Hydroquinone lattices with

Once such a molecular complex with hydroquinone has been formed it may persist under conditions where it is no longer thermodynamically stable. Because the molecules of the second component are enclosed in the cavities they cannot escape without breaking a number of hydrogen bonds in the -hydroquinone lattice. This corresponds to a considerable energy of activation which may prevent the attainment of thermodynamic equilibrium. 

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. 

P-hydroquinone lattice cavity with methylisocyanide guest. 

Figure 7.20 (a) Schematic diagram showing the hydroquinone cage structure, (b) Structure of the P-hydroquinone lattice cavity with methylisocyanide guest. 

HYDROQUINONE HOST LATTICES WITH ENCLATHRATED FULLERENES... 

In the hydroquinone clathrate compounds the guest molecules are separated by the rigid hydroquinone lattice. The interaction among them is primarily electrostatic, perhaps with an additional effect of the van der Waals force. 

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... 

The lattice of the host in the form it takes in the clathrate is usually thermodynamically unstable by itself—that is, with the holes empty. It is stabilized by inclusion of the guest molecules, and it is of obvious interest in connection with the nonstoichiometry of clathrates to consider the extent to which the cavities in the host lattice must be filled before the system achieves thermodynamic stability. The cavities in the host lattice may all be identical in size and environment, as in the hydroquinone clathrates, or they may be of more than one kind. The gas hydrates, for example, have two possible structures, in each of which there are two sorts of cavity, van der Waals and Platteeuw (15) have developed a general statistical theory of clathrates containing more than one type of cavity. 

The hydroxylation of phenol on TS-1 is normally operated in a slurry reactor, at temperatures close to 100°C, with total consumption of the oxidant. The selectivities on phenol and H2O2 are generally in the ranges 90-95% and 80-90%, respectively. The hydroquinone to catechol ratio is well in excess of the statistical value of 1 2, owing to lower steric requirements for / -hydro-xylation and the faster diffusion of the p-substituted product (Table 2). Yields and kinetics are strictly related to the content of lattice Ti. It should be emphasized that any extra-framework Ti species, present as impurities on TS-1, are the major source of unproductive side reactions, such as H2O2 decomposition and unselective radical chain oxidations. 

Hydroquinone, p-C6H4(OH)2, also known as quinol in the older literature) exists in three polymorphic forms (or crystalline modifications). a-Hydroquinone is the stable form at room temperature, whereas the metastable monoclinic y-form can be prepared by sublimation or rapid evaporation of a solution in ether. Crystallization of hydroquinone from a common solvent such as methanol generally yields a clathrate with guest solvent inolecules trapped inside cavities of the (3-hydroquinone host lattice.". The existence of the (3 polymorph of hydroquinone (empty P-hydroquinone clathrate), which can be obtained by crystallization from n-octane, was reported in 1981. Described in this article are the structural features of these compounds, and some recent developments are also covered. 

The architecture of the single P-hydroquinone host lattice of 3C6H4(OH)2 C6o may be compared with that of empty P-hydroquinone, with hexagonal unit cell edges... 

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