Variation of carbon isotopic composition with formation of clathrate hydrate
T. Inoue, A. Kinjo, R. Netsu and T. Ikeda-Fukazawa
Department of Applied Chemistry, Meiji University, Japan

In interstellar molecular clouds, various molecules such as H2O, CO, CO2, CH3OH, H2CO,and NH3 are condensed onto dust grains. Water exists as amorphous ice on the dust grains andis transformed into various structures depending on thermal conditions and compositions ofcoexisting molecules. From the results using transmission electron microscopy and Fouriertransformed infrared (IR) spectroscopy, it was found that the vapor deposited amorphous ice
including CH3OH and CO2 is transformed into clathrate hydrate at around 120 K [1]. Thissuggests that the cometary ice exists as clathrate hydrate. Clathrate hydrates are inclusioncompounds consisting of water molecules and a variety of guest molecules. From infrared
astronomical observation, it was found that the 12C/13C isotopic ratio of comets is larger thanthat of interstellar matter [2]. To investigate the effect of clathrate hydrate formation on
12 13
C/ C isotopic ratio, IR spectra of vapor deposited amorphous ice including 12CO2 and13
CO2 were measured. Using the spectra, the variation process of 12CO2/13CO2 ratio during
warming was analyzed.
Amorphous ice including 12CO2 and 13CO2 was prepared with vapor deposition of mixture of
12
CO2, 13CO2 gases and distilled and degassed water on a substrate of oxygen-free copper at
43 K. In the gas mixture, the 12C/13C isotopic ratios were 4–92, and (12CO2 +13CO2)/ H2O
ratio was 1. The total pressure in the vacuum chamber was kept at about 5.0 × 10 –5 Pa during
the deposition. After the deposition of amorphous ice, the substrate was warmed to 180 K at arate of 1–4 K/min. The IR spectra were measured using a spectrometer (ShimadzuIRPrestage-21) at 2 K intervals during warming. To analyze the sublimation behaviors, thetemperature programed desorption (TPD) curves were also measured using quadrupole mass
spectrometer (Pfeiffer QME220) during warming.
The results show that the wave number of the O–H stretching modes of H2O, 12C=O
asymmetric stretching mode of 12CO2, and 13C=O asymmetric stretching modes of 13CO2
during the warming significantly change at 100±10 K. The wave number of newly appeared
peak at this temperature was close to that of 12C=O asymmetric stretching mode of 12CO2 inclathrate hydrate. Furthermore, a gas release was also observed at this temperature from theTPD curves. These results indicates the formation of CO2 clathrate hydrate at 100±10 K.
From the analyses of integrated intensities of 12CO2 and 13CO2 peaks, it was found that the12CO2/13CO2 ratio increases at around 100 K. These results suggest that the formation ofclathrate hydrate from amorphous ice with warming can be a cause of 12C condensation.
Furthermore, it was found that the changing rate of 12CO2/13CO2 with formation of clathrate
hydrate depends on initial 12C/13C rate in amorphous ice. The present results have important
implications for water states and their role in materials’ evolution in the universe.

References
[1]
[2]
[3]

D. Blake, L. Allamandora, S. Sandford, D. Hudgins, F. Freund, 1991, Science 254, 548
V. Vanysek, J. Rahe, 1978, The Moon and the Planets 18, 441
F. Fleyfel, J. P. Devlin, 1991, J. Phys. Chem. 95, 3811