Structure and Thermal Expansivity of Tetrahydrofuran Deuterate Determined by Neutron Powder Diffraction

Abstract

The crystal structure of tetrahydrofuran deuterate, a clathrate hydrate, has been refined from neutron powder diffraction data at five temperatures in the range 7-265 K. The thermal expansivity was shown to be greater than that of ice Ih in the same range of temperature (T), as observed in previous studies of other clathrates. The overall effect of T has been resolved into contributions from different geometrical parameters in the structure. Thus, an increase in T results in expansion of the host-lattice framework with increases in both the D-D and O-O distances and out-of-plane tilting of water molecules. The greatest dependence on T is exhibited by the D-D distances and the distortion of the hexagonal faces from planarity, which is particularly pronounced in the range 75-140 K. The cage volumes show a complex dependence on T: from 7 to 140 K, the volume of the small cage decreases slightly and that of the large cage increases, and between 140 and 205 K, the trend is reversed. The most pronounced structural changes occur in a similar regime of T as changes in guest dynamics observed in spectroscopic and thermodynamic studies. The temperature dependences of the structure and R(T), when considered along with the relation of R(T) to the degree of anharmonicity in bonding, 34 could be formulated to provide a sensitive test of molecular models of clathrate hydrates.

Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

“…Figure 1 b shows an HRPD pattern recorded at 120 K under atmospheric pressure for (THF+HD)*. With a lattice parameter of a = 17.13445(78) (wRp = 4.06 %, c 2 = 6.08), the [D 8 ]THF occupancy in (THF+HD)* resulted in a value of unity, which is in agreement with the findings of an earlier diffraction study, [15] whereas the HD occupancy was refined to 0.724 (11). The HRPD patterns of (THF+H 2 )* and (THF+D 2 )* are also shown in Figure S1a In Figures S2 and S3 in the Supporting Information, the internuclear distances in the sII hydrates are plotted as a function of temperature.…”

“…[14] The diffraction patterns did not include significant reflection peaks resulting from H 2 O (ice I h ); thus, we neglected the ice phase during the refinement process. The initial host-lattice atomic positions and lattice parameters of the sII clathrate hydrate were taken from Jones et al [15] The guest molecule was inserted as a rigid body ( Table S1 in the Supporting Information). A rigid body of THF and the aforementioned materials (H 2 , D 2 , and HD) was created with distance and geometry restraints and inserted into the large (3/8, 3/8, 3/8) and small (0, 0, 0) cage centers with an arbitrary orientation.…”

Linking microscopic guest properties to macroscopic observables in clathrate hydrates: Guest-host hydrogen bonding

“…Experimental lattice vectors for THF and THP clathrates are also given for comparison. 38,39,3 The SPC/E-AMBER force field simulation underestimates the experimental unit cell vectors by ϳ2%. The lattice vectors generally increase according to the size of the guest molecules.…”

Experimental Evidence for a Crossover between Two Distinct Mechanisms of Amorphization in IceIhunder Pressure

“…An example of PIA due to mere mechanical melting may be found in the case of THF clathrate hydrate [25,26]. This system shows no NTE in the entire temperature range [27,28], and since NTE requires an overall softening of part of the PDOS, thermal melting may be excluded. In fact, specific TA branches of THF hydrate exhibiting pressure-induced phonon softening are being identified using coherent inelastic neutron scattering [29].…”