Li Keyao, Shi Ruili, Tang Lingli, Huang Yingying, Cao Xiaoxiao, Su Yan
Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China.
Phys Chem Chem Phys. 2019 May 8;21(18):9150-9158. doi: 10.1039/c8cp07207h.
Water-cage clusters encapsulating guest molecules are the basic components of hydrate crystal structures. Herein, we investigated the fusion process from bi-cages to tri-cages to probe the nucleation mechanism at the initial stage of CH4 hydrate formation by employing dispersion-corrected density functional theory. We found that tri-cages possess high stability by sharing three, rather than two, polygonal faces. In addition, any mono-cage combined with a nonstandard 4151062 cage could achieve considerable stability regardless of which face is shared; this finding illustrates that 4151062 cages are more likely to appear at the early stages of CH4 hydrate nucleation than other nonstandard cages. We then simulated the Raman spectra of CH4 molecules in water-cage to characterize the spectral characteristics of the CH4 hydrate. The C-H symmetric stretching frequency of encapsulated CH4 molecules red-shifted with increasing mono-cage size, which follows the prediction of the "loose cage-tight cage" model. The symmetric stretching vibrational frequencies of trapped CH4 molecules in the tri-cage revealed a clear red-shift compared with those in the component mono- and bi-cages. The cage fusion process and spectroscopic properties described in this work are expected to provide new atomistic insights into CH4 hydrates at the initial nucleation stage.
包裹客体分子的水笼簇是水合物晶体结构的基本组成部分。在此，我们采用色散校正密度泛函理论研究了从双笼到三笼的融合过程，以探究甲烷水合物形成初始阶段的成核机制。我们发现，三笼通过共享三个而非两个多边形面而具有高稳定性。此外，任何单笼与非标准4151062笼结合，无论共享哪个面，都能获得相当的稳定性；这一发现表明，在甲烷水合物成核的早期阶段，4151062笼比其他非标准笼更有可能出现。然后，我们模拟了水笼中甲烷分子的拉曼光谱，以表征甲烷水合物的光谱特征。被包裹的甲烷分子的C-H对称伸缩频率随着单笼尺寸的增加而红移，这与“松笼-紧笼”模型的预测一致。与组成单笼和双笼中的情况相比，三笼中被捕获的甲烷分子的对称伸缩振动频率有明显的红移。这项工作中描述的笼融合过程和光谱性质有望为甲烷水合物初始成核阶段提供新的原子层面的见解。
