Dynamics of hydrogen guests in ice XVII nanopores

Abstract

The present high-resolution inelastic neutron scattering experiment on ice XVII, containing molecular hydrogen with different ortho/para ratio, allows to assign the H2 motion spectral bands to rotational and center-of-mass translational transitions of either para-or ortho-H2. Due to its structure, ice XVII confines H2 molecules to move in spiral channels of molecular size. Reported data demonstrate that H2 molecules rotate almost freely in these nanometric channels, though showing larger perturbation than in clathrate hydrates, and perform a translational motion exhibiting two low frequency excitations. The agreement between experimental spectra and corresponding molecular dynamics results clearly enables to portray a picture of the confined motions of a hydrophobic guest within a metastable ice framework, i.e. ice XVII.PACS numbers: 78.70. Nx, 63.20.Pw Ice XVII is a newly discovered form of solid water obtained from the so-called C 0 -phase of the H 2 -H 2 O binary compound, quenched at a temperature T = 77 K and ambient pressure, after letting the hydrogen molecules diffuse out of the crystal 1 . It is a pure water crystal, metastable at ambient pressure if maintained below 130 K. Its crystal structure is intrinsically porous and presents accessible channels where hydrogen molecules have been located during the production and where other molecules (belonging to hydrogen or another gas) can be absorbed again, confined in an essentially one dimensional geometry 2 . The diameter of these channels, measured from the center of the oxygen atoms, is about 6.10 A. This exotic and low-density water crystal adds to the list of solid structures of water possibly stable at negative pressure 3 .The study of the dynamics of hydrogen molecules in nano-confinement, that is intrinsically quantummechanical, is of great importance from both a practical and fundamental point of view. In hydrogen clathrate hydrates, which possess a similar chemical composition, although a different structure and stoichiometry, the hydrogen molecules confined in nearly spherical cages 4 perform an almost free rotation and a deeply non-harmonic center-of-mass (CoM) vibrational motion (rattling), both of which have been experimentally investigated by inelastic neutron scattering 5-9 and Raman scattering 10-13 . In this paper we discuss the results of a combined experimental and simulation study on the dynamics of the H 2 guests in D 2 O ice XVII and of the D 2 O host lattice.Samples for the present Inelastic Neutron Scattering (INS) experiment were produced at ISC-CNR using D 2 O and H 2 , as described in Ref.1. This isotopic composition is chosen in order to exploit the intrinsic advantage of the large incoherent neutron scattering cross-section of the proton (compared to both D and O), thus allowing for a relatively simple access to the self dynamics of molecular hydrogen. Measurements have been performed at T = 15 K on TOSCA, a high energy resolution (∆E) spectrometer at ISIS (UK) characterized by 1.5%∆E/E i 2.5%, with E i being the incoming ne...

Computational analysis of vibrational spectrum and hydrogen bonds of ice XVII

“…The experimental INS data can be compared with the simulated PDOS curves, because the INS measurements collected the phonon signals throughout the whole BZ [25]. A comparison of the INS data ( [26]) and calculated PDOS of deuterated ice XVII is shown in figure 2 to manifest the validity of the simulation in this work.…”

“…Thus, the physical mechanism of H-bonding in ices is best understood in terms of the local tetrahedral structure. In figure 3, two examples of normal modes at 209 and 301 cm −1 are [26] were measured using deuterated ice XVII, and the calculations were also performed using a 48-molecule model of deuterated ice XVII. illustrated, with a frequency ratio of 1.44.…”

Cubic ice Ic without stacking defects obtained from ice XVII

“…14,15 This low density solid water phase has the characteristic of being highly porous, and, unique among the various stable and metastable phases of ice, exhibits a structure comprising only pentagonal rings of water molecules. 15,16 Ice XVII can be maintained at room pressure only up to about 130 K, above which it undergoes a phase transition similar to that mentioned above for the amorphous 10 and high-pressure crystalline 11,12 forms. Whilst the end-product of all of these transitions, above 200 K, is the ordinary hexagonal form of ice (ice Ih), the remarkable difference between ice XVII and the other forms is the nature of the intermediate state, where, instead of stacking-disordered ice, we find a structurally-pure form of cubic ice (true ice Ic).The transition can be easily detected by Raman spectroscopy, which is also a valuable method to study the transition kinetics as a function of either temperature or time.…”

On a new nitrogen sX hydrate from ice XVII

“…Finally, Amos et al found that sX also exists in the carbon dioxide:water system and determined the full structure of both hydrogen and carbon dioxide hydrates in situ using neutron diffraction 15 . Due to the high guest-host ratio, the reversibility of the filling and emptying, and the Nitrogen sX Hydrate high mobility of hydrogen in the cavities, the sX/ice XVII system has been proposed as a possible hydrogen storage material 34,35 . Other possible applications, however, have not been discussed.…”

“…Nitrogen sX Hydrate As shown by del Rosso et al, guest molecules in the sX structure are highly mobile and are not locked into one position 34,35 . This results in a rather continuous nitrogen density along the helical channels.…”