Neutron scattering study of polyamorphic THF·17(H2O) – toward a generalized picture of amorphous states and structures derived from clathrate hydrates

From crystalline tetrahydrofuran clathrate hydrate, THF–CH (THF·17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF–CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77–140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF–CH and liquid THF·17H2O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water–water correlations (less dense local water structure) and guest–water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest–host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H2O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H2O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ∼18 H2O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.