Evaluation of the criteria of hydrogen bonding in highly associated liquids

Evaluation of the geometric and energetic definitions of a hydrogen bond have been performed for the liquid water under ambient (T = 298 K, ρ = 0.998 kg dm−3) and sub-critical (T = 573 K, ρ = 0.72 kg dm−3) conditions using the flexible BJH model. It has been shown that single-condition geometric definitions only controlling the intermolecular separation are not capable to reject molecular pairs having repulsive interaction energy and are inapplicable at any temperature. The geometric definitions which additionally account for the allowable orientation of the H-donor molecule are reliable under ambient conditions. They are, however, not sufficient to exclude pairs which are unbound or weakly bound in the physical sense at elevated temperatures. The deficiency of the simple energetic and the mixed energy-distance definition consists in the inability to distinguish pairs having improper relative orientation. The extended energetic definition controlling the interaction energy, intermolecular O⋯H separation and orientation of the H-donating partner seems the most reliable. The purely geometric and energetic definitions have been shown to overestimate the connectivity, statistics and lifetime of hydrogen bonds. The overestimation of the connectivity and statistics is substantial in the sub-critical region whereas more significant differences in the dynamics of hydrogen bonds have been found for ambient water.