Guest repulsion

Clathrate hydrates form when small (<0.9 nm) non-polar molecules contact water at ambient temperatures (typically <3(X) K) and moderate pressures (typically >0.6 MPa). On a molecular scale, single small guest molecules are encaged (enclathrated) by hydrogen-bonded water cavities in these non-stoichiometric hydrates. Guest repulsions prop open different sizes of water cages, which combine to form three well-defined unit crystals shown in Figure 1. 

One of the aims of the crystallographic studies is to visualize the spatial conditions of non-H-bond type of interactions. Van der Waals forces (dispersion and exchange repulsion) and polarization are representatives of such interactive forces. They are governed by geometric features such as contact surfaces and volumes of the host and guest matrices. 

Bandyopadhyay and Yashonath (31), in an extension of their work on MD studies of noble gas diffusion, presented MD results for methane diffusion in NaY and NaCaA zeolites. The zeolite models were the same as those used in the noble gas simulations (13, 15, 17, 18, 20, 28, 29) and the zeolite lattice was held rigid. The methane molecule was approximated as a single interaction center and the guest-host potential parameters were calculated from data of Bezus et al. (49) (for the dispersive term) and by setting the force on a pair of atoms equal to zero at the sum of their van der Waals radii (for the repulsive term). Simulations were run for 600 ps with a time step of 10 fs. 

In discussions to this point, no significant interaction between a guest and its medium has been considered. This is probably the case in the reaction cavity model of Cohen [13] as well, since product selectivity was attributed mainly to the presence or absence of free volume within the cavity. The analogy of guests in hosts to balls in boxes is very deficient, but is really not different from the situation in the kinds of crystal systems which first inspired the Cohen nomenclature. Interatomic attraction and repulsion was important in analyzing those systems and was even critical to the crystal engineering used to assemble some of the systems used in the studies by Schmidt and his co-workers [1,48,89]. In addition to being stiff or flexible, cavity walls must... 

Thus far, attention has been focused on the guest molecules in their ground states. This is so because it is relatively easy to predict and visualize the geometry and orientation of molecules within reaction cavities based on attractive and repulsive interactions between ground state guest molecules and the host structure. However, electronic excitation frequently lead to changes in molecular geometry and polarizability [97], For example, it is well known that formaldehyde becomes pyramidal upon excitation and the C—O... 

Comparatively, the walls of a reaction cavity of an inclusion complex are less rigid but more variegated than those of a zeolite. Depending upon the constituent molecules of the host lattice, the guest molecules may experience an environment which is tolerant or intolerant of the motions that lead from an initial ketone conformation to its Norrish II photoproducts and which either can direct those motions via selective attractive (NB, hydrogen bonding) and/or repulsive (steric) interactions. The specificity of the reaction cavity is dependent upon the structure of the host molecule, the mode of guest inclusion, and the mode of crystallization of the host. 

Occupation of hydrate cavities and the hydrate structure is determined to a large degree by the guest size in structures I and II. In structure H both size and shape considerations are necessary for a guest molecule. The repulsive interactions between guests and hosts stabilize the hydrate structure. 

A typical potential m (r) is shown in Figure 5.2. Note that the potential is more negative (high attraction) in the center of the cell, or at some distance from the cell wall, with high repulsion (positive values) at the cell wall. As the guest molecule approaches one wall of the cavity, it is both repulsed by that wall and attracted by the opposite wall, causing it to exist in the center. Recent work in molecular simulation suggests that smaller molecules are located in local minima away from the center, and that the repulsive portion of the potential is more important than the attraction (see the molecular simulation discussion in Section 5.3). 

The compositional dependence of the volume of hydrates is solely in the vo term. The compositional dependence was assumed to be a Langmuir type expression that accounts for a guest molecules repulsive nature with each hydrate... 

Guests larger than ethane cannot fit into the small cage of sll and therefore the repulsive constants are zero. Due to the lack of si compositional data, the repulsive constant for only one of the hydrate cages could be regressed. Due to lack of sH compositional data, the volume of sH hydrates was assumed to be independent of composition (Table 5.5). 

Regressed Repulsive Constants and Guest Diameters for Hydrate Volume... 

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