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Sometimes a compound can be trapped inside another molecule rather like
a ball in a cage. Organic chemists in the US who have looked at such compounds
have found that they are held in place unexpectedly tightly, and are unable
to move about freely. They believe that these cage compounds could one day
be made to behave like liquid crystals.

Donald Cram and his colleagues at the University of California, and
Marco Vincenti of the Guido Donegani Institute at Novara, Italy, made seven
new compounds in which one, or even two, ‘host’ molecules, such as methanol
(CH₃OH) or acetonitrile (CH₃CN), are trapped inside
large ‘guest’ molecules (Journal of the Chemical Society: Chemical Communications,
p 1403).

Cram’s group has been making cage molecules since 1985. Cages form in
a reaction in which the two halves come together. In the process, a compound
may be trapped. For example, when the two halves of a cage are joined in
a solution such a acetonitrile or methanol, one or two of the solvent molecules
may end up incarcerated.

Cram and his colleagues used the technique of nuclear magnetic resonance
(NMR) spectroscopy to probe the inner secrets of the cages. The technique,
which involves placing the cages in an intense magnetic field and subjecting
them to radio waves, identifies the hydrogen atoms of the trapped molecules.
The results reveal just how the prisoners are arranged. When two acetonitrile
molecules are trapped, their CN parts are together at the host’s equator,
and their CH₃ groups in the temperate regions.

Cram’s team found that acetonitrile molecules are not entirely stable
in this arrangement. When the chemists heated the complex to 110 °C,
one of the molecules escaped from the cage, probably through a hole at one
of the poles. But the chemists found that they were unable to dislodge the
remaining molecule, even by heating the cage at 215 °C for five days.

Cram believes he has an explanation. He believes that the first molecule
is expelled only when it is struck by the other, rather like when two billiard
balls collide. However, this mechanism no longer operates when the second
molecule is left alone. It simply cannot accumulate enough energy to escape.

Cram and his colleagues find that when the trapped molecules are both
methanols, even the billiard ball trick won’t allow either to escape. Heating
the cage at 110 °C for five days has no effect. Cram believes that methanol
molecules stay together because a hydrogen bond connects their oxygen atoms.
However, it is a peculiar hydrogen bond. When the chemists take an NMR spectrum,
the sign that this hydrogen atom creates resonates in a different part of
the spectrum from any other hydrogen bond.

Cram’s group hopes to exploit the ability of host molecules to control
their guests in order to make compounds that behave like liquid crystals.
At the moment, the chemists are investigating host molecules in which the
guest responds to visible light which is transmitted through the cage.

If the trapped molecules are polar – that is, there is a separation
of positive and negative charge within the molecule – and an external electric
field is applied, they will line up in the same direction temporarily. In
this regular arrangement, they will diffract light in a regular pattern.
When the electric field is removed, however, the molecules will relax into
random positions and scatter light in all directions.

Inorganic chemists have also been encapsulating other chemicals, including
chloride, bromide, iodide and carbonate ions. Achim Muller and his colleages
at the University of Bielefield have discovered that vanadium and oxygen
atoms will form ‘spherical cluster shells’ which hold negatively charged
atoms captive. Muller points out that this is remarkable because the shells
themselves are negatively charged and should repel their captives.

Muller and his colleagues made a typcial shell by treating a solution
of caesium vanadate (C_sVO₃) with hydrazinium hydroxide. (N₂H₅OH)
at 95 °C for an hour. To the dark brown solution they then added hydrobromic
acid, and left the mixture for a further hour-and-a-half at the same temperature.
On cooling, black crystals precipitated out with the formula CS₉(H₄V₁₈O₄₂Br).
12H₂O.

Muller’s group analysed these using the techniques of infrared spectroscopy
and X-ray diffraction. These revealed that the bromide ion was inside the
ball. The chemists have discovered that the same cage will also trap a larger
iodide ion.