H-Bonds in Liquid Water

We may wonder what happens at molecular level when ice, a crystal, melts and becomes a liquid, hi most hquids the correlations of positions and orientations of constituent molecules rapidly fall off when the distance between them increases. Furthermore, correlations between nearby molecules display rapid fluctuations that make the hquid a fluid. Most of these hquids, however, consist of molecules that mainly interact via weak Van der Waals forces, which are, at room temperature, much less directional than H-bonds, the only molecular interactions established by H2O molecules in liquid water. The fnst idea that usually comes to mind is then that a relatively great proportion of H-bonds are broken in liquid water, so that HjO molecules may gain some independence, making these correlations fall off rapidly with distance. Experiments teU us that the proportion of broken H-bonds is much too small in hquid water to be at the origin of its fluidity. We rapidly examine the fnst type of such experiments, thermodynamics, and describe in more detail a more informative type of experiment IR spectroscopy, from which will emerge an image of the H-bond network of hquid water 

As already seen in Ch. 8, WhaUey (1) found that the energy of formation HB of H-bonds in ice is 

This energy is that retrieved when transforming one H-bonded O- H - O group in ice into one free 0-H group. It compares quite well with the H-bond energy of formation of a water dimer, which falls in the vicinity of 21kJmol (2, 3). On the other hand, the heat of fusion of ice, A//j is equal to 

Supposing that the concentration of broken H-bonds (or free 0-H groups) in liquid water is C, and that H-bonds have same energy in water and ice, we write  

Intermonomer bands, due to relative vibrations of H-bonded H2O molecules, both of a translational type (the intermonomer stretching band) and of a rotational type (librations Pg o)  

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By comparing the heats of melting and sublimation one finds that only 13% of H-bonds in ice are broken upon melting A similar result (19%) was recently suggested on the basis of a heuristic density-functional method. Many other estimates of the percentage of broken H-bonds are available in the literature. " These estimates, based both on experimental results obtained by various techniques and on theoretical models, provided values ranging from 2 to 72%, " which are dependent on the definition used for an H-bond in liquid water. 

As already seen in Ch. 4, IR spectroscopy is by far the most precise method to provide information on H-bonds. It is therefore interesting to examine this particular point of the existence of free O H groups, or of broken H-bonds in liquid water. This is the object of Figure 9.1 where the IR absorption spectrum of a 1 jim thick film of liquid water is displayed together... 

The IR spectra of ice and liquid water thus clearly show that the main difference between the H-bond networks of liquid water and ice is an appreciable increase, in liqnid water, of the amplitude of librations. Their especially important role has also been recently emphasized in analyses of femtosecond IR spectra (24). Let us note that this conclnsion does not contradict the usual simplified picture that H-bonds in liquid water are weaker than H-bonds in ice, as great rotational amplitudes of individual H2O molecules imply weakening of the H-bonds. It is more precise, as it points to the mechanism at the origin of this weakening, and will consequently allow a more precise description of the H-bond network of liquid water. It implies that the strengths of H-bonds in liquid water extend over a wide range and rapidly vary with time, at the frequency of librations. 

The present Chapter aims to elucidate our current understanding of H-bonding in liquid water, and is organized as follows. We start with a description of H-bonding in liquid water... 

At r > 273 K(I), the abrupt shifts at cooling of the from 75 to 220 cm and the (Ou from 3,200 to 3,140 cm indicate ice formation. The cooperative col blueshift and (Ojj redshift indicate that cooling shortens and stiffens the 0 H bond but lengthens and softens the H-O bond in the liquid phase, which confirms the predicted master role of the 0 H bond in liquid water. 

Figure 1.2 (a) Structure of hexagonal ice (bond lengtlj 0 --H---0 2.76A, O—H 1 A, O H 1.75 A, angles H-O-H 109.5" [4]), (b) fraction of the molecules involved in various numbers of H bonds in liquid water at 10"C [5]) and (c) hydrogen bond network in liquid water (molecules are shown as points). Reprinted with permission from (5). Copyright 1973 American Chemical Society... 

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