Water molecular assemblies

T. S. T. liyama, K. Nishikawa, K. Kaneko, Study of the structure of a water molecular assembly in a hydrophobic nanospace at low temperature with in situ X-ray diffraction, Chem. Phys. Lett. 274 (1997)152-158. 

As for the mechanical response of thin lipid films, surface pressure(fl)-surface area(A) characteristics of lipid monolayer at air/water interface have been well studied under quasi-static conditions. It has been established that different phases are observed for the ensemble of lipid molecules in a two-dimensional arrangement, similarly to the gas, liquid, and solid phases and some other intermediate phases as in three-dimensional molecular assemblies. 

We have recently demonstrated the ability of six resorcin[4]arenes and eight water molecules to assemble in apolar media to form a spherical molecular assembly which conforms to a snub cube (Fig. 9.3). [10] The shell consists of 24 asymmetric units - each resorcin[4]arene lies on a four-fold rotation axis and each H2O molecule on a three-fold axis - in which the vertices of the square faces of the polyhedron correspond to the corners of the resorcin[4]arenes and the centroids of the eight triangles that adjoin three squares correspond to the water molecules. The assembly, which exhibits an external diameter of 2.4 nm, possesses an internal volume of about 1.4 A3 and is held together by 60 O-H O hydrogen bonds. 

In this review article, the functions of polymers and molecular assemblies for solar energy conversion will be described including photochemical conversion models, elemental processes for the conversion such as charge separation, electron transfer, and catalysis for water decomposition, as well as solar cells. 

Polymers are attracting much attention as functional materials to construct photochemical solar energy conversion systems. Polymers and molecular assemblies are of great value for a conversion system to realize the necessary one-directional electron flow. Colloids of polymer supported metal and polynuclear metal complex are especially effective as catalysts for water photolysis. Fixation and reduction of N2 or C02 are also attractive in solar energy utilization, although they were not described in this article. If the reduction products such as alcohols, hydrocarbons, and ammonia are to be used as fuels, water should be the electron source for the economical reduction. This is why water photolysis has to be studied first. 

Langmuir-Blodgett was the first technique to provide a practical route for the construction of ordered molecular assemblies. These monolayers, which provide design flexibility both at the individual molecular and at the material levels, are prepared at the water—air interface using a fully computerized trough (Fig. 1). Detailed discussions of troughs (4) and of surface pressure, 7T, and methods of surface pressure measurements are available (3,6). 

As a result many different functioning structures may appear within the transient architecture of liquid water. This makes it possible for liquid water to perform in several roles. Water is thus comparable to a supramolecular assembly, and indeed it has been postulated to act as a template for macromolecular systems (e.g. nucleic acids) which have evolved and have breathed life into non-purposive molecular assemblies. In Table 1 we compare some characteristics and attributes of bulk water and the molecule of H2O. 

One point to address concerns the use of the words s pramolecular and supermolecule. The concept of supramolecular chemistry has become a unifying attractor, in which areas that have developed independently have spontaneously found their place. The word supramolecular has been used in particular for large multiprotein architectures and organized molecular assemblies [1.16]. On the other hand, in theoretical chemistry, the computational procedure that treats molecular associations such as the water dimer as a single entity is termed the supermolecule approach [1.34,1.35]. Taking into account the existence and the independent uses of these two words, one may then propose that supramolecular chemistry be the broader term, concerning the chemistry of all types of supramolecular entities from the well-defined supermolecules to extended, more or less organized, polymolecular associations. The term super molecular chemistry would be restricted to the specific chemistry of the supermolecules themselves. 

The Use of Molecular Assemblies in Water Splitting and Related Reactions 525... 

The Co(III)—C bond in the natural coenzymes is resistant to cleavage in protic solvents. However, the bond length [20] is similar to that in models. Indeed, there appear to be no special corrin ring electronic properties necessary for such water-stable Co—C bonds even Co(III)—CH3 compounds with classical ligands such as ammonia or ethylenediamine have now been discovered [21], Although such non-Bi2-related systems are outside the scope of this review, I believe that the main reason that few such compounds are known lies in the paucity of synthetic routes. Since the Co—C bond, once formed, is relatively inert, such compounds could be used for multiple types of applications such as in molecular assemblies or devices [22], The natural compounds and some models are photosensitive, however [23]. It is this photosensitivity that delayed the discovery of the coenzymes, leading instead to the isolation and characterization of the vitamin [1]. 

Summary The analysis of supramolecular structures containing polymers, and the discussion about the effect of polymeric materials with different chemical structures that form inclusion complexes is extensively studied. The effect of the inclusion complexes at the air-water interface is discussed in terms on the nature of the interaction. The entropic or enthalpic nature of the interaction is analyzed. The description of these inclusion complexes with different cyclodextrines with several polymers is an interesting way to understand some non-covalent interaction in these systems. The discussion about the generation and effect of supramolecular structures on molecular assembly and auto-organization processes is also presented in a single form. The use of block copolymers and dendronized polymers at interfaces is a new aspect to be taken into account from both basic and technological interest. The effect of the chemical structure on the self-assembled systems is discussed. 

Fig. 10 Plots of anisotropy factor g versus the mole fraction of chiral 16 for helical supra-molecular assembly of chiral 16 and achiral 14a in water. (Cited from [66].)...

Although the formation of amphiphilic molecular assemblies in water is facilitated by hydrophobic interactions, micelles and bilayers are available even in... 

To investigate the possibihty of ordered molecular assemblies forming in ionic liquids, the solubilities of glycolipids. Schemes 11.3 and 11.4, were tested. L-Glutamate derivatives 4 (n= 12, 16) were employed because they are insoluble in water, even at a concentration of 1 mM. It was found that these water-insoluble... 

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