Path integral molecular dynamics study of small H2 clusters in the large cage of structure II clathrate hydrate

Temperature dependence of quantum spatial distributions

Alexander Witt, Francesco Sebastianelli, Mark Tuckerman, Zlatko Bacic

Research output: Contribution to journalArticle

Abstract

We report a path integral molecular dynamics (PIMD) study of the temperature dependence of the spatial distribution of two and four H2 molecules inside the large cage of the structure II clathrate hydrate. The PIMD calculations were performed at five temperatures ranging from 25 to 200 K. Their results were combined with those from an earlier diffusion Monte Carlo (DMC) study of this system at T = 0 K [Sebastianelli, F.; Xu, M.; Bačić, Z. J. Chem. Phys. 2008, 129, 244706]. The spatial distribution of the confined H2 molecules at each of the temperatures considered was characterized with the help of several one-dimensional (1D) and three-dimensional (3D) distribution functions of suitably chosen intermolecular coordinates, generated by the PIMD and DMC calculations. The 1D distribution that proved to be the most strongly temperature dependent, and also the most revealing about the structural properties of the system as a function of temperature, involves the H2-cage center-H2 angle. In the case of four caged H2 molecules, this angular distribution provides clear evidence that between 50 and ∼100 K the system undergoes a qualitative change. At 50 K and below, the system is fully localized in the global minimum of the intermolecular potential, corresponding to a tetrahedral configuration of H2 molecules with a unique orientation relative to the cage frame. At temperatures of 75-100 K and higher, nearly degenerate local minima ∼200 cm-1 above the global minimum become accessible and are increasingly sampled by the system. The 3D spatial distributions also show this growing delocalization above 75-100 K. Our findings are in accord with the localization-delocalization transition observed experimentally to occur at 50 K for four D2 molecules in the large cage.

Original languageEnglish (US)
Pages (from-to)20775-20782
Number of pages8
JournalJournal of Physical Chemistry C
Volume114
Issue number48
DOIs
StatePublished - Dec 9 2010

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clathrates
Hydrates
hydrates
Spatial distribution
Molecular dynamics
spatial distribution
molecular dynamics
temperature dependence
Molecules
molecules
Temperature
temperature
molecular diffusion
Angular distribution
angular distribution
distribution functions
Distribution functions
Structural properties
configurations

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Electronic, Optical and Magnetic Materials
  • Surfaces, Coatings and Films
  • Energy(all)

Cite this

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title = "Path integral molecular dynamics study of small H2 clusters in the large cage of structure II clathrate hydrate: Temperature dependence of quantum spatial distributions",
abstract = "We report a path integral molecular dynamics (PIMD) study of the temperature dependence of the spatial distribution of two and four H2 molecules inside the large cage of the structure II clathrate hydrate. The PIMD calculations were performed at five temperatures ranging from 25 to 200 K. Their results were combined with those from an earlier diffusion Monte Carlo (DMC) study of this system at T = 0 K [Sebastianelli, F.; Xu, M.; Bačić, Z. J. Chem. Phys. 2008, 129, 244706]. The spatial distribution of the confined H2 molecules at each of the temperatures considered was characterized with the help of several one-dimensional (1D) and three-dimensional (3D) distribution functions of suitably chosen intermolecular coordinates, generated by the PIMD and DMC calculations. The 1D distribution that proved to be the most strongly temperature dependent, and also the most revealing about the structural properties of the system as a function of temperature, involves the H2-cage center-H2 angle. In the case of four caged H2 molecules, this angular distribution provides clear evidence that between 50 and ∼100 K the system undergoes a qualitative change. At 50 K and below, the system is fully localized in the global minimum of the intermolecular potential, corresponding to a tetrahedral configuration of H2 molecules with a unique orientation relative to the cage frame. At temperatures of 75-100 K and higher, nearly degenerate local minima ∼200 cm-1 above the global minimum become accessible and are increasingly sampled by the system. The 3D spatial distributions also show this growing delocalization above 75-100 K. Our findings are in accord with the localization-delocalization transition observed experimentally to occur at 50 K for four D2 molecules in the large cage.",
author = "Alexander Witt and Francesco Sebastianelli and Mark Tuckerman and Zlatko Bacic",
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T1 - Path integral molecular dynamics study of small H2 clusters in the large cage of structure II clathrate hydrate

T2 - Temperature dependence of quantum spatial distributions

AU - Witt, Alexander

AU - Sebastianelli, Francesco

AU - Tuckerman, Mark

AU - Bacic, Zlatko

PY - 2010/12/9

Y1 - 2010/12/9

N2 - We report a path integral molecular dynamics (PIMD) study of the temperature dependence of the spatial distribution of two and four H2 molecules inside the large cage of the structure II clathrate hydrate. The PIMD calculations were performed at five temperatures ranging from 25 to 200 K. Their results were combined with those from an earlier diffusion Monte Carlo (DMC) study of this system at T = 0 K [Sebastianelli, F.; Xu, M.; Bačić, Z. J. Chem. Phys. 2008, 129, 244706]. The spatial distribution of the confined H2 molecules at each of the temperatures considered was characterized with the help of several one-dimensional (1D) and three-dimensional (3D) distribution functions of suitably chosen intermolecular coordinates, generated by the PIMD and DMC calculations. The 1D distribution that proved to be the most strongly temperature dependent, and also the most revealing about the structural properties of the system as a function of temperature, involves the H2-cage center-H2 angle. In the case of four caged H2 molecules, this angular distribution provides clear evidence that between 50 and ∼100 K the system undergoes a qualitative change. At 50 K and below, the system is fully localized in the global minimum of the intermolecular potential, corresponding to a tetrahedral configuration of H2 molecules with a unique orientation relative to the cage frame. At temperatures of 75-100 K and higher, nearly degenerate local minima ∼200 cm-1 above the global minimum become accessible and are increasingly sampled by the system. The 3D spatial distributions also show this growing delocalization above 75-100 K. Our findings are in accord with the localization-delocalization transition observed experimentally to occur at 50 K for four D2 molecules in the large cage.

AB - We report a path integral molecular dynamics (PIMD) study of the temperature dependence of the spatial distribution of two and four H2 molecules inside the large cage of the structure II clathrate hydrate. The PIMD calculations were performed at five temperatures ranging from 25 to 200 K. Their results were combined with those from an earlier diffusion Monte Carlo (DMC) study of this system at T = 0 K [Sebastianelli, F.; Xu, M.; Bačić, Z. J. Chem. Phys. 2008, 129, 244706]. The spatial distribution of the confined H2 molecules at each of the temperatures considered was characterized with the help of several one-dimensional (1D) and three-dimensional (3D) distribution functions of suitably chosen intermolecular coordinates, generated by the PIMD and DMC calculations. The 1D distribution that proved to be the most strongly temperature dependent, and also the most revealing about the structural properties of the system as a function of temperature, involves the H2-cage center-H2 angle. In the case of four caged H2 molecules, this angular distribution provides clear evidence that between 50 and ∼100 K the system undergoes a qualitative change. At 50 K and below, the system is fully localized in the global minimum of the intermolecular potential, corresponding to a tetrahedral configuration of H2 molecules with a unique orientation relative to the cage frame. At temperatures of 75-100 K and higher, nearly degenerate local minima ∼200 cm-1 above the global minimum become accessible and are increasingly sampled by the system. The 3D spatial distributions also show this growing delocalization above 75-100 K. Our findings are in accord with the localization-delocalization transition observed experimentally to occur at 50 K for four D2 molecules in the large cage.

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