Hydrogen molecule, anomalous properties

Ludwig s (2001) review discusses water clusters and water cluster models. One of the water clusters discussed by Ludwig is the icosahedral cluster developed by Chaplin (1999). A fluctuating network of water molecules, with local icosahedral symmetry, was proposed by Chaplin (1999) it contains, when complete, 280 fully hydrogen-bonded water molecules. This structure allows explanation of a number of the anomalous properties of water, including its temperature-density and pressure-viscosity behaviors, the radial distribution pattern, the change in water properties on supercooling, and the solvation properties of ions, hydrophobic molecules, carbohydrates, and macromolecules (Chaplin, 1999, 2001, 2004). 

That the molecules in solid and liquid water interact by hydrogen bonding and that hydrogen bonding is responsible for the anomalous properties of water compared with the other hydrides of Group 16... 

Although water is an excellent solvent and the most popular, it has somewhat anomalous properties that come from the hydrogen bonding ability of water to form three-dimensional networks (Fig. 1.2, Section 1.1.3). Large molecules and ions are often difficult to dissolve in water, unless they have hydrophilic site(s). Therefore, water is not suitable as a medium for reactions involving large hydro-phobic molecules or ions. In contrast, most dipolar aprotic solvents are non-struc-tured or only weakly structured and can dissolve many large molecules and ions. This is another major reason why dipolar aprotic solvents are often used instead of water. 

We can anticipate that this change will have a marked effect on the properties of H20 when these are compared with other substances (for example, it clearly will change the relative behaviour of the curves discussed in Figure 21.1 Frame 21) and we later show that our predictions are borne out experimentally by the, so called, anomalous properties of pure water, which are, in turn, largely brought about by the existence of hydrogen bonding in water molecules. 

All of these anomalous properties can be attributed to the polarity and hydrogen bonding ability of the water molecule. Because of the molecular structure, and the ability to act as both hydrogen bond donor and acceptor, the most favourable interactions occur in a tetrahedral... 

The unique features of individual water molecules (discussed in the preceding chapter) give rise to many anomalous properties of liquid water. Commonly attributed to the presence of an extensive hydrogen-bond network, these anomalies teach us a lot more about water itself Anomalies are observed in many properties, ranging from a density maximum at 4°C, the temperature dependence of isobaric specific heat and isothermal compressibility to a host of dynamic properties. Here we discuss some of them, with the emphasis on collective properties that are relevant to our study of complex systems discussed later. Understanding these anomalies is still the subject of considerable research activity. 

One of the apparently anomalous properties is that water is one of the few known substances where the solid form is less dense than the hquid form. The variation of the volume of water with temperature is shown in Figure 5.1. The expansion between -4 and 0 °C is due to the formation of hydrogen-bonded aggregates. Above 4 °C, thermal expansion occurs as O-H bond vibrations become more vigorous, which tend to increase the mean distance between the molecules. 

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