Water reorientation

Garcia-Araez N, Climent V, Feliu JM. 2008. Evidence of water reorientation on model electrocatalytic surfaces from nanosecond-laser-pulsed experiments. J Am Chem Soc 130 3824-3833. 

There are two processes that must cooperate for a successful proton transfer, the basis of proton mobility. The first is water reorientation and then the second is proton tunneling. Hence the rate of proton transfer will be limited by whichever of the two processes is slower. One must therefore suspect that the water reorientation is the rate-determining step in the process of proton transfer (because the tunneling through the barrier has dready been shown to be too fast to be consistent with the mobility observed). 

Thus, in the new theory by CBL, a model was formulated in which the librational properties of water played an important part. Even that was not the whole story. The calculation of the specific rate of water reorientation is a complex task. One cannot consider the reorienting water molecule as an isolated entity. If that were so, then one could work on the only basis of the rate of rotation of a gas molecule and calculate the rate. However, the water molecule is hydrogen bonded to other water molecules within the 3D lattice and therefore the reorientation involves the torsional stretching and breaking of the hydrogen bonds—an attempt that seldom succeeds. 

How Well Does the Field-Induced Water Reorientation Theory Conform with the Experimental Facts ... 

Can the CBL theory predict experimental values The answa- is a resounding yes, and the results justify printing the lengthy story of an advance made in 1956. The rate of this field-induced water reorientation was faster than the rate of the spontaneous thermal rotation, but turned out to be much slower than the proton tunneling rate. Thus, it is thQ field-induced rotation of water that determines the ovo all rate of proton transfer and the rate of proton migration through aqueous solutions. According to the theory, the estimated value of the proton mobility is 28 x 10 and that observed experimentally is 36 x 10 cm s" V ... 

The anomalous decrease in the heat of activation with an increase in temperatnre also follows from the model. An increase in temperature causes increased disorder in the water structure, and consequently there are on avCTage, fewer H bonds to break when water molecules reorient. Since the water reorientation increases, the heat of activation becomes smaller. 

Halle et al. (1981) measured NMR relaxation for solutions of several proteins as a function of frequency and protein concentration. They estimated hydration by use of a two-state fast-exchange model with local anisotropy and with assumed values of the order parameter and several other variables. The hydration values ranged from 0.43 to 0.98 h for five proteins, corresponding approximately to a double layer of water about a protein. The correlation time for water reorientation was, averaged over the set of proteins, 20 psec, about eight times slower than that for bulk water. A slow correlation time of about 10 nsec was attributed to an ordering of water by protein at very high concentration. 

Co-condensed EtOH-water mixtures reveal the formation of distinct EtOH hydrate phases in different temperature domains. A hydrate 1 appears in the 130 K - 163 K range depending on the EtOH content. It is proposed to have a cubic lattice similar to that of the clathrate type I. Hydrate 2 is found to crystallize at 158 K or 188 K-193 K in correlation with the absence or the presence of ice Ic and EtOH content. Its composition seems to correspond to the monohydrate. The deposited solids undergo crystallization 10 K lower in comparison to frozen aqueous solutions. This reflects the remarkable ease with which water molecules initiate molecular rearrangement at low temperature. This seems most likely due to EtOH generating defects that facilitate the water reorientation . This may also reflect the generation of clusters (in the vapour phase before deposition) having a different nature relative to those encountered in the liquid solutions. These unusual structures may have implications in atmospheric chemistry or astrophysics. 

The dynamics of the molecular rotation of 2-pyridone in toluene, carbon tetrachloride, methanol, and water have been investigated at 305 K by 13C and 2H NMR spectroscopy. Both chemical shifts and relaxation times show that it forms stable hydrogen-bonded complexes in methanol and in water, reorienting as a complete unit and taking with it two solvent molecules. These solvated species are stable within the liquid-state temperature range, and reorient according to the hydrodynamic law as indicated by the 14N line width measurements (85MRC460). 

The equations for the heat of adsorption of water on a metal as a function of the separation distance and of the angular orientation of the water will be used in future work to find the optimum free energy of adsorption for anions on these metals. In one theory of ionic adsorption on mercury the separation distance for water was assumed to be equal to the radius of the molecule and the energy difference in primary, adsorbed waters reorienting in the field of the adsorbed ion would be one-half the experimental A77 . It is expected that this work will not only make it possible to extend this theory to other metals but it will improve the assumption of the theory by making more accurate predictions of the changes in number of molecules of water of hydration during adsorption. 

Now we turn to a detailed discussion of rotational motion in water. As already mentioned, the Debye rotational diffusive model was initially widely employed to describe water reorientation. As explained above, it describes the reorientation as an... 

D. Laage and J. T. Hynes, A molecular jump mechanism of water reorientation. Science, 311 (2006), 832-835. 

B. Jana, S. Pal, and B. Bagchi, Hydrogen bond breaking mechanism and water reorientational dynamics in the hydration layer of lysozyme. J. Phys. Chem. B, 112 (2008), 9112. 

Okishiro, K. Yamamuro, O. Tsukushi, I. Matsuo, T. Nishikiori, S. Iwamoto, T. Calorimetric and dielectric studies on the water reorientation in the two-dimensional hydrogen-bond system of Cd(H20)2Ni(CN)4-4H20. J. Phys. Chem. 1997, 101, 5804. 

Water reorientation is usually predicted by theoretical models, but for Ag(lll), MD simulations do not confirm the noticeable increase in water population near a... 

Laage D, Hynes JT (2006b) A Molecular Jump Mechanism of Water Reorientation. Science 311 832-835... 

Balbuena PB, Johnston KP, Rossky PJ, Hyun J-K (1998) Aqueous ion transport properties and water reorientation dynamics from ambient to supercritical conditions. J Phys Chem B 102 3806-3814 Barthel J, Buchner R, Eberspacher P-N, Miinsterer M, 8tauber J, Wurm B (1998) Dielectric relaxation spectroscopy of electrolyte solutions, recent developments and prospects. J Mol Liq 78 83-109... 

Fig. 25.5. Aclivalion energy ( ,) vs. relaxation time t for various materials at 300 K (for symbols see Figs 25.3 and 25.4). Dashed lines Join values corresponding to higher temperature. Polyatomic ion reorientation, water reorientation proton and ion jump domains are shown. The value for liquid H2O (white cross) is shown. The same picture is observed for V2O5. l.bH O gels containing various types of cation (HjO, Li , Na, K, Ba ). In this case intra-site motions give rise to low activated relaxations ...

Quantitative determination of water reorientational and diffusional modes by NMR and 0 tracer experiments on the analogous HUAs provided no correlation with the data for proton conductivity . Instead a good correlation between molecular diffusion and proton conductivity indicated a mechanism in which H2O and HjO are mobile as a whole in the conduction plane without a significant amount of proton transfer between them. The proton conductivity in HUP and HUAs is somewhat higher than that of other monovalent cations in corresponding compounds. This was one of the reasons why molecular diffusion was not... 

In conclusion, the present results show that the entropy of the interfacial water network on Pt(lll) is nearly dominated by the free charge density. The state of maximum disorder corresponds to Om 0, i iipzfc. Then, at om < 0, the entropy of the interfacial water network decreases, due to the fact that the interfacial water molecules achieve a net polarization with the hydrogen end towards the metal. Conversely, at om > 0, the orientation with the oxygen end towards the metal becomes the most favorable. Noteworthy, the fact that water reorientation on Pt(l 11) takes at om 0, E evidences that specific interactions between water molecules and the Pt(l 11) surface play a minor role on the orientational behavior of the interfacial water network. That is, water stmctures with net orientations with the hydrogen and with the oxygen towards the metal seem to experience similar specific interactions with the Pt(lll) surface. These conclusions will be discussed in more detail in Section V.3. 

V. WATER REORIENTATION ON SINGLE-CRYSTAL ELECTRODES FROM NANOSECOND LASER-PULSED EXPERIMENTS. POTENTIAL OF MAXIMUM ENTROPY OF DOUBLE-LAYER FORMATION... 

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