The far-infrared spectra of H2O and D2O ices Ih and Ic at 4.3 °K are reported. Many weak features are observed and for ice Ih are shown to correspond well with features in the published Raman spectra. Differences between the spectra of ices Ih and Ic are reported for the first time; they are due to the different crystal structures and can be understood in more detail from published calculations of dispersion curves and densities of translational vibrational states. These results and the Raman spectra provide much information with which to test calculations of the translational vibrations in ice I. Extra lines in powder diffraction patterns of ice Ic have generally been attributed to ice Ih impurity. From the measurements reported in this work it is suggested that this may not be the case.

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The far infrared transmission spectra of the structure II clathrate hydrates of cyclopropane, oxetane, 1,3-dioxolane, tetrahydrofuran, and cyclobutanone at 4.3, 50, and 100 °K have been recorded. The features have been assigned to translational vibrations of the water molecules and rotational vibrations of the enclathrated guest molecules. The ring puckering vibration of cyclobutanone may contribute to the spectrum, but no evidence was seen of that of oxetane. The density of translational vibrational states of the water molecules is similar to, but shows more pronounced features than, that of the structure I hydrates. The force constants for rotational displacements of the guest molecules arise mainly from steric forces, as is the case in the structure I hydrates. The force constants for 1,3-dioxolane are anomalously high in spite of the fact that the Arrhenius activation energy for reorientation of 1,3-dioxolane in the hexakaidecahedral cage is not.

The infrared spectra between 330 and 15 cm-' of the struct~lre I clathrate hydrates of ethylene oxide, cyclopropane, and trimethylene oxide, at 4.3 K are presented. The spectra have an unusually high signal-to-noise ratio made possible by a Michelson interferometer and a silicon bolometer detector which operates at 1.2 K. Rotational vibrations of the guest molecules were observed at 65.0 and 35.6 cm-' for ethylene oxide and at 69 and 50 cni-' for trimethylene oxide. Inter-guest coupling of rotational vibrations is small and the two frequencies are assigned to vibrations about different inertial axes. The resulting force constants are 487 and 264 ferg rad-2 for ethylene oxide and 1190 and 1130 ferg rad-2 for trin~ethylene oxide and are discussed in relation to the barriers to reorientation of the guest molecule. The bands due to thesevibrations are fairly sharp at 4.3 K, but are broad and poorly defined at 100 K. The guest and water vibrations interact predominantly through their transition dipoles, although the main contribution to the force constants of the rotational vibrations is from steric forces. The absorption by the water vibrations above 100 cni-' is very similar for ethylene oxide and cyclopropane hydrates but significantly different for trirnethylene oxide hydrate. Strong objections exist to the obvious interpretations of this differencc which remains uncxplained.

The far-infrared absorption spectrum of the structure I clathrate hydrate of xenon at 4.3 K is reported, with the x-ray powder diffraction pattern of the same compound at 110 K. The cubic lattice parameter is 11.84±0.02 Å at 110 K. The spectrum is similar to that of the isomorphous cyclopropane hydrate, confirming the previous conclusion that the latter is due to the translational vibrations of the disordered water lattice. The results also show, by comparison with the lattice parameters and spectra of the hydrates of cyclopropane, ethylene oxide, and trimethylene oxide, that a large guest molecule lowers the vibrational frequencies of the water molecules by 2.5% to 3% while increasing the lattice parameter, and therefore the lengths of the hydrogen bonds, by 0.5% to 1%.