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A Dense and Efficient Clathrate Hydrate Structure with Unusual Cages

2001; Wiley; Volume: 40; Issue: 7 Linguagem: Inglês

10.1002/1521-3773(20010401)40

1521-3773

K.A. Udachin, Christopher I. Ratcliffe, John A. Ripmeester,

CO2 Sequestration and Geologic Interactions

Filling space with a host of cages: completely hydrophobic interactions between organic molecules and water are present in the novel clathrate hydrate that has dimethyl ether guests in three different cages. One of these cages is the previously unreported tetrakaidecahedron (4151063) and is shown in the picture along with an ecapsulated dimethyl ether molecule. Clathrate hydrates, hydrogen-bonded networks of water molecules stabilized by the presence of guest molecules, are found in both natural1 and industrial settings2 and hence have considerable potential to affect human welfare.3 As well, they are seen as ideal models for the hydration of hydrophobic bioorganic materials.4 The three known hydrate structural families,5, 6 structure (str.) I, II, and H, have the pentagonal dodecahedron (512 cage) as a common feature. In fact, str. II and str. H are based on the stacking of layers of 512 cages so that other polytypes are likely to exist as well.7 Herein we report a new, highly complex hydrate structure that does not have 512 polyhedra and contains two unusual cages, one of which has not been reported before. This new structure is denser than the other hydrate structures because of the low number of empty small cavities, and is likely to be a preferred structure for guests of intermediate dimensions at ambient or somewhat elevated pressures. Dimethyl ether (DME) is one of a few guests8, 9 that forms two distinct hydrates.10 A phase diagram (not illustrated) shows a hydrate of composition DME⋅17 H2O melting incongruently at −20 °C (this hydrate was shown to be a str. II hydrate) and a lower hydrate of composition DME⋅7 H2O melting incongruently at −28 °C. Evidence from dielectric and NMR spectroscopic measurements11 suggested a highly anisotropic environment for the DME guest in the lower hydrate and this led to the supposition that the structure might be the same as that of the tetragonal bromine hydrate.12 Below, we present the actual structure of the DME hydrate, which although it does have a number of cages in common with bromine hydrate, is unique and has several unusual cages. The structure13 of this hydrate is trigonal, space group P321, a=34.995, c=12.368 Å, and can be described as 12 P⋅12 T⋅24 T′⋅12 U⋅348 H2O, where P is the 51263 cage, also known from bromine hydrate, T is the well-known 51262 cage from str. I hydrate, T′ is a previously unobserved cage designated as 4151063, and U is a small cage designated as 425861. The latter has been observed in the structure of one of the hydrates of propylamine.5 The DME molecules reside in all three types of large cages in the structure, giving an overall composition of DME⋅7.25 H2O. There is no interaction between guest and host other than the van der Waals' contacts usual for the true clathrate hydrates. The T′ cage has not been recorded in either the hydrate5 or clathrasil14 literature, and thus this structure represents a new way5 of filling three-dimensional space by stacking a novel combination of cages. The new hydrate falls outside the structural numbering scheme proposed by Jeffrey,5 and we propose that the structure be known as structure T (from trigonal) hydrate. The overall structure is shown in Figure 1, the constituent cages in Figure 2. The packing motifs resulting in the trigonal structure derive from several units with three-fold symmetry: clusters of three cages, where pairs of cages share a face so that the three have one common edge, or clusters of four where three cages are attached to a central cage by sharing a face. The guest positions in the three cages are disordered but quite well defined: a single position can be seen for each DME methyl group, whereas multiple positions are observed for the oxygen centers. As in previous work on hydrates,15 2H NMR spectroscopy has shown the disorder to be dynamic. For samples corresponding to the str. II composition, the lineshape in the 2H NMR spectrum is isotropic, as expected for DME undergoing rapid reorientation in the large pseudospherical 51264 cage. Str. T shows an anisotropic lineshape (Figure 3 a) characteristic of an axially symmetric electric-field gradient at the quadrupolar nucleus, which indicates the presence of DME in nonspherical cages. The doublet splitting of 17.5 kHz at 152 K is significantly less than the 38.5 kHz typical for a methyl group reorienting rapidly about its threefold axis. The reduction factor of 0.455 is consistent with further rapid reorientation of the whole molecule about an axis parallel to the C–C direction, as suggested by the disorder in the guest positions, with additional librational dynamics accounting for a further slight reduction. General view of the str. T hydrate as determined by single crystal X-ray diffraction. View of the cages in the str. T hydrate; the large cages show the location of the disordered DME guest. NMR spectra of DME hydrates: a) 2H NMR spectrum at 152 K of CD3OCH3/H2O, str. T; b) 129Xe NMR spectrum at 77 K, DME:Xe:H2O ratio 1:2:20, str. II; c) 129Xe NMR spectrum at 77 K, DME:Xe:H2O ratio 10:1:8.6, str. T. The 129Xe NMR spectra of mixed Xe/DME hydrates also provide indications of a structure different from those already known. At high water:guest ratios a spectrum (Figure 3 b) is observed which is characteristic of str. II hydrates16 with a small amount of Xe in the large cages giving the resonance at δ=76, and a strong signal at δ=232 for Xe in the small cages. When DME is in excess, however, a quite different spectrum with a signal, arising from a single small cage with a very slight asymmetry, is observed at δ=222, Figure 3 c. This shift is quite different from that of Xe in small cages in other hydrate structures: it falls between the shifts for the 512 cages in str. II and str. H at δ=232 and the 435663 cage of str. H at δ=212.4 (Xe in the 512 cages of str. I and the bromine hydrate has larger shifts than str. II). Thus this new signal may be assigned to str. T. The significance of the structure becomes apparent upon examining some of its properties. One striking feature is the relative number of small cages, the ratio of small to large cages is significantly smaller than for the other known structures. This means that if small-cage guests are absent, this structure is the most efficient in terms of minimizing vacant void space, as also reflected in the density of the hydrate structures (Table 1). The low hydrate is denser than the str. II hydrate, and also the hypothetical str. I hydrate by ∼2 %, with the implication that this structure may be favored over str. I or II for guests of intermediate size at somewhat elevated pressure. It is pertinent to refer to work by Dyadin et al.9 on the effect of pressure on the phase structure of a number of hydrates that form str. II at ambient pressure. At a few kbar pressure there is the appearance of a new phase with a hydration number of ∼7 water molecules per guest. This was interpreted as a transition of str. II to str. I plus ice, as str. II is inherently unstable at high pressure because of the large number of empty small cages. Based on density alone, str. T should be more stable than str. I. However, there are other factors, such as hydrogen-bonded water molecules arranged in squares, that must be energetically unfavorable because of ring strain. Therefore it may be postulated that some or all of the pressure-induced transitions could involve the structure reported here. Hydrate Small (S) cages[a] Large (L) cages[a] Ratio (S/L cages) Density [g cm−3][b] str. H 3 (512), 2 (435663) 1 (51268) 5 0.834[c] str. II 16 (512) 8 (51264) 2 0.938 str. I 2 (512) 6 (51262) 0.33 1.054[c] str. T 12 (425861) 12 (51263), 12 (51262), 24 (4151063) 0.25 1.074 Colorless single crystals (0.4×0.4×0.5 mm) of dimethyl ether (DME) hydrate were grown in a sealed tube at −40 °C from an aqueous solution containing excess dimethyl ether. Crystal data were collected on a Bruker SMART CCD diffractometer, (graphite monochromator) with MoKα radiation (λ=0.71073 Å, 2θmax=<57.5°, ω scan mode). The structure proved to be difficult to solve17 as the crystal was twinned and the unit cell is large.18 The NMR experiments were performed as described previously.15, 16 Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-151791. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: [email protected]).