Intermolecular cavity effect

Here, however, it is possible to obtain stabilization of the low-dense lattice build-up of bulky molecules via intermolecular adhesion and orientation forces. Molecules with planar structural elements are advantageous in this respect since they are apt to support the lattice aggregate, and at the same time they are able to partition off cavities effectively. It is very convenient to use aromatic units. 

Surface Pressure, Potential, and Fluidity Characteristics for Various Interactions in Mixed Monolayers. It is possible to distinguish various types of interactions which occur in mixed monolayers by measuring the surface pressure, surface potential, and surface fluidity of the monolayers. Deviation from the additivity rule of molecular areas indicates either an interaction between components or the intermolecular cavity effect in mixed monolayers. 

Intermolecular Cavity Effect. Figure 5a shows the general characteristics of mixed monolayers in which the "intermolecular cavity effect ... 

Hydrocarbon-Hydrocarbon Interaction. Figure 5c shows the general characteristics of mixed monolayers in which hydrocarbon-hydrocarbon interaction occurs—e.g., trimyristin-myristic acid monolayers (16). The average area per molecule shows a deviation, whereas the surface potential per molecule follows the additivity rule. Hydrocarbon-hydrocarbon interaction also increases the cohesive force in the lipid layer and therefore reduces the fluidity of the mixed monolayer. It is evident from Figures 3a and 3c that surface fluidity is the only parameter which distinguishes an intermolecular cavity effect from hydrocarbon-hydrocarbon interaction. 

If at a surface pressure, tt, the average area per molecule of two surfactants in their individual monolayers is Ai and A2, then in the mixed monolayer (1 1 molar ratio) of these two surfactants, the average area per molecule should be (Ax + A2)/2 at the same surface pressure provided the surfactant molecules occupy the same area in the mixed mono-layer as they do in their individual monolayers (8,9). However, in many cases, the average area per molecule in a mixed monolayer is greater or smaller than that expected from the simple additivity rule (10, 11, 12). A reduction in the average area/molecule in a mixed monolayer can be attributed to the molecular attraction between the surfactants or to the intermolecular cavity effect (9,13). An expansion in the average area/... 

An additional example of a cycloamylose-induced rate acceleration which may be reasonably attributed to a conformational effect is the facilitation of the transfer of the trimethylacetyl group from the phenolic oxygen of 9 to the aliphatic oxygen of the adjacent hydroxymethyl group to form 10. This intramolecular transesterification is remarkably enhanced relative to a comparable intermolecular reaction,6 and occurs, at pH 7.0 and 25.5°, with a rate constant of 0.0352 sec-1 (Griffiths and Bender, 1972). An even larger rate enhancement is achieved upon inclusion of this material within the cyclohexaamylose cavity—fc2 = 0.16 sec-1. This fivefold acceleration cannot be satisfactorily explained either by a microsolvent effect which would be expected to depress the rate of the reaction or, at this pH, by covalent... 

These expressions depend on approximating the effects of sorbate-sorbate interaction in a multiply occupied cavity as a reduction in free volume because of the finite size of the molecules (factor (1 — sfi/v)), together with a decrease in the average potential energy from the intermolecular attraction (exponential term). 

This is a crude assumption. However, it appears that a quantum picture of discrete rotational lines, placed in the submillimeter wavelength range (ca. from few to 150 cm-1), is essentially determined by a form of a molecule only for a gas. In the case of a liquid, discrete spectrum is not revealed, since separate rotational lines overlap due to strong intermolecular interactions, which become of primary importance. So, due to these interactions and the effect of a tight local-order cavity, in which molecules reorient, the maximum of the absorption band, situated in the case of vapor at 100 cm-1, shifts in liquid water to... 

The intermolecular parameters, eA and isolated pair of the guest molecules and can be evaluated by well-known methods, while q and ctq are characteristic of the quinol lattice, c is always associated with Z, the effective number of the elements in the wall of a cavity, and Z(coA)1/2 can be treated as a single parameter, which, together with ctq, must be fixed by using a suitable experimentally determinable property. The theory can then be quantitatively tested by calculating other properties and comparing the values obtained with the experimental ones. 

Accurate solvation procedures can be found in the family of continuum methods as well as in that of discrete methods. Now, the number of cases in which the application of methods belonging to the two families has given very similar results (with a good agreement with experimental data) is large. Continuum methods must take into account all the components of G, and they must use a realistic description of the cavity. Ad hoc parametriza-tions of cavities of simpler shapes such as to reproduce, for example, the desired value of an energy difference, often lead to considerable deformations of the reaction potential, and thus of the solute properties, which the interpretation of the phenomenon depends on. On the other side, discrete methods depend on the quality of the intermolecular potential as well as of the simulation procedure both are critical parameters. The simulation should also include the solvent electronic polarization, or some estimates of its effect. 

Although neutral methanol and ammonia are more stable in vacuo than their ions, the reaction field is capable of inverting this gap. At 3.0A as the spherical cavity radius, the diionic form becomes more stable. The tetrahedral substrate can approach the dyad to a shorter distance than the planar substrate. The repulsive barrier occurs at distances shorter than 2.5A for the planar, but only at 2.0A for the tetrahedral. The tetrahedral substrate is more stabilized by the reaction field effect than the planar substrate, due to an increase in the in-vacuo dipole moment of the tetrahedral. The reaction field is supposed to mimic the protein surrounding, and it is proposed that the protein stabilizes the diionic form even though the simulation of the reaction field is not sufficient to obtain a realistic interpretation. This study indicates a tendency to tetrahedralization of the model substrate at distances characteristic of the Michaelis-Menten complex formation. The authors believe that this must affect intermolecular interactions of large substrates. 

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