Dipole moment of water molecules in narrow pores

We use ab initio molecular dynamics simulation to calculate the dipole moment of water molecules in one-dimensional hydrogen bonded chains in narrow pores. The electronic charges are partitioned among the water molecules using maximally localized Wannier functions. For water molecules confined to the interior of a carbon nanotube we find an average dipole moment of about 2.7 D, almost 10 % lower than the dipole moment of water molecules in the liquid phase calculated with the same method. The value of the water dipole moment in hydrogen bonded chains determines the effective interaction of excess protons and hydrogen bonding defects in the water chain. Using the dipole moment from our simulations we obtain an effective charge of 0.4e and -0.4e for the D-defect and the L-defect, respectively, and an effective charge of 0.6e for an excess proton in the chain.