title: Insights on hydrogen-bond lifetimes in liquid and supercooled water.
abstract: We study the temperature dependence of the lifetime of geometric and geometric/energetic water hydrogen-bonds (H-bonds), down to supercooled water, through molecular dynamics. The probability and lifetime of H-bonds that break either by translational or librational motions and those of energetic broken H-bonds, along with the effects of transient broken H-bonds and transient H-bonds, are considered. We show that the fraction of transiently broken energetic H-bonds increases at low temperatures and that this energetic breakdown is caused by oxygen-oxygen electrostatic repulsions upon too small amplitude librations to disrupt geometric H-bonds. Hence, differences between geometric and energetic continuous H-bond lifetimes are associated with large H-bond energy fluctuations, in opposition to moderate geometric fluctuations, within common energetic and geometric H-bond definition thresholds. Exclusion of transient broken H-bonds and transient H-bonds leads to H-bond definition-independent mean lifetimes and activation energies, ~11 kJ/mol, consistent with the reactive flux method and experimental scattering results. Further, we show that power law decay of specific temporal H-bond lifetime probability distributions is associated with librational and translational motions that occur on the time scale (~0.1 ps) of H-bond breaking /re-forming dynamics. While our analysis is diffusion-free, the effect of diffusion on H-bond probability distributions where H-bonds are allowed to break and re-form, switching acceptors in between, is shown to result in neither exponential nor power law decay, similar to the reactive flux correlation function.
authors.affiliation: Grupo de Fı́sica-Matemática da Universidade de Lisboa , Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal.
authors.forename: H F M C
authors.initials: HF
authors.lastname: Martiniano
authors.forename: N
authors.initials: N
authors.lastname: Galamba
mesh_terms: D003080:Cold Temperature; D006860:Hydrogen Bonding; D056004:Molecular Dynamics Simulation; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water