In his Nobel Prize lecture, Odd Hassel particularly referred that there are Cl···N charge-transfer interactions which was widely studied by scientists later in cyanuric chloride crystals. As can be seen in figure 1, the molecules of C3N3Cl3 are forming planar layers parallel to the ab-plane. Within the layers, each molecule is linked to six neighbor molecules by halogen bonds. Owing to its outstanding structure, cyanuric chloride can be considered as a model system for studying the structural properties of halogen-bonded crystals under high pressure. Bo Zou’s group from State Key Laboratory of Superhard Materials at Jilin University has made an in-deep research in study the changes of crystal structure and halogen bonds under high pressure. The research result has been published in The Journal of Physical Chemistry B.

Fig.1 The halogen bonds in cyanuric chloride and diamond anvil cell

For studying the changes of cyanuric chloride under high pressure ,this group carried out a high pressure experiment at BSRF，the highest pressure reached in this study was about 30 GPa. Representative X-ray diffraction patterns of cyanuric chloride at various pressures are shown in Fig. 2 (left). With the increase of pressure, it is evident that all the diffraction peaks shifted to higher angles, indicating a decrease in unit cell volume. We note that the shift of the (002) diffraction peak has the largest value, due to the high compressibility along the c-axis. These compressional behaviors could be explained by taking into account the layered crystal packing. The planar cyanuric chloride molecules were connected together by multiple halogen bonds to form a layered structure in the ab-plane, while the main interaction between the different layers was π-π stacking interaction. Therefore, the small compressibility of the a-and b-axes was explained by the strength of the multiple intermolecular halogen bonds between molecules. Because of weak π-π stacking interaction between layers, the c-axis was the most compressible one as expected. As pressure increased, the peaks became broader and less intense and some merged together. From the above observations and analysis, we can conclude that there is no obvious phase transition that occurred up to 30 GPa.

Fig.2(left) Representative X-ray diffraction patterns of cyanuric chloride at elevated pressures; (right) The calculated geometry of halogen bonds and the “fish-scale” arrangement of halogen-bonded networks at 30 GPa

To understand the experimental results, the research group analyzed the changes of the crystal structure and the halogen bond under high pressure based on ab initio calculations. The crystal structure of the cyanuric chloride and the structure of halogen bond are shown in Fig. 2 (right). From the figure, we can see that a halogen bond is always along the b axis, and its bond angel maintains 180 ° under high pressure, while the bond angel of another halogen bond decreases gradually with increasing pressure. This indicates that the cyanuric chloride molecular rotates around the b axis under high pressure, which leads the originally nearly flat-shaped halogen bond network structure to become fish-scale arrangement.

In this study, synchrotron radiation light source provides sufficient experimental data for the stability of crystal structure under high pressure, which demonstrates the halogen bond in the cyanuric chloride crystal is an effective intermolecular interaction, especially the cooperation of multi-halogen bond. This research is important for understanding the changes of weak interaction of halogen bond under high pressure and the stability of halogen bond system under high pressure.

Article:

Kai Wang, Defang Duan, Mi Zhou, Shourui Li, Tian Cui, Bingbing Liu, Jing Liu, Bo Zou^*, and Guangtian Zou, Structural Properties and Halogen Bonds of Cyanuric Chloride under High Pressure，J. Phys. Chem. B, 2011, 115, 4639–4644.