Rotations of Methane Molecules in Amorphous and Crystalline Hydrates
Yamamuro Group
Methane hydrate has been studied extensively not only as a future energy resource but also from the interest in quantum and classical rotations of methane molecules. The crystalline methane hydrate (c-MH) forms a host water lattice of type I with two dodecahedral (12-hedral) and six tetradecahedral (14-hedral) cages per unit cell (space group: Pm3n), and each cage contains one methane molecule [1]. The chemical formula of c-MH is CH4･5.75H2O. The “amorphous” MH (a-MH) was first prepared with a vapor-deposition method by our group [2]. The radial distribution functions derived from the neutron diffraction data demonstrated that the methane molecules are accommodated in the cage-like spaces even in amorphous structures. It is also known that the a-MH crystallizes into the c-MH around 175 K [3].
In this study, we have investigated the rotations of methane molecules in a-MH and c-MH. Our main interest is how the rotation of methane molecules is affected by the cage (like) structure, e.g., size and symmetry. The a-MH was prepared by depositing the mixture of CH4 and D2O gases onto the substrate cooled at 5 K using a custom-made cryostat [4], and c-MH was obtained by annealing the sample at 170 K under 0.1 MPa of CH4 gas. Their quasi-elastic neutron scattering (QENS) and inelastic neutron scattering (INS) data were collected at 5 to 80 K using AGNES spectrometer at JRR-3, Tokai, Japan.
Figure 1 shows clear QENS broadening due to the rotational motion of CH4 accommodated in a-MH. It is worth noting that the QENS appeared even at 5 K. The data were well fitted to the combination of Delta and Lorentz functions as usual and the relaxation time τ was calculated from the peak width of the Lorentz function. Similar QENS data were observed also in c-MH. Figure 2 shows the Arrhenius plots for both a-MH and c-MH. There is a clear bending of the τ line around 20 K for both samples and the slope (activation energy ΔEa) on the low-temperature side is very small (ΔEa < 0.2 kJ/mol). We guess that there is a crossover between the classical (high-T) and quantum (low-T) rotations around 20 K; there is no temperature dependence of τ in a pure tunneling process. The a-MH has longer τ than that of c-MH. This is probably because the cage space in a-MH is narrower than that of c-MH. Additional neutron diffraction and calorimetric experiments are now going on to explain the QENS data more quantitatively.
Fig. 1. Quasielastic neutron scattering spectra of amorphous methane hydrate. The data from all detectors are summed up. The average Q value is about 1.7 Å−1.
Fig. 2. Arrhenius plots of the relaxation times at Qav = 1.7 Å−1 for the CH4 rotation in amorphous and crystalline hydrates.
References
- [1] D. W. Davidson et al., Nature 311, 142 (1984).
- [2] T. Kikuchi et al., J. Phys. Soc. Jpn. 81, 094604 (2012).
- [3] M. Z. Faizullin et al., Chem. Eng. Sci. 130, 135 (2015).
- [4] O. Yamamuro et al., J. Chem. Phys. 115, 9808 (2001).