Effect of confinement on water rotation via quantum tunnelling

Water exhibits different behaviors in confined space compared to free space, which is critical for desalination, biosensing, and many potential applications. Recent studies indicated that quantum tunnelling plays an important role in the orientation of H₂O molecules and the H-bond network of water clusters, but whether this effect is important in confined space remains elusive. Here, we studied the quantum tunnelling effect of water dimers in carbon nanotubes with different sizes by first-principles calculations. Our results show that though this effect may be negligible at room temperature, it becomes dominant at low temperatures up to ∼100 Kelvin. In particular, with the injection of a small amount of energy to excite a specific vibrational mode, the tunnelling rotation effect can be significantly enhanced, which provides a new strategy to tune the H-bond network of confined water.