Molecular polarizability anisotropy of liquid water revealed by terahertz-induced transient orientation

Peter Zalden,Robin Santra,Pankaj Kr Mishra,Haoyu Huang,Franz X Kärtner,Xiaojun Wu,Ralph Welsch,Christian Bressler,Oliver D Mücke,Joscha Reichert,Liwei Song,Michael Thorwart,Frederike Ahr

doi:10.1038/s41467-018-04481-5

Abstract

Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules. The simulation of these interactions requires a parametrization of the permanent and induced dipole moments. However, the underlying molecular polarizability of water and its dependence on ions are partially unknown. Here, we apply intense terahertz pulses to liquid water, whose oscillations match the timescale of orientational relaxation. Using a combination of terahertz pump / optical probe experiments, molecular dynamics simulations, and a Langevin dynamics model, we demonstrate a transient orientation of their dipole moments, not possible by optical excitation. The resulting birefringence reveals that the polarizability of water is lower along its dipole moment than the average value perpendicular to it. This anisotropy, also observed in heavy water and alcohols, increases with the concentration of sodium iodide dissolved in water. Our results enable a more accurate parametrization and a benchmarking of existing and future water models.

Highlights

Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules
To further support the existence of an underlying orientation mechanism of the permanent dipole moments, we have evaluated the Langevin model based on the fully anisotropic rotational diffusion tensor reported for liquid water in literature
We have shown that single-cycle electromagnetic pulses in the THz regime orient the dipole moments of liquid water along their electric field

Summary

Introduction

Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules. One such example is the polarizability tensor αij, which, together with the permanent dipole moment μi, governs the interaction of water molecules with an electric field Ei via their energy This fundamental interaction determines the outcome of chemical and biochemical reactions, e.g., in proteins[2,3], whose folding reaction was calculated using classical force field models[4]. Based on present-day literature, it is unclear whether Δα > 09 or Δα < 010 is the correct result, and an abundance of references exists for either case[11] This is mostly due to the small value of the anisotropy Δα It is worth mentioning that studies using DC electric fields commonly find a reduction of the refractive index in the direction of the applied field due to dielectric saturation[15,16,17]

Methods

Results

Conclusion