Rings and strain in pure silica zeolites

The stability of rings in pure silica zeolite frameworks is investigated through a new method. The energetic terms influencing the stability of rings are evaluated through the short-range plus electrostatics corresponding to SiO bonds and OSiO and SiOSi angles. Several atomistic force fields have been evaluated, and a reparametrization of the Vessal-Leslie-Catlow force field has been chosen because of its physical meaning and good structural accuracy. The ring energetics are analyzed and discussed in terms of local strain in the framework and in terms of geometric variables. This methodology allows to differentiate between the strain of rings of a given size in different zeolite structures. In particular, it is found that double four rings (D4Rs) are not necessarily, as previously stated, a strained secondary building unit. The analyses of D4Rs in the topologies AST, BEC, and LTA allow the calculation of its stability in the different structures showing high energy in BEC and LTA and low energy in AST. Implications of these results on nucleation of zeolites are drawn regarding the facility with which D4Rs are inserted in different frameworks.