Proton trapping and defect energetics in ice from FT-IR monitoring of photoinduced isotopic exchange of isolated D2O

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  • Paul J. Wooldridge
    Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
  • J. Paul Devlin
    Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078

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<jats:p>The photoexcitation of 2-naphthol as a trace impurity in ice results in the injection of excess protons into the ice network. These protons are immobile at temperatures &lt;100 K but warming to ∼120 K generates a near steady-state concentration of mobile protons which decays slowly. This behavior confirms the existence of shallow proton traps in ice which, following Kunst and Warman, are presumed to be intrinsic and, most probably, Bjerrum L defects. The quantity of mobile protons at a given temperature, in a pseudoequilibrium with immobile protons bound to the L defects, is controlled by the temperature coefficients of (a) the pseudoequilibrium constant and (b) the L-defect concentration. Since both the L and D defects are immobile below ∼130 K, the L-defect concentration can be taken to be temperature independent. Consequently, the temperature dependence of the rate at which D2O molecules isolated in H2O cubic ice are converted to (HOD)2 units by mobile protons is a direct measure of the binding energy between the excess protons and the L defects. This binding energy has been estimated at 10.0 kcal/mol. At the completion of each kinetic experiment at T&lt;126 K, the predominant deuterated species is (HOD)2. Such samples are ideal for observation of the ice L-defect activity which is thermally activated by warming to above 130 K. By following the rate of conversion of (HOD)2 to isolated HOD for the range 134 to 150 K, the activation energy for the L-defect formation and mobility has been determined to be 12.2 kcal for cubic ice. This is close to the value of 12.0 kcal previously determined for cubic ice from isotopic exchange rates, but is less than the accepted value for hexagonal ice of 13.1 kcal/mol. Further, the enthalpy change for ice self-ionization has been estimated as 16.8 kcal from a combination of the activation energies for proton transport (9.5 kcal) and L-defect formation (7.8 kcal) with the L-defect–proton binding energy of 10.0 kcal.</jats:p>

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