Raman Signatures and Thermal Expansivity of Acetylene Clathrate Hydrate.

The vibrational signatures for the υ2 C≡C and υ1 symmetric C-H stretches of acetylene in cubic structure I clathrate, synthesized under ambient pressure, are reported for the first time. The most diagnostic features are at 1966 for υ2 and 3353 cm-1 for υ1, respectively, and are assigned to acetylene trapped in the large 51262 cages. In addition, the υ2 mode for acetylene occupying the small 512 cages is observed at 1972.5 cm-1, a red shift of 1.5 cm-1 from its gas phase frequency. Unit cell parameters and thermal expansion coefficients are determined via powder X-ray diffraction between 195 and 225 K and are found to be in good correlation with previous single crystal data at 143 K. The calculated density for acetylene clathrate is also reported, with values ranging from 0.985 g/cm3 at 195 K to 0.976 g/cm3 at 225 K. These results are relevant for spectral detection of acetylene-containing compounds on planetary bodies, as well as providing additional insights on the thermal behavior and physical properties of acetylene clathrate.