Liquid structure coordination number

Clementi, E. Determination of Liquid Water Structure, Coordination Numbers for Ions and Solvation for Biological Molecules, in Lecture Notes in Chemistry, Vol. 2, Springer-Verlag, Berlin, Heidelberg, New York, 1976. 

Volume 2 E.Clementi Determination of Liquid Water Structure. Coordination Numbers for Ions and Solvation for Biological Molecules. 1976. 

E. dementi, Determination of liquid water structure, coordination numbers for ions and solvation for biological molecules, Lecture notes in chemistry, Vol. 2, Springer, Berlin, 1976. 

Table 8.53 shows the main features of XAS. The advantages of EXAFS over diffraction methods are that the technique does not depend on long-range order, hence it can always be used to study local environments in amorphous (and crystalline) solids and liquids it is atom specific and can be sensitive to low concentrations of the target atom (about 100 ppm). XAS provides information on interatomic distances, coordination numbers, atom types and structural disorder and oxidation state by inference. Accuracy is 1-2% for interatomic distances, and 10-25 % for coordination numbers. 

The structure of a simple mixture is dominated by the repulsive forces between the molecules [15]. Any model of a liquid mixture and, a fortiori of a polymer solution, should therefore take proper account of the configurational entropy of the mixture [16-18]. In the standard lattice model of a polymer solution, it is assumed that polymers live on a regular lattice of n sites with coordination number q. If there are n2 polymer chains, each occupying r consecutive sites, then the remaining m single sites are occupied by the solvent. The total volume of the incompressible solution is n = m + m2. In the case r = 1, the combinatorial contribution of two kinds of molecules to the partition function is... 

The rather low coordination in the (100) and (110) surfaces will clearly lead to some instability and it is perhaps not surprising that the ideal surface structures shown in Figure 1.2 are frequently found in a rather modified form in which the structure changes to increase the coordination number. Thus, the (100) surfaces of Ir, Pt and Au all show a topmost layer that is close-packed and buckled, as shown in Figure 1.3, and the (110) surfaces of these metals show a remarkable reconstruction in which one or more alternate rows in the <001 > direction are removed and the atoms used to build up small facets of the more stable (111) surface, as shown in Figure 1.4, These reconstructions have primarily been characterised on bare surfaces under high-vacuum conditions and it is of considerable interest and importance to note that chemisorption on such reconstructed surfaces can cause them to snap back to the unreconstructed form even at room temperature. Recently, it has also been shown that reconstructions at the liquid-solid interface also... 

A consequence of the intermolecular forces is that any molecule in the liquid state has a large coordination number so that the structure has a low energy configuration. In other words there is short-range order in the liquid state. A significant amount of work must be done to remove a molecule from this structure - the latent heat of vaporisation. 

The molecular structure of liquids is best analyzed using the concept of RDF. This is of particular importance in solute-solvent structures as it defines the solvation shells around the solute molecule. Therefore, we analyzed the solvation of the anion F using the RDF between the anion and the oxygen of the water molecules, as shown in Fig. 2. At least three solvation shells are well defined. The integration of these peaks defines the coordination number, or the number of water molecules in each solvation shell. The first shell that ends at 3.15 A with a maximum at 2.65 A has, on average, 6.6 molecules of water. The second shell,... 

Crystalline Silica. Silica exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a liquid. The literature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of silica, there are three main forms at atmospheric pressure quartz, stable below about 870°C tridymite, stable from about 870—1470°C and cristobalite, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the structures are based on Si04 tetrahedra linked in such a way that every oxygen atom is shared between two silicon atoms. The structures, however, are quite different in detail. In addition, there are other forms of silica that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of silicon is six rather than four. 

Raman spectral studies of solutions of metal nitrates in liquid ammonia show a coordination number of four for zinc and mercury, but six for cadmium. Dissolution of Znd2 and InCl3 in a 1 2 ratio in liquid HCN yields [Zn(NCH)6] [InCLJ2 with HCN coordination via nitrogen.137 A Raman study of the compounds [Cd(NH3)6]X2 (X = C1, Br or I) has been reported.138 Structural determinations of A2Zn(NH2)4 (A = Rb or K) reveal monomeric tetrahedral anions.139... 

The only clearly defined manner of describing liquid structure in use at present involves the concept of a set of probability density functions. p . for ascending numbers, ji. of molecules. The function p depends on the vector coordinates n. r .. .. r of n molecules, and... 

Labile cluster liquid water molecules are arranged around a dissolved solute molecule in a prehydrate structure, with essentially the correct coordination number. A conceptual hypothesis exists for clusters growing to larger structures at an interface. 

The density maximum at 4 °C is induced by the sum of two effects. The normal lowering of density with increasing T by the increase of the volume of thermal vibrations and the rise of p with T by an increase of the content of 0F. The structure near the orientation defects is more like non-polar liquids with their densed package and higher coordination number. 

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