New insights into the cohesive forces of crystal structures

The study focuses on the polyhedranes formed by carbon atoms surrounding hydrogen atoms, in particular their capacity to form stable crystal structures with melting temperatures of up to 400 ºC. According to Santiago Àlvarez, a professor with the Department of Inorganic Chemistry and researcher for the UB's Institute of Theoretical and Computational Chemistry, "In the study we have found that polyhedranes meet various chemical conditions that enable them to exhibit stronger hydrogen interactions than had been thought possible. In particular, we have seen that interaction is strongly favoured by the fact that the carbon atom holding the hydrogen is connected to a large skeleton structure containing more carbon atoms."

The team of researchers carried out a systematic computational study of homopolar hydrogen bonds (CH∙∙∙HC), the name given to the forces that bind polyhedrane structures. The results show that the flatter the surface of the polyhedrane, the stronger the intermolecular interactions in the structure, and the team also observed that the spherical form of the examples studied allows them to establish interactions with neighbouring molecules in multiple directions. "The combination of these factors explains the strong cohesive forces in polyhedranes, which require high temperatures to break the three-dimensional structure and form a liquid," explains Santiago Àlvarez.

"These types of interactions are ubiquitous in the molecular chemistry of organic, organometallic and coordination compounds, and we believe that this widespread presence will require us to reconsider previous studies of aspects such as the relative stabilities of the different crystal structures in a single compound," says Àlvarez. This is an important consideration in the design of synthetic compounds, particularly for the pharmaceutical industry, since each form of a single compound, or polymorph, exhibits different pharmacological properties and industrial patents cover only one of these polymorphs, making the identification of a new form a patentable discovery.