Effect of Cage Occupancies on Molecular Vibrations of Methane in Structure H Clathrate Hydrate: Ab Initio Molecular Dynamics Simulation,The Journal of Physical Chemistry B

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Effect of Cage Occupancies on Molecular Vibrations of Methane in Structure H Clathrate Hydrate: Ab Initio Molecular Dynamics Simulation
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2024-06-04 , DOI: 10.1021/acs.jpcb.4c01790
Ken Yoshida _{1,

2} , Shinnosuke Suhara ₁ , Naoki Noguchi ₁

Affiliation

The structure H (sH) of methane hydrate, which has a distinctive structure with large (LL) cages capable of encapsulating multiple methane molecules, has been suggested as a methane reservoir in large icy bodies such as Titan, making it important in planetary science. This high-pressure phase, which exists in the GPa range, lends itself to the study of methane states and dynamics using powerful experimental techniques such as IR and Raman spectroscopy. However, the interpretation of the vibrational spectra of methane in the sH structure has been challenging because of the spectral complexities. The signals attributed to the methane molecules in the LL cage, as well as those of the other two cage types, overlap in the spectra. In this study, we investigated the microscopic origins of the shape of the C–H stretching vibration spectrum of methane in the LL cage using ab initio molecular dynamics (AIMD) simulations. For a single methane molecule in the LL cage, the ν₃ band of the C–H stretching mode was observed at a higher frequency typical of isolated molecules in vacuum due to the large size of the LL cage. As the number of methane molecules in the LL cage increased beyond one, a tendency to blue-shift with increasing methane occupancy was observed, consistent with a loose-cage–tight-cage model. By characterizing the time correlation function of methane stretching vibrations based on the solvation number of methane and water molecules proximal to methane within the LL cage, we showed that the complicated spectral line shape observed in cases of higher methane occupancy in the LL cage resulted from the wider variation of the solvation shell states. Analysis of the solvation structures of the AIMD trajectories provided interpretations of the experimental spectral line shape, demonstrating the complementary nature of AIMD to the experiment and its effectiveness in analysis.

中文翻译：

笼占据对结构 H 笼形水合物中甲烷分子振动的影响：从头算分子动力学模拟

甲烷水合物的 H (sH) 结构具有独特的结构，具有能够封装多个甲烷分子的大型 (LL) 笼，已被认为是泰坦等大型冰体中的甲烷储层，这使其在行星科学中发挥着重要作用。这种存在于 GPa 范围内的高压相有助于使用红外和拉曼光谱等强大的实验技术来研究甲烷状态和动力学。然而，由于光谱的复杂性，sH 结构中甲烷振动光谱的解释一直具有挑战性。 LL 笼中甲烷分子的信号以及其他两种笼类型的信号在光谱中重叠。在这项研究中，我们利用从头算分子动力学 (AIMD) 模拟研究了 LL 笼中甲烷 C-H 伸缩振动谱形状的微观起源。对于 LL 笼中的单个甲烷分子，由于 LL 笼的尺寸较大，在真空中孤立分子典型的较高频率下观察到 C-H 拉伸模式的 ν ₃带。当 LL 笼中的甲烷分子数量增加到超过 1 时，观察到随着甲烷占据量的增加而出现蓝移的趋势，这与松笼-紧笼模型一致。通过基于 LL 笼内甲烷和接近甲烷的水分子的溶剂化数来表征甲烷伸缩振动的时间相关函数，我们发现在 LL 笼中甲烷占据率较高的情况下观察到的复杂谱线形状是由于溶剂化壳层状态的更广泛变化。对 AIMD 轨迹溶剂化结构的分析提供了对实验谱线形状的解释，证明了 AIMD 对实验的补充性质及其分析的有效性。

更新日期：2024-06-04

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