Closure molecules

The synthesis principle we are using is based on the fact that molecules can diffuse into individual channels [3,4,31]. This means that, under appropriate conditions, they can also leave the zeolite by the same manner. In some cases, however, it is desirable to block their way out, so as to stabilize the structure. This can be done by adding a closure molecule after the synthesis has been completed. A variety of closure molecules which seal the channels completely or only partially can be used, depending on the requirement. An example already used is the addition of fluorenone which enters readily but leaves the structure reluctantly. If more complete sealing is needed, molecules bearing appropriate reactive groups can be used. Functionalization of the closure molecules is an option for tuning wettability, refractive index, chemical reactivity, and other properties. 

A particular advantage of the low-mode search is that it can be applied to botli cyclic ajic acyclic molecules without any need for special ring closure treatments. As the low-mod> search proceeds a series of conformations is generated which themselves can act as starting points for normal mode analysis and deformation. In a sense, the approach is a system ati( one, bounded by the number of low-frequency modes that are selected. An extension of th( technique involves searching random mixtures of the low-frequency eigenvectors using Monte Carlo procedure. 

It is not clear exactly when the association illustrated above actually takes place. It is certainly involved by the final ring closure stage, but it seems reasonable to assume that there is some cation-glyme type interaction taking place from the instant of solution. The fact that wrapping occurs in such a way that the two ends of the molecule are held in proximity, allows the reaction to be conducted at much higher concentrations than might otherwise be practical. Tlie evidence for the operation of such a template effect is presented and considered below. 

We should mention here one of the important limitations of the singlet level theory, regardless of the closure applied. This approach may not be used when the interaction potential between a pair of fluid molecules depends on their location with respect to the surface. Several experiments and theoretical studies have pointed out the importance of surface-mediated [1,87] three-body forces between fluid particles for fluid properties at a solid surface. It is known that the depth of the van der Waals potential is significantly lower for a pair of particles located in the first adsorbed layer. In... 

A spectacular application of the acyloin ester condensation was the preparation of catenaries like 11. These were prepared by a statistical synthesis which means that an acyloin reaction of the diester 10 has been carried out in the presence of an excess of a large ring compound such as 9, with the hope that some diester molecules would be threaded through a ring, and would then undergo ring closure to give the catena compound ... 

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