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Encyclopedia of Physical Science and Technology EN007G-334 June 30, 2001 15:23

720 Inclusion (Clathrate) Compounds

nickel may be replaced by another square planar metal or and Y type spaces may both be filled by a small molecule.
by a tetrahedral one (for example, cadmium in Cd(CN) 2 ). For example, the ideal stoichiometry is 8CH 4 · 46H 2 O for
Third, bulky substituents may be introduced into the the gas clathrate hydrate of methane. It is also possible to
amine. form mixed hydrates in which a smaller guest is trapped
in the smaller cavity and the larger in the more spacious
void.
C. Gas Clathrate Hydrates
Structure I belongs to the cubic space group Pm3n and
These substances, first characterized by Faraday in 1823, can be viewed as a close packing of 12- and 14-hedra shar-
are based on the ice structure. The gases are included in ing faces in the ratio of 1:3. Structure II falls in the cubic
voids in the hydrogen-bonded network. There is a geomet- Fd3m group and is viewed as 12- and 16-hedra sharing
rical similarity between these materials and the zeolites in faces in the ratio of 2:1. A hexagonal hydrate structure
that both are based on three-dimensional four-connected (type H) requiring both large and small guests to stabi-
nets. The guests range from dioxane down to argon. Also lize the lattice has been found to be isostructural with
considered under this topic are the quaternary ammonium dodecasil-1H. Lower symmetry structure types III–VII
salt hydrates and the alkylamine hydrates. With a number have been observed for the ammonium salt hydrates and
of guests the gas clathrate hydrate structure is thermo- alkylamine hydrates. Description of these structures is be-
dynamically more stable than that of ice. Indeed, several yond our scope, but it is instructive to note the differences
◦
◦
have melting points in the range of 0 –15 C and one is between the ammonium or amine structures and those of
◦
reported to melt at 31.5 C. the gases. In the case of simple salts that contain anions
−
The true gas clathrate hydrates, with few exceptions, such as F , the latter is not surprisingly found involved
crystallize in one of two structures, I and II. In general, the with the hydrogenbond framework. For anions such as
smaller guests belong to structure I and the larger ones to benzoate, the oxygen atoms are a part of the framwork,
structure II, while those of intermediate size may belong while the phenyl group is located in a void. The cations
to either under slightly different conditions. The ideal are entrapped, but much larger voids are required when
+
unit cell contents for a type I is 6X · 2Y· 46H 2 O and for a an ion as large as [N(i-C 5 H 11 ) 4 ] is utilized. Alkylamine
type II, 8X · 16Y · 136H 2 O, where Y refers to the guests hydrates are hydrogen bonded to the framework and pene-
in a 12-hedra and X to those in 14-hedra or higher. The trate the cavities as well. Figure 5 illustrates this situation.
polyhedra can be understood with reference to Fig. 4. X Gas clathrates are of considerable importance industri-
ally, even though they have gained some infamy by be-
ing held responsible for plugging the Alaska natural gas
pipeline. The methane clathrate has a melting point well
above that of ice itself, so the substance can crystallize

FIGURE 5 Structure of the diethylamine hydrate 12(CH 3 CH 2 ) 2
FIGURE 4 Clathrate hydrate voids: (a) 12-hedra, (b) 14-hedra, NH · 104H 2 O. The nitrogen atom is the dark sphere; the carbon
(c) 15-hedra, and (d) 16-hedra. atoms of the ethyl groups are shaded spheres.