Revisiting orientational relaxation in bulk water: Observation and mechanistic understanding of an intermediate time scale (around 1 ps).

The longer time scale of water reorientation is intimately coupled to H-bond exchange, as elucidated by the jump reorientation mechanism proposed by Laage and Hynes. Here, we revisit this mechanism to probe the short-to-intermediate time scales of the reorientation process. Analysis of the reorientation time correlation function (TCF) of bulk water reveals an intermediate time scale of ∼1.15 ps contributing nearly 20% of the total decay. To elucidate its molecular origin, we further analyze the TCF of the donor O*-H* vector over the interval preceding an H-bond exchange event, removing contributions from the transition-state region, which reveals the same ∼1.15 ps component along with a slower ∼6 ps frame-tumbling motion. A wobbling-in-a-cone analysis reveals that this intermediate relaxation time scale originates from an effective coupling between the second wobbling motion and the final orientational randomization process. This regime involves a broader angular exploration of the donor O*-H* bond (∼33°-35°) with an associated relaxation time scale of ∼1.38 ps before randomization. Further trajectory analysis reveals acceptor-free dangling states of the donor between successive successful exchange events, where short-lived states (≤20 fs) arise mainly from angular fluctuations, while longer dangling stretches (≥50 fs) involve increased donor-acceptor distance and populate a near transition-state-like region (∼3.5 Å, ∼30°), although distinct from an exact transition-state configuration. The average waiting time of such long-lived dangling events is ∼1.07 ps. Interestingly, the time scale of the second wobbling motion closely matches the average waiting time of long-lived dangling stretches, indicating unsuccessful hydrogen-bond switching attempts drive intermediate reorientation dynamics. We believe similar analysis will be useful for breaking down time scales observed in various confined systems.