Strength of London dispersion forces in organic structure directing agent – zeolite assemblies

Herein, we study the London dispersion forces between organic structure directing agents (OSDAs) and silica zeolite frameworks (FWs) among the ensemble of possible intermolecular interactions. We consider tetraalkyl-ammonium or -phosphonium OSDAs that are fully templating different silica FWs. Starting with calculations using density functional theory with dispersion correction (DFT-D3), we demonstrate that the interaction energy for dispersion forces between the OSDA and the silica FW is strictly correlated to the number of H atoms in the OSDA, irrespective of the specific structures of OSDAs or FWs. We also show that this correlation is independent of variations in formal charges and thermal motions. All calculations considered – DFT-D3 and ab-initio molecular dynamics with Born-Oppenheimer molecular dynamics (BOMD) scheme undertaken by us, and molecular mechanics (MM) from an accessible database (OSDB) – lead to the same trend. We estimate the energy of these dispersion forces to be ca. -2 kcal.mol-1 per H for efficient H-O contacts. This value is twice the energy of dispersion forces between n-alkane molecules, obtained using the same level of calculations. Such a deviation that might be explained by differences in electron polarizabilities highlights the great attraction existing between alkyl groups and siloxane bonds forming OSDA-zeolite host-guest assemblies.