SELF-LIMITING DISTORTION BY ANTIROTATING HINGES IS THE PRINCIPLE OF FLEXIBLE BUT NONCOLLAPSIBLE FRAMEWORKS

Microporous frameworks formed by TO4 coordination tetrahedra (with T = Si or Al) in zeolites are systems composed of rigid parts, the tetrahedra, and of joint oxygen atoms serving as flexible hinges between the tetrahedra. It is known that these frameworks can be flexible or inflexible (or even rigid). Some of the most open tetrahedral frameworks display a pronounced resistance to collapse, even when undergoing radical changes in the chemistry of their embedded guests (ions or molecules). A study of empirically observed angular distortions in zeolite A (LTA) and a computer simulation show that this property is due to a self-regulating mechanism which allows changes in the dimensions of the framework only within limits set by chemically possible values of the TOT angles of the joint bridging oxygen atoms and which depends solely on the topological, symmetrical, and geometrical properties of the three-dimensional nets underlying the frameworks. In collapsible frameworks the hinges corotate, and compression or tension at all hinges is exerted simultaneously in the same sense. In noncollapsible frameworks the hinges antirotate, and compression at one hinge necessitates tension at another hinge and vice versa. © 1992.