Effects of central protonation on relationships of structures and properties of the Keggin-type <italic>α</italic>-[H<italic><sub>x</sub></italic>W<sub>12</sub>O<sub>40</sub>]<sup>(8-<italic>x</italic>)-</sup> (<italic>x</italic>=1-4): The electronic structures with constraint hydrogen bonds in the cage (Ⅰ)

Abstract

Based on the previous theoretical research, the DFT/O3LYP method has been further used to investigate the geometric and electronic structures of α -[H x W 12 O 40 ] (8- x )- ( x =1-4) influenced by the different central protons. All the title species concerned have been fully optimized in the gas phase at the O3LYP/LanL2DZ theory level, thereafter, NBO analysis were performed on the optimized structures. It has been shown that, central geometric structures of the title Keggin cages were greatly influenced by the central protons, which not only surrounded by the metal-oxygen cage, but also formed different center tetrahedral structures with the four center oxygen atoms. Besides, the central protons have been formed different strength and orientation hydrogen-bonds in the Keggin cage with the four central oxygen atoms. These hydrogen-bonds are the constrained ones caused by the restraining from rigid metal-oxygen cage (~0.7 A diameter). The differences between the central tetrahedrons and the special intramolecular hydrogen-bonds have been fully analyzed and compared based on the bond valence (BV) model. The saturability of central protons in the α -[H x W 12 O 40 ] (8- x )- ( x =1-4) metal-oxygen cage is put forward for the first time. And the maximum number of protons accommodated by the of a -Keggin type metatungstates is supposed to be six. So, the puzzle question how many protons can be accommodated in the α -Keggin cavity is solved scientifically from the theoretic point of view.