School of Mathematical Sciences, Dalian University of Technology, Dalian 116024, China.
J Chem Phys. 2012 Jun 14;136(22):224508. doi: 10.1063/1.4728157.
Endohedral CH(4)@(H(2)O)(n) (n = 16, 18, 20, 22, 24) clusters with standard and nonstandard cage configurations containing four-, five-, six-, seven-membered rings were generated by spiral algorithm and were systematically explored using DFT-D methods. The geometries of all isomers were optimized in vacuum and aqueous solution. In vacuum, encapsulation of methane molecules can stabilize the hollow (H(2)O)(n) cage by 2.31~5.44 kcal/mol; but the endohedral CH(4)@(H(2)O)(n) cages are still less stable than the pure (H(2)O)(n) clusters. Aqueous environment could promote the stabilities of the hollow (H(2)O)(n) cages as well as the CH(4)@(H(2)O)(n) clusters, and the CH(4)@(H(2)O)(n) clusters possess larger stabilization energies with regard to the pure (H(2)O)(n) clusters except for n = 24. The lowest energy structures of the CH(4)@(H(2)O)(20) and CH(4)@(H(2)O)(24) cages are identical to the building units in the crystalline sI clathrate hydrate. All of the low-energy cages (including both regular and irregular ones) have large structural similarity and can be connected by "dimer-insertion" operation and Stone-Wales transformation. Our calculation also showed that in the range of cluster size n = 16-24, the relative energies of cage isomers tend to decrease with increasing number of the adjacent pentagons in the oxygen skeleton structures. In addition to the regular endohedral CH(4)@(H(2)O)(20) and CH(4)@(H(2)O)(24) cage structures, some nonstandard CH(4)@(H(2)O)(n) (n = 18, 20, 22, 24) cages have lower energies and might appear during nucleation process of methane hydrate. For the methane molecules in these low-energy cage isomers, we found that the C-H symmetric stretching frequencies show a red-shift trend and the (13)C NMR chemical shifts generally move toward negative values as the cavity size increases. These theoretical results are comparable to the available experimental data and might help experimental identification of the endohedral water cages during nucleation.
通过螺旋算法生成了具有标准和非标准笼状结构的内包 CH(4)@(H(2)O)(n)（n = 16、18、20、22、24）团簇，并使用 DFT-D 方法对其进行了系统研究。所有异构体的几何形状均在真空中和水溶液中进行了优化。在真空中，甲烷分子的包封可以通过 2.31~5.44 kcal/mol 稳定空的（H(2)O)(n)笼；但内包 CH(4)@(H(2)O)(n)笼仍然不如纯（H(2)O)(n)簇稳定。水相环境可以促进空（H(2)O)(n)笼以及 CH(4)@(H(2)O)(n)簇的稳定性，并且 CH(4)@(H(2)O)(n)簇相对于纯（H(2)O)(n)簇具有更大的稳定能，除了 n = 24 之外。CH(4)@(H(2)O)(20)和 CH(4)@(H(2)O)(24)笼的最低能量结构与晶体 sI 笼状水合物的构建单元相同。所有低能量笼（包括规则和不规则笼）都具有较大的结构相似性，可以通过“二聚体插入”操作和 Stone-Wales 转化连接。我们的计算还表明，在簇大小 n = 16-24 的范围内，笼异构体的相对能量随着氧骨架结构中相邻五边形数量的增加而趋于降低。除了规则的内包 CH(4)@(H(2)O)(20)和 CH(4)@(H(2)O)(24)笼结构外，一些非标准的 CH(4)@(H(2)O)(n)（n = 18、20、22、24）笼具有更低的能量，并且可能在甲烷水合物成核过程中出现。对于这些低能量笼异构体中的甲烷分子，我们发现 C-H 对称伸缩频率呈红移趋势，而（13)C NMR 化学位移值随着腔尺寸的增加通常向负值移动。这些理论结果与现有的实验数据相当，可能有助于在成核过程中识别笼内水笼。
