Lattice clathrates

MgTPP has also been examined as a substrate for constructing porphyrin sponges, i.e., lattice clathrates that can reversibly absorb and release guest molecules. Such guests as methyl benzoate, propanol and (R)-phenethylamine) have been structurally authenticated other examples are known. " ... 

Those combinations which arise from lattice considerations alone do not follow the pattern of combination of coordination compounds. Hence, they are excluded from consideration here. These have been variously designated as lattice, clathrate (85), or occlusion (89) compounds, or as adducts (54). 

Fig. 2. Classification/nomenclature of host—guest type inclusion compounds, definitions and relations (/) coordinative interaction, (2) lattice barrier interaction, (J) monomolecular shielding interaction (I) coordination-type inclusion compound (inclusion complex), (II) lattice-type inclusion compound (multimolecular/extramolecular inclusion compound, clathrate), (III) cavitate-type inclusion compound (monomolecular/intramolecular inclusion...

Extramolecular Cavity Inclusions Lattice-Type Inclusion Compounds, Clathrates... 

A wide variety of guest molecules may be trapped by the Wemer-type crystalline host lattice, ranging, eg, from noble gases to condensed aromatic hydrocarbons. These clathrates may be formed from solution or by sorption. Kinetics of sorption—desorption have been studied (83). 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

Probably the most familiar of all clathrates are those formed by Ar, Kr and Xe with quinol, l,4-C6H4(OH)2, and with water. The former are obtained by crystallizing quinol from aqueous or other convenient solution in the presence of the noble gas at a pressure of 10-40 atm. The quinol crystallizes in the less-common -form, the lattice of which is held together by hydrogen bonds in such a way as to produce cavities in the ratio 1 cavity 3 molecules of quinol. Molecules of gas (G) are physically trapped in these cavities, there being only weak van der Waals interactions between... 

A common feature of all clathrates discussed so far is a host lattice, by itself thermodynamically unstable, which is stabilized by inclusion of the second component. The forces binding this component must be similar in nature to the intermolecular forces in liquids. It seems natural, therefore, to regard a clathrate compound as a solid solution of the second component in the (meta-stable) host lattice. 

In the present review a description is given of the phase behavior of clathrates on the basis of a solution theory. The treatment is restricted to those cases where the empty host lattice ( solvent") is indeed unstable, although many of the present considerations also apply to the few cases known where the host lattice is stable. An example of the latter is the chroman complex first discovered by Dianin9 and recently examined by Baker and McOmie and Powell and Wett ers.34... 

Although it is difficult to predict exactly which solute molecules will form clathrate solutions in any given host lattice, the general principle is quite clear. All molecules which fit into the cavities will be able to stabilize the host lattice, unless they show a specific chemical interaction with the solvent molecules. HC1 (or the other hydrogen halides), for instance, does not form a clathrate with water, but rather the stoichiometric compounds HC1 H20,... 

Fig. 1. The crystal structure of a hydroquinone clathrate according to Palin and Powell. 8 The balls inside the transparent spheres represent argon atoms encaged in the cavities formed by the two interpenetrating lattices, (photograph kindly supplied by Dr. Powell).

The foregoing lattice is that which is found in the clathrates of hydroquinone with small molecules (C2H2, HC1, etc.). Powell24 further showed that if larger molecules are included, the hydroquinone lattice is distorted to form oblong cavities this distortion increases in the series CH3OH, S02, C02 and has become extreme for CH3CN. 

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

The assumptions (a) - (d) are believed to give an adequate description of the physical situation in the great majority of clathrates. Assumption (a) implies that the spectrum of the host lattice is not affected by the presence of the solute molecules. Little is known about this, but since the host lattice in general is a com-... 

The equilibrium clathrate of methanol has the much higher value yA = 0.47 at 25°C. This is to be expected since the methanol molecule is so large that it distorts the lattice contrary to assumption (a) of Section II.A, thereby increasing the value of Ay to be taken in Eq. 25. The methyl cyanide molecule distorts the lattice even more, and as already noted by Powell,24 its equilibrium clathrate must therefore have a value of yA still higher than that for the methanol clathrate. (The CH3CN clathrate investigated by Powell, however, was not an equilibrium clathrate, cf. point B in Fig. 5). 

To conclude this section it may be worthwhile to remark that, although the L-J-D method is generally applied to a face-centered cubic lattice (z = 12), it is equally valid for the cavities in a clathrate (z = 20—28), as long as one restricts oneself to first-neighbor interactions. 

One important difference between the present and the previous case should be noted. For the hydroquinone clathrates, where the wall of a cavity consists of 12 OH groups, 6 adjacent carbon atoms, and 6 CH groups in ortho position to the OH groups, it seemed best to consider the product z qjk) as one unknown. For hydrates one may not do this the walls of both types of cavities consist exclusively of tetrahedrally-coordinated water molecules. Hence, one should use the same value of (,eg/k) —characteristic for a water molecule in a hydrate lattice—for both types of cavities and multi-... 

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

Acetylene, clathrate in hydroquinone, 7 hydrate thermodynamic data and lattice constants, 8 Acrylamides, polymerization of, 181 Acrylonitrile, 155 Activity coefficients, 125... 

Hindered rotation, 33, 34 internal, 367 Homopolymer, 168, 183 Hot bands, 374 Hot lattice, 4, 11, 21 Hydrates, 7, 9, 21, 31, 41 crystallization, 44 Hydrochloric acid clathrates, 2 in hydroquinone, 7 Hydrogen, bound, 4, 175 bromine hydrate, 35 4- carbon dioxide system, 110 4 carbon monoxide system, 96, 108 chloride hydrate, 35 clathrates, 2 chloride, 30... 

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