On the short time motion of hydrogen-bonded molecules in supercooled water

Abstract

The short time dynamics of tagged pairs of molecules that, at the initial time, are in the first coordination shell, is investigated in supercooled liquid water at 245 K by using the molecular dynamics technique with the four-points transferable intermolecular potential of Jorgensen et al. [J. Chem. Phys. 79, 926 (1983)]. The instantaneous normal mode approach and the results of the local structure investigations are exploited to build up a correlation function of the relative displacements that represents the projection of normal modes along the initial center of mass separation vector. By imposing simple constraints to the initial dynamical conditions, localized damped oscillations of the centers of mass are detected along the hydrogen bond directions. The corresponding density of states shows a maximum around the frequency of 230 cm−1 and its shape agrees with the frequency contributions expected from the translational phonon branches of ice. Total and radial correlation functions of the relative velocity are also computed. The radial component is dominated by oscillations at the frequency of 230 cm−1; it compares fairly well with that derived from the translational density of states, thus supporting the reliability of the employed method. A slower component of motion, absent along the hydrogen bond direction, is derived; in the density of states, it produces a contribution peaked around 50 cm−1.

Vibrational motions of hydrogen bonds from the short time relative pair dynamics

“…3͒. They are damped oscillations with amplitude of about 0.08 Å, significantly smaller than the initial pair separation ͗r 12 (0)͘ϭ2.8 Å, and frequency of 6.9ϫ10 12 Hz, about one-half of the librational frequency. 1 The oscillation persists till about 0.4 ps.…”

“…To select the relative pair orientations, the set of the Eulerian angles ϭ͕,⌿,⌽͖ in the local reference frame was used, and the total angular space of a pair, ⍀ ϭ͕ 1 , 2 ͖, was divided in subspaces ⍀ ⌫ which collect a set of distorted dimer geometries; each set is referred to as a configuration state ⌫ ͑or simply configuration or state͒. The main physical quantities concerning the translational motion are R 2 (r 0 ,⍀ ⌫ ;t)ϭ͗r 12 2 (t)͘ 0,⌫ Ϫ͗r 12 2 (0)͘ 0,⌫ and S 2 (r 0 ,⍀ ⌫ ;t) ϭ͉͗r 12 (t)Ϫr 12 (0)͉ 2 ͘ 0,⌫ , where r 12 is the intermolecular center-of-mass ͑c.m.͒ separation vector and ͗͘ 0,⌫ stands for averages over tagged pairs of molecules which, at the time tϭ0, are in the region of the configuration space characterized by the distance r 0 ϭr 12 (0) and by the orientations of the subspace ⍀ ⌫ . These functions are the extension to molecular fluids of the functions used to investigate the relative motion in monatomic fluids.…”

Temperature evolution of the translational density of states of liquid water

“…2 If the short-time motions of the molecules were purely harmonic oscillations, ⌬B(r 0 ;t) should yield the average of the normal modes projected over the space direction R i j (0) selected by the configuration ⌫. 1,2 If nonharmonic motions give a significant contribution, one can suppose that the computed ⌬B(r 0 ;t) is the sum of the harmonic, ⌬B h (r 0 ;t), and nonharmonic, ⌬B nh (r 0 ;t), contributions:…”

“…In Ref. 2, it has been suggested that the DOS of HB can be reproduced by Gaussian functions that arise from broadened phonon branches of the solid. To analyze the short-time dynamics we have used a damped harmonic oscillator ͑DHO͒like model.…”

“…1,2 The method was used to derive a meaningful description of the translational motions in liquid supercooled water (T ϭ243 K) and is here applied to liquid water at the melting and boiling points ͑273 and 373 K͒. Many studies have been devoted to the microscopic dynamics of water and to the relaxation mechanisms that bring a liquid towards a glass behavior as the temperature decreases and the temperature of the glass transition is reached.…”