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.