Computational Approach for Determining Molecular Diametersand Access to Pores

Proper use of shape-selectivity effects requires (1) spatially resolved locations of active sites and (2) an optimized ratio between molecular and pore diameters. While effective pore diameters depend on the individual framework stability, in particular at increased temperatures, the diameters of (probe) molecules can be derived on a rather comparable basis. Nevertheless, large discrepancies in describing molecular dimensions are reported. We thus herein provide unified diameters of common probe molecules and reactants that are frequently applied for determining shape-selectivity effects in microporous materials. Our computational approach screens possible conformers of a molecule and yields the smallest diameters and anisotropy descriptors. By applying accepted empirical considerations, we then provide diameters for minimum pores that can be accessed. Now, the access of catalytic sites by different probe molecules can be compared using a consistent diameter and access scale. This is a significant contribution for the spectroscopic and catalytic application of molecules for investigating shape-selectivity effects in catalysis.