Infrared Spectra of the Clathrate Hydrates
Authors: Othen D. A. , Wright P. G. , Bates F. E. , Hendricksen D. K. , Jacobs S. M. , Bertie J. E. 
Published: September 2017

Detailed mid- and far-infrared spectra have been recorded for authenticated samples of several clathrate hydrates with the two main structures, I and II, at temperatures between 150 and 4 K. The systems are complicated, yet a detailed analysis of the many interesting spectral features is required before reliable, detailed information can be obtained. Consequently only rather general conclusions can be drawn at present.

The mid-infrared spectra have been recorded for the structure I hydrates and deuterates of ethylene oxide, trimethylene oxide, and cyclopropane, and the structure II hydrate and deuterate of trimethylene oxide, all at 100 K. The OD stretching vibrations of isolated HDO molecules, vOD (HDO), and of the fully deuterated forms, vOD(D2O), have also been recorded for cyclopropane and trimethylene oxide structure I hydrates at 40 K, and the absorption by the guest molecules has been studied for all of the structure I hydrates at temperatures down to 40 K.

The absorption by the water molecules at 100 K is similar to that in ice, with frequencies that vary in the expected way with the lattice parameter and, hence, the hydrogen bond lengths. The shapes of the vOH(H2O)vOD(D2O), and vR(D2O) bands are essentially the same for structures I and II, while the vOD(D2O) band varies only slightly and depends more on the guest molecule than on the structure. The water absorption changed only slightly when the samples were cooled from 100 to 40 K.

The vOD(HDO) bands provide clear evidence that the distribution of hydrogen bond lengths in cyclopropane hydrate differs from those in ethylene oxide and trimethylene oxide structure I hydrates, even though powder X-ray methods indicate that the three hydrates are isostructural. The difference is attributed to an interaction between the hydrogen bonds and the dipole moment of the guest which is too irregular to be sensed by powder X-ray methods.

Some absorption bands of the guest molecules are visible in the spectra of structure I hydrates and, as expected, fewer are visible in the spectra of structure II hydrates, which have a higher water-to-guest ratio. For both structures more guest bands are visible in the spectra of deuterates since the D2O absorption is weaker than that of H2O. The guest bands are single in most cases, with half-widths of 1 to 5 cm-1 for ethylene oxide and cyclopropane and 5 to 15 cm-1 for trimethylene oxide in its deuterates of both structures. One band of ethylene oxide at 100 K and below is a doublet, and one degenerate and one non-degenerate vibration of cyclopropane yields a doublet at 40 K. This must mean that there are two non-equivalent positions for the guest molecule in the tetrakaidecahedral cage of the structure I hydrate at these temperatures.

Far infrared spectra have been recorded of the structure I hydrates of ethylene oxide, cyclopropane, trimethylene oxide, and xenon, and of the structure II hydrates of trimethylene oxide, cyclopropane, tetrahydrofuran, cyclobutanone, and 1,3-dioxolane, all at 4.3 K. The spectra of the hydrates and corresponding deuterates have enabled the absorption by the rotational vibrations of the guest molecules in the cage to be identified. The absorption above 100 cm-1 by the translational vibrations of the water molecules is significantly different for the two structures, but is rather insensitive to the guest molecule within one structure.

A careful search was made for evidence of the transition in trimethylene oxide structure I hydrate at which the guest molecules partly order, that was detected by Davidson from dielectric studies. No spectral changes due to the transition were detected.

Papers describing this work have been published in Canadian. Journal of Chemistry, Vol. 51, No. 8, 1973, p. 1159-68; Vol. 53, No. 1, 1975, p. 71-75; Vol. 55, No. 10, 1977, p. 1777-85. A further paper is accepted for publication in Journal of Chemical Physics and others arc in preparation.