Proton Mobility in Ice

If extra confirmation of the CBL model is needed, it is given by interpretation of the fact that in pure ice the proton s mobility (not its conductance) is approximately 1(X) times greater than it is in water at the same temperature. Does this means that water molecules turn faster in ice than in water Intuitively, the reverse might be expected. 

Bemal and R. E. Fowler, The Structure of Water and Proton Conduction, /. Chem. Phys. 1 515 (1933). 

Conway, J. O M. Bockris, and H. Lynton, Solvent Orientation Model for Proton Transfer, J. Chem. Phys. 24 834 (1956). 

Erdey Gruz and S. Langyel, Proton Transfer in Solution, in Modem Aspects of Electrochemistry, No. 12, J. O M Bockris and B. E. Conway, eds., p. 349, Plenum, New York (1978). 

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In addition to the freeze-concentration effect, a catalytic role for ice crystals, a favorable orientation of substrate and biocatalyst, the markedly lower dielectric constant of ice compared with water, and the high proton mobility in ice, have been discussed as further factors that possibly influence reactions in frozen systems. In summary, the reverse action of hydrolases provides an attractive alternative to the chemical synthesis of peptides but this approach could also be verified for the synthesis of oligosaccharides and oligonucleotides using glycosidases and ribonu-cleases, respectively11631. 

However, it now seems doubtful whether these spectra of aqueous acids can actually be attributed to the hydronium ion. The detection of a vibrational frequency of 1205 cm " demands a lifetime of at least 1/(3 X 10 ° X 1205) = 3 X 10 s. The observed breadth of the infrared bands suggests a lifetime rather less than this, and measurements of proton mobility in ice lead to an estimate of between 0.8x10 and 1.0 X 10 s for the lifetime of an individual H30 ion. ... 

If, however, a small amount of DgO is included in a crystal made from H2O (or vice versa) then an interesting effect occurs. Because of proton or deuteron mobility the added deuterons rapidly diffuse to form HDO molecules and, since the 0-D stretching vibration frequencies are very different from those of 0-H, there is very little coupling between them. An examination of the 0-D vibration band in a crystal which is mostly HjO therefore gives an indication of the sort of environment found by an isolated deuteron. In most ices the 0-D bands found in this way are fairly broad and featureless, reflecting the continuous variety of average environments created by the proton disorder. In Ice II, however, the... 

The majority of studies of collective proton motion in ice or at interfaces resorted to model systems represented by infinite quasi-one-dimensional hydrogen-bonded chains (Gordon, 1990 Pnevmatikos, 1988 Tsironis and Pnevmatikos, 1989 Weiner and Askar, 1970). The chain consists of two coupled sublattices of alternating mobile protons and heavy anions. Each mobile proton is located between a pair of heavy anions, referred to, as before, as SGs. SGs are considered to be fixed with their tail group but their anionic head groups are allowed to fluctuate about their equilibrium positions. They act as proton relay groups. 

The relatively high proton mobility in water is related to spontaneous release of ballistic protons from H20-H+. The life-time of a ballistic proton in water is related to the period of microwave photons in the range of lOOpsec, as recently verified [23]. A longer range and life-time of proton mobility is anticipated within the ice phase, lacking microwave absorption by dimer precession. 

Eigen pointed out that the mobility of a proton in ice at 0°C is about 50 times larger than in water. In ice, H20 molecules already occupy fixed positions suitable for accepting a proton, so that the proton mobility is directly proportional to the rate of tunnelling. 

The mobilities of H+ and OH- ions in these aqueous mixtures are of some importance but their understanding presents problems. It is recalled that in ice the mobility of H+ dramatically exceeds that of H+ in water, and that in water the proton mobility is dependent on the ability of water molecules to rotate into a configuration which allows ready proton transfer (Hills et al., 1965). It is therefore noteworthy that addition of t-butyl alcohol to H+ in water lowers the proton diffusion coefficient, a trend expected if this co-solvent enhances water-water interactions (Lannung et al., 1974). 

At elevated temperatures, where the electron lifetime was much shorter than the pulse lengths of a few nanoseconds used, a second mobile species could be observed as a slowly decaying after-pulse conductivity component for large pulses. This was attributed to proton conduction with a proton mobility of 6.4 x 10 cm /Vs in H,0 ice and a somewhat lower value in D2O ice. ° In the case of the proton, the mobility was found to have an apreciable negative activation energy of 0.22 eV. The motion and trapping of protons was tentatively explained in terms of an equilibrium between free protons and a proton complexed with an orientational L-defect. °... 

The measurements of conductivities and dielectric constants furnish data for the computation of concentrations of the diflFerent types of defects as a function of solute concentration and of temperature, as well as interpretations in terms of lattice position, thermodynamics, and kinetics of these defects (77, 79). The quantitative evaluation of these measurements depends critically on the determination of the proton mobility, ion concentration, and dissociation constant in pure ice (Table IV) made by Eigen and coworkers (46, 47). 

HF molecules are held to occupy oxygen sites in the ice lattice. Since only one proton is associated with the fiuoride ion, each acid molecule introduces into the lattice one L type valence defect. In addition, HF is held to ionize according to the mass-action law, so that at a given temperature the number of ions increases with the square root of the concentration. Thus, in ice doped with HF there are hydronium ions from the dissociation of both the water molecules and the HF impurities. The negative ions, OH and F", are believed to have a very much lower mobility and are therefore neglected. Table III gives the relations obtained... 

Even though the chemical reaction rates are expected to slow at low temperatures following Arrhenius kinetics, due to freeze concentration, which decreases the distance between reactants and changes the pH of the medium in addition to the possible catalytic effects of ice crystals [60] (and given that proton mobility is higher in ice than in water due to the organized crystal structure), some enzyme-catalyzed chemical reaction rates do increase by orders of magnitude in the frozen state as compared to supercooled state... 

H-bridges in ice extend to a larger sphere than in water (see the following section). The mobility of protons in ice is higher than in water by a factor of 100. 

NMR Measurement of The Residual Water. The residual water obtained in Experiments No. 5 and No. 6 was measured with a broadline NMR spectrometer, in which, proton signal of the water gives a very broad line. The full width at half height of the NMR signal is 15750 Hz and 18000 Hz for samples obtained in Experiments No. 5 and No. 6, respectively. The line width for liquid water is normally less than 5 Hz while the line width for polycrystalline ice is 56000 Hz (39). Therefore, the residual water is expected to have a mobility closer to ice than to liquid water. The wider line given by sample obtained in Experiment No. 6 seems to agree with the expectation that the water is more immobile at higher electrolyte concentration. 

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