Water Clusters Confined in Nonpolar Cavities by Ab Initio Calculations

Encapsulation of (H2O)n clusters (n = 1−22) in fullerene cages of different diameters (0.73−1.41 nm) has been investigated using gradient-corrected density functional theory. A linear relationship between cavity volume and maximum number of the encapsulated water molecules has been obtained. The interaction between water molecules and the fullerene wall was identified as physisorption with an adsorption energy of about 1.1 kcal/mol per molecule. The equilibrium configurations of small confined water clusters (n < 12) roughly resemble those of gas-phase clusters, whereas larger water clusters tend to adopt cage-like configurations when they are encapsulated in fullerene cages of sufficiently large diameter (i.e., >1.4 nm). The dipole moments of water clusters in the confined phase are smaller than those in the gas phase due to the screening effect of the outer fullerene cage. These results might shed some light on the behavior of water clusters confined in the nonpolar cavities of biological interests.

本研究采用梯度校正密度泛函理论（gradient-corrected density functional theory），对不同直径（0.73~1.41 nm）的富勒烯笼封装(n=1~22)水分子团簇的过程进行了探究。研究得到了空腔体积与封装水分子最大数目之间的线性关系。水分子与富勒烯壁之间的相互作用被鉴定为物理吸附（physisorption），单分子吸附能约为1.1 kcal/mol。受限小尺寸水分子团簇（n<12）的平衡构型与气相团簇的平衡构型大体相似；而当大尺寸水分子团簇被封装进直径足够大（即大于1.4 nm）的富勒烯笼中时，其构型往往会呈现笼状结构。由于外层富勒烯笼的屏蔽效应，受限相内水分子团簇的偶极矩小于气相中的水分子团簇。本研究结果可为理解生物相关非极性空腔内受限水分子团簇的行为提供一定参考。