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Structure of cubane and solid cubane |
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Originally there was doubt whether
cubane does exist. The geometry at each carbon atom is far from tetrahedral.
Only later, we found out that there is no kinetically viable paths exist
for the thermal rearrangement of cubane. At same time, orbital symmetry
considerations shows that the energy of concerted two-bond ring-opening
reactions is very high. There will be very little gain in energy by breaking
just one bond, as the concomitant change in geometry is small, and the resulting
biradical is still very strained. |
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In 1964 Fleischer showed that cubane forms a
stable solid at room temperature with a crystalline structure composed of
cubane molecules occupying corners of the rhombohedral primitive unit cell
(space group R3). The cubic molecular geometry gives the solid many unusual
electronic,structural, and dynamical properties compared to the other hydrocarbons.
For example, solid cubane has a relatively high melting point temperature
about 405 K! and a very high frequency for the lowest-lying intramolecular
vibrational
mode (617 cm-1). Recent work related to cubane has focused on
solid cubane and cubane based
derivatives.Because of relatively weak intermolecular interaction the cohesive
energy relative to the constituent C8H8 is expected
to be small, and most of the physical properties of solid cubane are dominated
by the properties of the C8H8molecule. |
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