笼型甲烷水合物中单、双水分子缺位后的结构与稳定性

The structures and stabilities of clathrate methane hydrates with water molecule vacancies

采用量子化学计算方法,对甲烷水合物基本空腔(H2O)20缺失水分子后形成的(H2O)19和(H2O)18笼型结构特征及其甲烷水合物的稳定性等进行了深入探讨。结果表明,只有相邻且同面的两个水分子缺失,甲烷才有溢出笼型的可能,笼型结构中O—O边长的变化主要源于氢键键长的改变,O—H共价键键长以及H—O—H键角的变化很小;同类结构异构体的稳定性主要由水分子间的结合能决定,非同类异构体的稳定性主要取决于相邻五元环的个数,而相邻四元环数目的增加将降低其稳定性。

Quantum chemical calculations at the （B3LYP/6-31 ＋ G ＊ ＊ ） level have been employed to study the characteristic structures and stabilities of methane hydrates （CH4@ （H20） 19 and CH4@ （H2O） 18 ） formed by loss of water molecules from CH4 @ （H20）20- The results show that methane can escape from the hydrate cage only when two water molecules are removed from vicinal positions in the same ring. In these hydrate cages, the O--O side length changes can be mainly attributed to the hydrogen bond lengths, since the O--H covalent bond and H--O--H angle varies only slightly. Furthermore, for the isomeric hydrates formed with equivalent rings, the sta- bilities may involve a contribution from the binding energy between water molecules, but for those formed with non equivalent rings, the stabilities may be related to changes in the five-membered ring.