The Floppiness of It All: Bond Lengths Change with Atomic Displacement Parameters and the Flexibility of Various Coordination Tetrahedra in Zeolitic Frameworks. An Empirical Structural Study of Bond Lengths and Angles

Abstract

We have evaluated more than 7000 crystal structures of zeolites in terms of the values of their bond-lengths T–O (T = Si, Al, P, Zn, Be, Ge, B, As, Ga, Co) and their variability as well as the flexibility of their bond angles O–T–O. Out of these known crystal structure descriptions of zeolites, we have selected 1179 which have estimated standard deviations of their T–O bond lengths of 0.01 A or less. For the most common bond lengths, we obtain 1.603(11) A for 1323 mean tetrahedral Si–O, 1.736(8) A for 416 Al–O, and 1.522(9) A for 228 P–O. It is unsettling that the spread of each of these values is large: about 0.07 A within the population studied by us. Furthermore, these values disagree by several hundredths of an Angstrom from some of the mean values of Si–O, Al–O, and P–O compiled from nonzeolitic types of compounds. This is at variance with the widespread conviction that such T–O distance values should be relatively constant across different types of inorganic compounds. Ever since Cruickshank (Acta C...