Intermolecular interactions of inorganic and organic molecules embedded in zeolite-type materials probed by near-infrared Fourier transform Raman spectroscopy

Near-infrared Fourier transform Raman spectroscopy represents an excellently suited tool to investigate spectroscopically inorganic and organic molecules occluded in zeolite-type materials as well as interactions between them. Two examples are presented: First, insertion compounds of iodine in various microporous SiO₂ modifications (deca-dodecasil 3R, all- silica theta-1 and silicalite-1) are discussed. Intermolecular interaction between the inserted molecules is prevented by occlusion of iodine in the cages of deca-dodecasil 3R, but is allowed in the insertion compounds of hosts with higher pore dimensionalities. The intermolecular coupling is confirmed by an appreciable reduction of the Raman shifts, as observed similarly for liquid and amorphous iodine. The second example deals with pyridine and n-alkylamines (n-propyl-, n-butyl- or n-pentylamine) occluded during synthesis in all-silica ferrierite. Raman spectra reveal for all compounds, regardless of the n-alkylamine used, an interaction between the n- alkylamine and neighboring pyridine molecules, with both amines being located in the ten-membered ring channels. For this reason, it is proposed that bimolecular complexes, consisting of an n-alkylamine weakly bound to a pyridine molecule act as structure-directing agents during synthesis.