利用密度泛函理论中的杂化泛函M06-2X研究了受限于不同管径单壁碳纳米管(SWCnT)内水分子团簇(H2O)n=3~6的结构、能量以及振动频率.结果显示由于SWCnT的限域效应,水分子团簇的几何构型与在真空相比发生了巨大变化,如受限(H2O)6能形成单链锯齿型水分子构型.随着管直径的增大,纳米管与水之间的相互作用逐渐减弱,但水分子之间的氢键相互作用能变化不大.对比受限和真空下水分子O—H振动频率发现,绝大部分O—H的振动频率由于碳纳米管与水的相互作用而发生了红移.AIM理论分析显示O—H振动频率的红移应归因于其电子密度的减小.这也表明碳纳米管绝非简单的几何限制效应,而是与水分子之间存在弱电子相互作用,主要包括H…π氢键作用和O…π轨道作用,从而导致水分子的小部分电荷转移到了SWCnT上.The study of confined water molecules has attracted much attention because of their fascinating properties.In the present work, small water clusters(H2O)n=3~6 encapsulated in different diameter of single-walled carbon nanotubes(SWCNT) were investigated using density functional theory(DFT).The DFT-based M06-2X method along with mixed basis sets(6-311+G(d, p)//6-31+G(d)) was employed in all calculations for structure, interaction energy, charge transfer and vibrational frequency.The results indicate that as compared to water molecules in vacuum, the geometries of confined water molecules changed significantly due to the confinement effect of carbon nanotubes, for example, the(H2O)6 cluster can form chain-like configuration via hydrogen bond.The tube-water interaction energy decreased with the increase of the nanotubes diameter, while the hydrogen bond energy between water molecules changes slightly.Vibrational analysis reveals the red shift of the majority of the O—H stretching modes inside SWCNT compared to vacuum vibrational frequencies due to the tube-water interaction.The Bader's theory of atoms in molecule(AIM theory) has been used to characterize the tube-water interactions.Electron density ρ that reflects the strength of a bond has been used to explain the phenomenon of the red shift.The AIM analysis suggests that the red shift of O—H vibrational frequencies for confined water molecules should be attributed to the decrease of the electron density ρ.This fact demonstrates that the tube-water interaction is not a simple effect of geometry confinement, but weak electronic interaction between the molecular orbital of the confined water molecules and the delocalized π electrons from carbon nanotube.It mainly includes the H…π hydrogen bond interaction and O…π orbital interaction, resulting in a slight charge transfer from water molecules to SWCNT.Therefore, when constructing carbon nanotube devices, one should not ignore the existence of water molecules.We hope that our results can provide a guidance to the understanding of the behavior of water molecules encapsulated inside nanoscale environment.国家重点基础研究发展计划(No.2012CB214901); 国家自然科学基金(No.51274197)资助~
