THE INFRARED SPECTRUM OF ICE: TEMPERATURE DEPENDENCE OF THE HYDROGEN BOND POTENTIAL FUNCTION,

The infrared spectrum of ice has been studied in the spectral region from 2500 to 200cm-1 and over the temperature range —180 to —20°C Absorption near 220 cm-1 has been detected. Infrared spectra of H20–D20 ice mixtures show that crystal splittings are not responsible for the observed frequency of v2, the bending mode of H20, in ice. Two temperature effects are noted, one an irreversible change evidently caused by annealing. In addition the absorptions near 1600, 800, and 220 cm-1 of annealed ice shift with temperature reversibly. A simple normal coordinate analysis provides estimates of the potential function for stretching and bending the hydrogen bonds, about 0.16–105 and 0.09–105 dyn/cm, respectively. The temperature dependencies of these force-constants are attributed to the change in hydrogen bond length with temperature. The low bending force constant of H20 in ice is discussed in terms of rehybridization. Recent additions1,2,3,4 to the voluminous literature on the properties of ice have indicated that there remain questions in the interpretation of the infrared and Raman spectra. Giguere and Harvey report a significant temperature dependence of the librational mode, vu near 800cm-1 and discuss the uncertain degree of crystallinity of samples of ice condensed at very low temperatures 1 . Ockman2 confirmed these observations on vL but disagrees on the temperature behavior of the bending mode, v2. Whereas Giguere and Harvey find no change of v2 with temperature, Ockman concludes that v2 shifts downward as the sample is cooled. Hornig, et al., note that the liberational frequency permits a prediction of the bending frequency in ice, 1780cm -1, much higher than the observed value, 1620 cm-1,4. The crystallinity of ice condensed at a low temperature has been studied by calorimetry, X-ray diffraction, and electron diffraction. Hornig, et al., review much of this work4 and observe that ice condensed from the vapor at a sufficiently low temperature (below about —130 to — 140°C) has uncertain form, either being amorphous or a mixture of cubic and hexagonal ice5. Hence it must be regarded a disadvantage that most of their spectra related to ice samples condensed at —190°C. The translational modes of ice are considered to be below 300 cm-1, mostly on the basis of low frequency bands with suitable isotopic shifts detected in Raman studies2. Cartwright6 long ago reported an absorption in the infrared spectrum of ice at 160cm-1. Apparently no other infrared study has been reported which extends below about 250cm-1. We present here spectral evidence of the effect of temperature on the infrared spectra of ice samples, pertaining both to the irreversible changes which accompany annealing and the reversible temperature changes shown by annealed, hexagonal ice. With this experience we felt it desirable to re-examine the spectra of H2O—D2O mixtures as a possible aid in clarifying the assignments. These results permit some interesting amplification of the conclusions drawn by earlier workers1,4,7 concerning the potential function for hydrogen bond deformation. Finally, we present infrared spectra extending to 190cm-1.