Structure and Dynamics of Propylene Oxide and Trimethylene Oxide Clathrate Hydrates
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Structure and Dynamics of Propylene Oxide and Trimethylene Oxide Clathrate Hydrates Inmaculada Peral,1,2 Joseph E. Curtis,1 Bryan C. Chakoumakos,3 and Camille Y. Jones1 1
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-8562 U.S.A. 2Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115 U.S.A. 3Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 U.S.A.
ABSTRACT We present results from studies of the structure and dynamics of clathrate hydrates of three cyclic ethers by neutron diffraction and preliminary results on molecular dynamics simulations. Recent results from neutron powder diffraction and quasielastic neutron scattering of studies of propylene oxide (PO, C3H6O) and its isomer trimethyelene oxide (TMO, C3H6O), are compared with structural results obtained previously for tetrahydrofuran (THF, C3H5O). Experimental evidence of distortions of the host structures with temperature is discussed in light of the findings from quasielastic neutron scattering, which indicate distinct regions of hightemperature and low-temperature rotational dynamics and a temperature dependence related to the size of the guest. Preliminary MD results indicate a general expansion of the lattice with temperature resulting in increased volume available to PO. INTRODUCTION Clathrate hydrates [1] are inclusion compounds made of a hydrogen-bonded water "host" containing atoms or small molecules as "guests." The three-dimensional host network of spherical polyhedra of a clathrate hydrate (CH) is stable only when guest molecules are trapped within the polyhedra or cages. Restrictions on the size of a guest arise from the limited number of cage types that can encapsulate guest molecules while maintaining the necessary tetrahedral geometry for hydrogen bond formation. The guest-host (g-h) interactions are primarily repulsive, but for a guest with a permanent dipole, dipolar interactions may also be important and can affect, e.g., the rotational mobility of the guests [2] and the temperature at which the host H2O molecules begin to rotate [3]. The nature of these interactions can be explored through a study of the changes in the crystal structures and rotational dynamics of the guests with temperature (T). Guest rotations in the range 106 < ν < 1012 s-1 are in the fast motion limit of NMR, but are within the dynamic range of neutron scattering. In the present paper, we report results from neutron powder diffraction and quasielastic neutron scattering experiments on the CHs of three cyclic ethers, propylene oxide (PO, C3H6O) and its isomer trimethylene oxide (TMO, C3H6O), and tetrahydrofuran (THF, C3H6O), whose structures are shown in Fig. 1. The guest TMO forms clathrate hydrates with both the structure I (sI) and structure II (sII) frameworks (Fig. 2) depending on the composition of the initial mixture of TMO and water [4]. In each structure type, TMO occupies the large cage, i.e., the 51262 cage in sI
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