Hydrogen bonds, per water molecule

Number of hydrogen bonds per water molecule. Bxistence probabiliLy as defined in the text. Taken from Ref, 200. 

Table V shows the number of hydrogen bonds per water molecule calculated by integrating the goH(r) up to the first minimum, in the conditions of the four different simulations.

Figure 5. Panel a density profiles of water (solid line) and of nitrobenzene (dotted line) at 7= 300K. The liquid/liquid interface is near Z = OA, and there is also a water liquid/vapor interface near Z = -30 A. Panel b Average number of hydrogen bonds per water molecule (solid line, left vertical axis) and the fraction Pjj of unbroken hydrogen bonds (dotted line).

As temperature is decreased from above (say, from 20 °C) towards 0 °C, the number of hydrogen bonds per water molecule increases as the 2- and 3-coordinated water molecules get replaced predominantly by more stable 4-coordinated water molecules. In the process some 5-coordinated water molecules also form. Computer simulation studies show that at 10°C, about 70% of the molecules are 4-coordinated while 3- and 5-coordinated are nearly equally populated at about 14% each. As mentioned above, conversion of 2- and 3-coordinated water molecules to 4-coordinated ones is the main reason for the increase in density on lowering the temperature of water. However, as we approach 4°C, energetic reasons now favor 4-coordinated water molecules over 5- or 6-coordinated water molecules. These higher coordinated... 

In order to form an extended (that is, percolating) network that connects a large fraction of molecules of the entire system, there should be three or more hydrogen bonds per water molecule (unless molecules form large disconnected linear chains, which are unlikely and not seen in liquid water). Since each water molecule can easily form four hydrogen bonds, it can support such a network. Indeed this very ability to form a hydrogen-bond network has always been hypothesized to be the main reason for many anomalies exhibited by water (as shall be discussed later) [1-6]. 

Figure 4. Pressure dependence of the average number of hydrogen bonds per water molecule according to the geometric (G), energetic (E), and combined (E+G) criteria. The corresponding open symbols for ambient water are plotted at P = l(X)0MPa, at which the supercritical density is Ig/cm. ...

In the case of contact of hydrated silica surface with air (water vapor), the structure of adsorbed water is determined not only by the adsorbent surface (i.e., phase boundary of silica/water) but also by the phase boundary of water/air. Appearance of these phase boundaries leads to reduction of the free energy of the interfacial water accompanied by lowering of its freezing temperature. The information on a structure of the adsorption complexes at a surface of oxide adsorbents can be obtained from temperature dependences of chemical shift of protons of interfacial water molecules. It is necessary to take into account that 5h is defined by strength of the hydrogen bonds between water molecule and active surface sites and depends on the amounts of hydrogen bonds per water molecule. 

Fig. 6. (A) The average number of the hydrogen bonds per water molecule in the first shell around various ions. (B) Snapshots of waters in the first (shaded) and second shell (white) around an ion (black). Reproduced with permission from Hiibar, B. et al., /. Am. Chem. Soc. 2002,124,12302. 2002 American Qiemical Society.

In the gas phase, the molecules are too far apart, but hydrogen bonds are also found in ice. Ice has a very ordered structure, with the molecules lining up in an open, hexagonal structure to give four hydrogen bonds per water molecule. As a result, the molecules are slightly further apart in ice than in liquid water, which means that it is less dense than the liquid, and floats on it. Similarly, when you fill an ice cube tray with water, as it freezes, the ice cubes expand above the level of the tray. 

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