Structural transition in one-dimensional water unveils the origin of multifurcated hydrogen bond weakening.

Water confined to low dimensions exhibits even more anomalous behavior than bulk water. Since these anomalies are primarily dictated by hydrogen bonds (hbs), we investigate hb behavior in quasi-one-dimensional water chains using density functional theory. We show that a water chain with two molecules per unit cell transforms from a single-file (low-linear-density) to a double-file (high-linear-density) structure under uniaxial compression. Furthermore, we show that repulsive interactions emerge when hbs become multifurcated. These additional repulsive forces weaken the hbs and play a crucial role in driving the structural transition. We demonstrate that the many-body interactions associated with hb cooperativity, as well as the repulsive forces from multifurcated hbs, are captured by classical dipole-dipole interactions. This indicates that both hb cooperativity and the weakening of multifurcated hbs are classical electrostatic phenomena, even though quantum effects dictate the strength of the effective dipoles involved. Finally, we discuss how these results provide insights for understanding the flow mechanism of water along pores of sub-nanometer diameter and aid in developing a simple, universal water model.