Solving the 80-Year Structure Mystery: Definitive Crystal Structure of TBAB Hydrate Resolved with Synchrotron Radiation

For decades, tetra-n-butylammonium bromide (TBAB) hydrate has been widely used due to its ease of preparation─simply cooling its aqueous solution. However, its precise crystal structure has remained an unsolved mystery for 80 years, hindering both scientists and engineers in their efforts to fully design materials and optimize applications. This highlights the practical complexity and significance of understanding water-based functional materials and underscores the high priority placed on understanding the structuring of water. In this paper, the definitive crystal structure of TBAB·26H₂O hydrate was resolved with synchrotron radiation. The structure was revealed to have a tetragonal lattice with a space group $P\overline{4}{2}_{1}c$ and to incorporate TBA in the dodecahedral water clusters (D cages) activated as a cation holder. The present finding proposes additional options for designing hydrates, and its insights are further applicable to the scaled design of other clathrates, such as clathrates of Group 14 elements, surfactants, and functional polymers.