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Water as a Lévy Rotor
Journal article   Open access   Peer reviewed

Water as a Lévy Rotor

David A Faux, Arifah A Rahaman and Peter J McDonald
Physical Review Letters, Vol.127(25), pp.256001-1-256001-5
17/12/2021
PMID: 35029422

Abstract

Chemical bonding Classical transport Diffusion Molecular Dynamics Nuclear Magnetic Resonance
A probability density function describing the angular evolution of a fixed-length atom-atom vector as a Lévy rotor is derived containing just two dynamical parameters: the Lévy parameter α and a rotational time constant τ. A Lévy parameter α<2 signals anomalous (non-Brownian) motion. Molecular dynamics simulation of water at 298 K validates the probability density function for the intramolecular ^{1}H─^{1}H dynamics. The rotational dynamics of water is found to be approximately Brownian at subpicosecond time intervals, becomes increasingly anomalous at longer time intervals due to hydrogen-bond breaking and reforming, before becoming indistinguishable from Brownian dynamics beyond about 25 ps. The Lévy rotor model is used to estimate the intramolecular contribution to the longitudinal nuclear-magnetic-resonance (NMR) relaxation rate R_{1,intra}. It is found that R_{1,intra} contributes 65%±7% to the overall relaxation rate of water at room temperature.
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