Hiratsuka Masaki, Ohmura Ryo, Sum Amadeu K, Alavi Saman, Yasuoka Kenji
Department of Mechanical Engineering, Keio University, 3-4-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
Phys Chem Chem Phys. 2015 May 21;17(19):12639-47. doi: 10.1039/c4cp05732e.
Clathrate hydrates are typically stabilized by suitably sized hydrophobic guest molecules. However, it has been experimentally reported that isomers of amyl-alcohol C5H11OH can be enclosed into the 5(12)6(4) cages in structure II (sII) clathrate hydrates, even though the effective radii of the molecules are larger than the van der Waals radii of the cages. To reveal the mechanism of the anomalous enclathration of hydrophilic molecules, we performed ab initio and classical molecular dynamics simulations (MD) and analyzed the structure and dynamics of a guest-host hydrogen bond for sII 3-methyl-1-butanol and structure H (sH) 2-methyl-2-butanol clathrate hydrates. The simulations clearly showed the formation of guest-host hydrogen bonds and the incorporation of the O-H group of 3-methyl-1-butanol guest molecules into the framework of the sII 5(12)6(4) cages, with the remaining hydrophobic part of the amyl-alcohol molecule well accommodated into the cages. The calculated vibrational spectra of alcohol O-H bonds showed large frequency shifts due to the strong guest-host hydrogen bonding. The 2-methyl-2-butanol guests form strong hydrogen bonds with the cage water molecules in the sH clathrate, but are not incorporated into the water framework. By comparing the structures of the alcohols in the hydrate phases, the effect of the location of O-H groups in the butyl chain of the guest molecules on the crystalline structure of the clathrate hydrates is indicated.
笼形水合物通常由尺寸合适的疏水性客体分子来稳定。然而，实验报道显示，戊醇C5H11OH的异构体能够被包封在结构II（sII）型笼形水合物的5(12)6(4)笼中，尽管这些分子的有效半径大于笼的范德华半径。为了揭示亲水性分子异常包封的机制，我们进行了从头算和经典分子动力学模拟（MD），并分析了sII型3 - 甲基 - 1 - 丁醇和结构H（sH）型2 - 甲基 - 2 - 丁醇笼形水合物的客体 - 主体氢键的结构和动力学。模拟结果清楚地表明了客体 - 主体氢键的形成，以及3 - 甲基 - 1 - 丁醇客体分子的O - H基团纳入到sII型5(12)6(4)笼的框架中，戊醇分子其余的疏水部分也很好地容纳在笼中。计算得到的醇类O - H键的振动光谱显示，由于强烈的客体 - 主体氢键作用，频率发生了很大的位移。2 - 甲基 - 2 - 丁醇客体与sH型笼形水合物中的笼状水分子形成了强氢键，但没有纳入到水框架中。通过比较水合物相中醇类的结构，表明了客体分子丁基链中O - H基团的位置对笼形水合物晶体结构的影响。
