The flexibility of the T–X–T hinges between the coordination tetrahedra in various zeolitic frameworks: an empirical structural study

In recent years, a large number of precisely determined crystal structure determinations of zeolites, that is compounds based on open frameworks of coordination tetrahedra, has accumulated. This gives us the opportunity to evaluate them in terms of the flexibility of their bond angles T–X–T which serve as hinges between the coordination tetrahedra in zeolites containing various SiO₄, AlO₄, PO₄ and similarly sized ions. Out of about 7,000 known crystal structure descriptions of zeolites we have selected 1187 which are based on single crystal diffraction studies and have estimated standard deviations of their T–O bond length of 0.01 Å or less. The mean Si–O–Si angle in zeolitic frameworks with T-sites fully occupied by one type of T atoms is 152.9° and these angles range from 127.0° to 180°, the mean for Al–O–Si is 141.9°, ranging from 124.8° to 179.3°. Straight angles Si–O–Si do occur in some zeolites.