Effects of defects on structure of amorphous ice
A. Yoshimura and T. Ikeda-Fukazawa
Department of Applied Chemistry, Meiji University, JapanIn interstellar molecular clouds, various molecules (e.g., H2O, NH3, CO, CO2, and so on) areformed from elements such as H, C, O, and N [1]. Most of H2O in interstellar molecularclouds exists as a thin shell of amorphous ice around dust grain. The molecules, which areadsorbed or included in amorphous ice, undergo chemical evolutions to organic molecules
through various reaction processes. The photochemical reaction caused by ultraviolet (UV)photon irradiation is one of the dominant reaction processes [2]. Thus, the variation instructure of amorphous ice with UV-irradiation is an important factor to understand theprocesses of molecular evolutions. Recently, Tachibana et al. [3] found that the viscosity of
the UV-irradiated amorphous ice decreases at around 50 K, and becomes a lower value thanthat at the glass transition temperature. The defects formed by UV-irradiation are expected asa cause of the decrease in viscosity. To investigate the effects of defects on the structure ofamorphous ice, the molecular dynamics (MD) calculations of amorphous ice including OH
and OH− were performed.
The MD calculations were performed using an atom-atom potential model with MXDORTO
program [4]. We used an interatomic potential model (KAWAMURA potential model) for thepotential parameters of water molecule [5]. The potential parameters of OH and OH− weredetermined by constraining molecular structure to reproduce the energy surfaces obtained
from molecular orbital calculations. The amorphous ice was prepared by quenching of aliquid phase consisting of 360 water molecules from 290 K to 10 K [5]. The system including
OH (or OH−) with 5% in concentration was prepared by replacing 18 H2O molecules with 18
OH (or 18 OH−) at 10 K. To equilibrate fundamental cell, the MD code was run for 2 ns at 10K. Then, the system was warmed to 90 K with 2.5×10−5 K/fs in rate. The viscosity ofamorphous ice was estimated using Stokes–Einstein relation.
The result shows that the density of amorphous ice including OH with 5% in concentration islower than that of pure amorphous ice, whereas that including OH− is higher. For the viscosity,
the amorphous ices including OH and OH− have lower and higher values in comparison withthat of pure amorphous ice, respectively. In order to investigate the mechanisms of thevariations in density and viscosity with formation of defects, the local structures around thedefects were analysed. The results show that the mean coordination number of OH is 1.29,
while that of OH− is 4.35. For H2O, the coordination numbers for both of the states are closeto the value of pure amorphous ice. This indicates that the variations in density and viscosity
are resulted from the local structures around the defects. The density of amorphous ice
decreases as the coordination number decreases. The enhancement of thermal vibrations ofOH due to the release from restrictions of surrounded molecules can be a cause of decrease inviscosity.
References[1] J. M. Greenberg and J. I. Hage, Astrophys. J. 361, 260 (1990).
[2] F. J. Ciesla and S. A. Sandford, Science 336, 452 (2012).
[3] S. Tachibana et al., Sci. Adv. 3, eaao2538 (2017).
[4] K. Kawamura, MXDORTO, Japan Chemistry Program Exchange, #29 (1990).
[5] Y. Kumagai and T. Ikeda-Fukazawa, Chem. Phys. Lett. 678, 154 (2017).