figure

Only a subtle difference in molecular structure resulted in striking influence on thermal properties of a Schiff base compound. The event of crystallization was suggested to require an appropriate ratio of OH/NH tautomers and sufficiently low glass transition point so that an efficient nucleation and crystal growth can proceed.

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