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Vibrations of ice I and some clathrate-hydrates below 200°K Hardin, Arvid Holger

Abstract

The vibrations of H₂O, HDO and D₂O molecules participating in the hydrogen bonding of vitreous and crystalline solids, and some alkyl halides and halogens encaged in these solids, were studied by infrared spectroscopy between 4.2 and 200°K over the 4000 to l60 cm⁻¹ frequency range. Four kinds of 0-H‧‧‧‧0 hydrogen bonding lattices were investigated, vitreous and annealed (cubic) ice I and vitreous and annealed clathrate-hydrate mixtures. In vitreous ice I the effects on the molecular and lattice vibrations were observed in detail for H₂O between 77 and l80°K during the phase transformation to cubic ice I, and the results of the transformation for HDO and D₂O were recorded. As well, the effects on the molecular and lattice vibrations of H₂0, D₂0, H₂0 (5-9W HDO), and D₂0 (4.00% HDO) cubic ices I were studied during warming from 4.2 to 200°K. Similar studies were made for the vibrations of H₂0, HDO, D₂0 and guest molecules, during the vitreous-crystalline phase transformation of seven clathrate-hydrate mixtures and during warming of the resulting annealed mixtures. For ice I the method involved condensation of the vapour at 77°K, observation of the spectra during warming in stages to 185 ± 5°K, cooling to 4.2°K, and observation of the cubic sample spectra during warming to 200°K. The results were plotted as a function of temperature and were correlated to calculated distances and RMS amplitudes of translation. As well four models for molecular libration were investigated. Three approaches were taken to the clathrate-hydrate problem. In parallel to the ice I method gaseous stoichiometric mixtures were condensed, observed during transformation, cooled to 4.2°K and observed during warm-up. Other gaseous clathrate mixtures were condensed in an isolated sample chamber, to prevent sample fractionation, and treated as before. Finally, low temperature mulls of solid clathrate-hydrate mixtures were prepared and observed at 83 ± 3°K. The results show that on warming the ice I phase transformation occurred between 120 ± 5 and 135 ± 5°K and required, less than 18 minutes at 135 ± 3°K. Weak peaks due to oligomeric H₂O and D₂O units disappeared during annealing, while all hydrogen bonded H₂O molecular modes shifted to lower frequency and all lattice modes shifted to higher frequency. The half-height widths of the composite H₂O band (v₂/2vR) appeared to increase upon annealing and to decrease upon warming while the (VR, VR + vT) and (v₁, v₃, v₁ + vT) bands had the opposite behaviour. This was interpreted, as indicating a weak 2vR band underlying the stronger v₂ absorption near 1600 cm⁻¹. The frequency-temperature dependences of all cubic ice I bands were interpreted on a bilinear, high and low temperature basis (the lattice modes shifted to lower frequency and the molecular modes to higher frequency with increasing temperature). For HDO above 86°K [formula omitted] was 0.200 ± 0.005 cm⁻¹/°K, [formula omitted] was 0.123 ± 0.005 cm⁻¹/°K, the frequencies were "frozen-in" at 80 ± 5°K and 65 ± 5°K and had irregular behaviours between 50 and 70°K. The low temperature dependences were 0.047 ± 0.005 cm⁻¹/°K in both modes. An explanation is given for the apparent displacement of the HDO stretching frequencies from the H₂O and D₂O frequencies. The HDO results also permitted the accurate determination of [formula omitted] as 1921 cm⁻¹/Å and [formula omitted] as 128l cm⁻¹/Å above 150°K and as 8202 cm⁻¹/Å and 6629 cm⁻¹/Å below 100°K. As well, the HDO stretching frequencies gave an anharmonicity which increased from 4.2 to 80°K and then decreased between 80 and 200°K. The clathrate-hydrate mixtures transformed on warming in the temperature range 125 ± 5 to 145 ± 5°K and required less than l8 minutes at 135°K as for ice I. Similarly, the weak oligomeric and guest absorptions disappeared upon annealing. From the comparison of the three sets of "clathrate" results and the behaviour of annealed sample peaks we concluded that cubic ice I and not clathrate-hydrate was probably formed.

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