HYDROGEN BONDING IN LIQUID AND SUPERCRITICAL WATER

Density and temperature dependencies of the average number of H-bonds, hb), as well as those of average energy distance i o h)hb, and angle = ZO-H O) 

we may conclude that the increase of temperature from ambient to supercritical affects the characteristics of H-bonding in water much more dramatically than the changes in density from 0.02 to 1.67 g/cm along any supercritical isotherm. Compared to hydrogen bonds in ambient liquid water, H-bonds at 773 K are almost 2kJ/mol weaker, have by 0.1 A longer O H bond distances, and are about 10° less linear. 

We calculated the probability for the largest observed cluster to percolate depending on the hydrogen bond concentration, hb), under various thermodynamic conditions (Churakov and Kalinichev 2000). From these data, the size of a percolating cluster at r = 1.12 is estimated to be 65 molecules. This value divides cluster size distributions into two parts. One of them represents infinite percolating clusters, while another part represents smaller isolated clusters (Fig. 15). 

H-bonded molecular clusters containing open chain-like or tree-like structures are found to predominate over clusters containing cyclic ring-like elements. Average H-bonding angles, ZO O 0 and ZO H-0 , distances, (Ro-o) and h), and 

The dynamic nature of hydrogen bonding and the effects of temperature and density on the persistence of H-bonds in supercritical water were recently studied in MD simulations by Mountain (1995) using the ST2 and RPOL intermolecular potentials, and by Mizan et al. (1996) using the SPC potential and its flexible version. These authors use two different approaches to the estimation of the hydrogen bonding lifetime, but both of 

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Figure 11. Average parameters of hydrogen bonds in liquid and supercritical water.

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