Guest molecule diffusion

The guest molecules experience different potential depending on the nature and the spatial distribution of the ions and the structural modifications in the aluminosilicate framework associated with the Si-Al substitution. Accordingly, the diffusive process can be different [1], The efficiency of migration of guest molecules depends on several factors the Si/Al ratio, the nature of the extra framework cations, the presence of sorbed water molecules, the temperature, and the sorbate concentration [1]. 

This technique was employed to study the binding dynamics of Pyronine Y (31) and B (32) with /)-CD/ s The theoretical background for this particular system has been discussed with the description of the technique above. Separate analysis of the individual correlation curves obtained was difficult since the diffusion time for the complex could not be determined directly because, even at the highest concentration of CD employed, about 20% of the guest molecules were still free in solution. The curves were therefore analyzed using global analysis to obtain the dissociation rate constant for the 1 1 complex (Table 12). The association rate constant was then calculated from the definition of the equilibrium constant. 

The diffusion of entrapped guest molecules out of dendritic boxes was un-measurably slow over a period of several months. However, Meijer and coworkers have demonstrated that the shape-selective liberation of encapsulated guests could be achieved by removing the closed shell in two steps (Scheme 3) [15]. First, hydrolysis of the tBOC groups of the dense shell with formic acid gave a partly open dendritic box . At this point, small guests such as p-nitroben-zoic acid and nitrophenol could diffuse out of the box by dialysis. Susequent and complete removal of the outer shell by refluxing with 12 AT HC1, lead to the... 

A variety of spin probe methods have also been used to study the morphological features of the nano-channels present within MCM 41, as well as dynamical aspects connected to molecular diffusion in the inner pores,186-188 EPR has been used to investigate the adsorption and interactions of nitroxide-labelled de-ndrimers within porous silica.181 This method allows one to investigate the effective porosity of a solid surface (as a host) which is determined by the accessibility of the host surface to an adsorbed guest molecule. Information on the adsorption and interaction of dendrimers with the porous surface arises from computer-aided analysis of the EPR spectra based on of the well-established procedure proposed by Schneider and Freed.189... 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

The calculations led to predictions of adsorption sites for the nonpolar compounds that are in good agreement with those determined experimentally. The cation site is preferred over the window site. The activation barrier for movement between two cation sites was calculated to be 30 kJ/ mol and that for movement between a cation and a window site 43 kJ/mol. Experimental measurements of activation barriers to diffusion of benzene in faujasites are between 17 and 27 kJ/mol (24). The calculations provide strong support for the mechanism of surface-mediated diffusion for all guest molecules in the limit of infinite dilution and 0 K. The MEPs show that molecules slide along the wall of the supercage, with the plane of the aromatic ring almost parallel to the pore wall. 

Illustrative examples of substances which can behave as porous hosts in one of the above ways are also given. For instance, water readily forms open ice lattices which incorporate guests in clathrate hydrates of types I and II (see later text). Ordinary ice also possesses considerable porosity so that, as shown in Table I, He and Ne can readily diffuse through it. Ice below 0°C is zeolite-like in that it has a permanent, somewhat porous structure which (unlike the open-ice frameworks of the clathrate hydrates) does not require guest molecules for stabilization. 

Thus the boundaries of the enclosures in organized media may be of two types they may be stiff (i.e, none of the guest molecules can diffuse out and the walls do not bend), as in the case of crystals and some inclusion complexes, or flexible (i.e., some of the guest molecules may exit the cavity and the walls of the cavity are sufficiently mobile to allow considerable internal motion of the enclosed molecules), as in the case of micelles and liquid crystals. In these two extremes, free volume needed for a reaction is intrinsic (built into the reaction cavity) and latent (can be provided on demand). 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

Relaxation times T, and T2 depend on the motion of molecules which contain the nuclei (236) and their measurement often leads to the various kinetic parameters for the adsorbed molecules, the knowledge of which is essential for the understanding of the mechanism of many zeolite-mediated processes. The diffusion coefficient of the reactants and products in a catalytic reaction, which can be determined from NMR, is often rate limiting. Relaxation studies can also determine surface coverage by the sorbed species and provide information about the distribution of adsorption energy between the different sites on the surface of a catalyst. For these reasons a great deal of NMR work has been done with adsorbed species in zeolites in the course of the last twenty years. From the applied viewpoint the emphasis is on water and hydrocarbons as guest molecules from the fundamental viewpoint species such as Xe, SF6, H2, CH4, and NH3 are of special interest. 

An excited molecule X can pass into the basic level again. This can be performed by radiation—i.e., by emission of the energy difference as a photon hvx—or by nonradiation by internal quenching where the energy difference is dissipated as thermal energy. Furthermore there is the possibility that the excitation energy is not dissipated by radiation but by interaction with a guest molecule Y. This interaction can be described by diffusion of excitons and/or dipole-dipole resonance. 

Each cavity can contain at most one guest molecule, which cannot diffuse from the cavity. 

Water mobility from molecular reorientation and diffusion. Evidence for the motion of the water molecules in crystal structures is typically provided by XH NMR (Davidson and Ripmeester, 1984). At very low temperatures (<50 K) molecular motion is frozen in so that hydrate lattices become rigid and the hydrate proton NMR analysis suggests that the first-order contribution to motion is due to reorientation of water molecules in the structure the second-order contribution is due to translational diffusion. 2H NMR has been also used to measure the reori-entational rates of water and guest molecules in THF hydrate (Bach-Verges et al., 2001). Spin lattice relaxation rates (fy) have been measured during THF hydrate... 

Non-covalent bonding of guest molecules in the interior of a dendrimer molecule can also proceed dynamically. In micelle-like amphiphilic dendrimers, guests can diffuse into and out of the interior of the dendrimer scaffold via hydrophilic and hydrophobic interactions (Fig. 6.18). 

---