First-Principles Study of Water Chains Encapsulated in Single-Walled Carbon Nanotube

Water molecules confined inside a single-walled (6, 6) carbon nanotube were investigated using density functional theory. In this narrow-sized carbon nanotube (of about 0.8 nm in diameter), the encapsulated water molecules form chain-like configurations via hydrogen bonding. As compared to the water chains in vacuum, the intramolecular charge transfer in the encapsulated water chain is enhanced and the dipole moment is reduced due to the screening effect of the carbon nanotube. The tube-molecule interaction characterized by the coupling energy is about 0.28 eV per water molecule by local density approximation and 0.1 eV by general gradient approximation; the latter one is close to the results by empirical potentials. Weak coupling between the molecular orbitals of the encapsulated water molecules and the delocalized pi electrons from the carbon nanotube was observed, implying that the tube-water interaction is not a simple effect of geometry confinement. Vibrational analysis revealed some unique hydrogen-bond modes for the water chains as well as red shift of the O-H stretching modes for the encapsulated water molecules with regard to the vacuum frequencies due to the tube-water interaction.