Interaction between guest molecules

Equation (3) provides a very direct means of assessing the cell theory, as it links the occupancy of the different cavities to the properties of a coexisting vapour phase. In particular, it implies that the occupancy and fugacity should follow the Langmuir isotherm 

This has been tested by using Gibbs Ensemble Monte Carlo (GEMC) simulations to calculate the equilibrium distribution of guest molecules between hydrate and vapour phases as a function of pressure pressures have then been converted to fugacities /using standard thermodynamic integration [43] over the properties of the simulated vapour. 

GEMC simulations (O) and fitted to Eq. (9) (------) small cavities from GEMC 

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Davidson (1971) determined that the most important rotation inhibition interactions between guest molecules (in adjacent cages) were the dipole-dipole interactions, but even they were of minor importance. Within a single cage, both nonpolar and polar guest molecules such as EO, THF, and acetone have only small barriers to rotational freedom, which approximates that in the vapor phase. The rotational freedom is probably due to the fact that the sum of the cage water dipoles effectively cancel near the center of each cage, and even the quadrupolar fields are relatively small. 

From a supramolecular perspective, cyclodextrins offer a preformed cavity of defined size within a water-soluble macrocycle. The interaction between guest molecules, or substituents to the cyclodextrin itself, leads to some useful results. Hydrophobic fluorescent substituents will be self-included by the macrocycle in aqueous solution until they are displaced by guests with even greater affinities [6]. The act of displacement will generate a fluorescent response that lends itself to sensor applications. It is also possible to use the hydrophobic core to induce rotaxane formation. This approach has been used by many groups, for example, Liu has recently used it to prepare water-soluble gold-polypseudorotaxanes that capture fullerenes such as C60 [7]. 

The absorption spectrum of atoms and molecules in low temperature solids is composed of a sharp zero-phonon line and a phonon side band (Table 2.12 ). The phonon side band corresponds to light absorption accompanied by phonon absorption or emission. The absorption shown in bold in Table 2.12 is a zero-phonon line. The sum of the absorption, drawn in a finer line, yields the phonon side band. The phonon side band appears on the higher energy side of the zero-phonon line at low temperatures. A measure of the interaction between guest molecule and host matrix is given by the Debye-Waller factor, DW(T), defined as a function of temperature, T, in Eq. (2.3), using the areas of the zero-phonon line, S0(T), and the phonon side band, SP(T). 

Interactions between guest molecules in adjacent cages are neglected. 

Polar interaction between guest and cyclodextrin molecules, 32 428-432,434-435 Polarized X-ray absorption spectroscopy, 35 15-17... 

The polar interaction between guest and cyclodextrin molecules (44). 

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. 

Figure 21 Changes in the absorption spectrum of ethidium bromide, observed at (a) low and (b) high guest loading, upon intercalation into 0.008% BAZrP. The splitting of the absorption spectrum is due to strong excitonic interactions between EB molecules as a result of the self-assembly of the chromophores. (From Ref. 55. Copyright 1996 Elsevier Publications.)...

This effect of cholesterol on the location of guest molecules has been cited for chlorophyll a (36) and tetracaine (5) both molecules with specific biological function. In the present work we have been able to observe this effect over the physiological ranges of temperature and cholesterol concentration. Future experiments with other molecules would clarify if this property of cholesterol is generally applicable to other systems and, furthermore, if it extends to interactions between two molecules solubilized in a membrane. 

Table 11.1 Calculated energy of intermolecular interactions (kcal/mol) between guest molecules in layered clathrate, in orientations as in Fig. 11.8...

In the absence of guest molecules, the water host framework is unstable. The stability stems from van der Waals interactions between guest and host and is dependent on the extent of occupancy of the cages by the guest. 

Clathrate hydrates are crystalline but nonstoichiometric compounds and all the cages are not always occupied. Clathrate hydrates are stable only when the interaction between guest and water molecules dominates over sum of the unfavorable two terms (1) entropy decrease arising from confinement of guest molecules in small void cages, and (2) free energy for formation of empty clathrate hydrate structure from ice or liquid water. 

Corrections to the last two assumptions have been the most beneficial in improving the theory. Together, the two assumptions (4. no interactions between guests, and 5. host molecules energy being independent of cavity occupation) comprise the ideal solid solution theory, which has been satisfactory for almost 40 years. However, a recent study showed that a 0.5% change in the lattice parameter can change the pressure prediction (at F-lOO bar) by 15%. 

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