Magic Numbers in Silicon Dioxide-Based Clusters

The generation of magic number silica clusters [(SiO 2 ) n O 2 H 3 ] - with n = 4 and 8 by XeCl laser (308 nm) ablation of porous siliceous materials is reported. The production of magic cluster [(SiO 2 ) 4 O 2 H 3 ] - can be enhanced by sample selection and experimental optimization so that it becomes the most prominent species in silica clusters. To study the structure of the magic cluster [(SiO 2 ) 4 O 2 H 3 ] -, we performed structural optimization for the neutral bare cluster (SiO 2 ) 4, the neutral complex cluster (SiO 2 ) 4 O 2 H 4, and the anionic cluster [(SiO 2 ) 4 O 2 H 3 ] - at the HF/6-31G** level. It was found that the ground state of the bare silica tetramer has a linear chain structure whereas a pseudotetrahedral cage-like structural isomer of S 4 symmetry is most stable for the complex cluster (SiO 2 ) 4 O 2 H 4 . The stabilization of the three-dimensional (3D) structure can be attributed to the active participation of the O 2 H 4 group in chemical bonding during cluster formation. Our theoretical calculation and bonding analysis indicate that the magic number anionic cluster [(SiO 2 ) 4 O 2 H 3 ] - might also take a pseudotetrahedral structure similar to (but with a different symmetry) that of the neutral precursor (SiO 2 ) 4 O 2 H 4 as the ground state in which the valence, coordination, and bonding characteristics of all the constituent atoms are nearly fully satisfied.