Uncooperative Effect of Hydrogen Bond on Water Dimer

The water dimer demonstrates a completely different protype in water systems, it prefers not forming larger clusters instead existing in vapor phase stably, which contracts the viewpoint of the cooperative effect of hydrogen bond (O–H\cdotsO). It is well accepted that the cooperative effect is beneficial to forming more hydrogen bonds (O–H\cdotsO), leading to stronger H-bond (H\cdotsO) and increase in the O–H bond length with contraction of intermolecular distance. Herein, the high-precision ab initio methods of calculations applied on water dimer shows that the O–H bond length decreases and H-bond (H\cdotsO) becomes weaker with decreasing H-bond length and O\cdotsO distance, which can be considered as the uncooperative effect of hydrogen bond (O–H\cdotsO). It is ascribed to the exchange repulsion of electrons, which results in decrease of the O–H bond length and prevents the decrease in the O\cdotsO distance connected with the increasing scale of water clusters. Our findings highlight the uncooperative effect of hydrogen bond attributed to exchange repulsion of electrons as the mechanism for stabilizing water dimer in vapor phase, and open a new perspective for studies of hydrogen-bonded systems.