Methane clathrate: CH4 quantum rotor state dependent rattling potential

• Published in: The Journal of Chemical Physics
• Main Authors: Prager, M., Press, W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: AIP Publishing 2006
• Subjects: Methane hydrate
• Online Access: http://dx.doi.org/10.1063/1.2400031; https://pubs.aip.org/aip/jcp/article-pdf/doi/10.1063/1.2400031/14738066/214703_1_online.pdf

In methane hydrate the dominant peak in the density of states above 3meV represents a rattling mode of the guest molecule CH4 in the large ice cages. This mode shifts from 6.7meV at T=4.5K to T=30K to 7.14meV with conversion of CH4 guest molecules into the tunneling ground state. The less symmetric angular density distribution ΨΨ* in the excited rotational state compared to the ground state allows the methane to fit better in the orientation dependent cage potential surface. This leads to a larger average distance to the cage-forming molecules with a weaker potential and a reduced rattling energy. A two state single particle model with characteristic rattling energies of 5.20meV for pure T-methane and 7.3meV for pure A-methane weighted by the population factors can fit the data.