Structural stability of giant polyoxomolybdate molecules as probed by EXAFS

Recent progress in synthesis of "giant" polyoxomolybdate (POM) molecules that have a structure of hollow spheres or wheels and size of several nanometers in diameter has emphasized the need in their accurate structural investigation. Our temperature-dependent EXAFS studies demonstrated that the first generation POM molecules containing 132 Mo atoms, undergo irreversible structural transformations at ca. 500K, while the recently synthesized second generation POM at ca. 400K. The number of different structural units in these molecules can be accurately obtained from EXAFS analysis and compared with model structure calculations. In our method of analysis, we use theoretical calculations of pair radial distribution functions from the model structures as initial approximations to analyze the structures of these molecules. Our analysis allowed us to identify the new POM structure as a giant sphere, similar to the {Mo₁₃₂} structure but containing 522 Mo atoms. Our results also demonstrate that the structural rigidity of different building blocks of these molecules can be quantitatively probed by EXAFS.