Observation of hydrogen in deuterated methane hydrate by maximum entropy method with neutron powder diffraction

• Published in: The Journal of Chemical Physics
• Main Authors: Hoshikawa, Akinori, Igawa, Naoki, Yamauchi, Hiroki, Ishii, Yoshinobu
• Format: Article in Journal/Newspaper
• Language: English
• Published: AIP Publishing 2006
• Subjects: Physical and Theoretical Chemistry; General Physics and Astronomy; Methane hydrate
• Online Access: http://dx.doi.org/10.1063/1.2215606; https://pubs.aip.org/aip/jcp/article-pdf/doi/10.1063/1.2215606/14793495/034505_1_online.pdf

The crystal structure of deuterated methane hydrate (structure I, space group: Pm3¯n) was investigated by neutron powder diffraction at temperatures of 7.7–185K. The scattering amplitude density distribution was examined by a combination of Rietveld method and maximum entropy method (MEM). The distribution of the D atoms in both D2O and CD4 molecules was clarified from three-dimensional graphic images of the scattering amplitude density. The MEM results showed that there were low-density sites for the D atom of D2O in a particular location within the D2O cage at low temperatures. The MEM provided more reasonable results because of the decrease in the R factor that is attainable by this method. Accordingly, the low-density sites for the D atom of D2O probably exist within the D2O cage. This suggests that a spatial disorder of the D atom of D2O occurs at these sites and that hydrogen bonds between D2O molecules become partially weakened. With regard to the CD4 molecules, there were high-density sites for the D atom of CD4, and the density distribution of the C and D atoms was observed separately in the scattering amplitude density image. Consequently, the C–D bonds of CD4 were not observed clearly because the CD4 molecules had an orientational disorder. The D atoms of CD4 were displaced from the line between the C and O atoms, and were located near the face center of the polygon in the cage. Accordingly, the D atoms of CD4 were not bonded to specific O atoms. This result is consistent with the hydrophobicity of the CD4 molecule. We also report the difference between the small and the large cages in the density distribution map and the temperature dependence of the scattering amplitude density.