Constrained conformations of 2,2 '-bipyridine occluded in non acidic-MFI zeolites

The diffuse reflectance UV-visible absorption and Raman scattering spectrom etry provide evidence of sorption of 2,2'-bipyridine (bpy, C10H8N2) as inta ct molecules into nonacidic MFI zeolites, silicalite-1, and Al-ZSM-5. The e ffect of the aluminum content n, the nature of the charge-balancing cation Mn+ as well as bpy loading in M-m/n(AlO2)(m)(SiO2)(96-m) (M(m/n)ZSM-5; m = 0, 3, 6; n =1, 2; M = Na+, Zn2+) were examined. The Raman scattering as wel l as the X-ray powder diffraction experiments provide evidence of weak stru ctural deformations of hosts upon bpy sorption. Clear evidence has emerged from Raman and UV-visible spectroscopic experiments to support the contenti on that sorption of bpy in dehydrated MFI type zeolites results in three di fferent bpy conformations according to the aluminum content of the framewor k and the nature of extraframework cations. The modeling investigations usi ng Monte Carlo simulations, molecular mechanics, and molecular dynamics cal culations provide coherent structural support to the interpretation of spec troscopic results. It appears that the size and shape of straight channel i tself of MFI type zeolites does not generate energetic and steric constrain ts to change the trans planar structure of free bpy molecule. Thus, the sor ption of 2,2'-bipyridine into purely siliceous silicalite-l retains the tra ns-planar conformation. The nonbonding interactions between the extraframew ork Na+ cation and occluded bpy in Na(n)ZSM-5 are the main driving forces t hat stabilize the cisoid nonplanar conformer, whereas the coordination bond ing between Zn2+ and N atoms in Zn(3)ZSM-5 requires the cis-planar conforma tion. The spectroscopic and modeling results were found to be in agreement with previous quantum calculations concerning the conformational behavior o f free 2,2'-bipyridine relative to the inter-ring rotation.