The special nature of C28
The origin of the C28 and isolated pentagon rule paper was a result by the Rice group.
The result by the Rice group indicated that multiphoton disassociation of C60 proceeded by elimination of C2 groups and cage closure sequentially, ie C58 > C56 > C54 down to C32. At this point, C32 essentially exploded. This led the Rice group to suggest that this was a smallest possible fullerene.
While sitting at a Lewes coffee table one Sunday afternoon playing with a molecular modelling kit, Kroto wondered what the possible structure of C32 might be. He produced a model and realised it was not C32, but C28.
He was suddenly exhilarated because he remembered that they had an experimental run in which the C28 signal was very strong indeed. Kroto wanted to publish the Rice/Sussex results but the request was refused.
When I examined the structure of the C28 molecule I had constructed, I realised that it might be very special in that it was tetrahedral. If it added four hydrogen atoms to the four tetrahedral C corner atoms, then it still retained four aromatic six-membered rings. The corner atoms would become sp3 and would be relaxed. This suggested that C28 might be a cluster super atom tetravalent analogue of the carbon atom.
Sir Harry Kroto
Several years later, the Rice group published a paper in which they showed that Kroto's tetravalency conjecture was correct, though it was not referenced as such. They observed U@C28 where the tetravalency was satisfied by an endohedral U atom.
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Download the IPR and C28 paper (PDF, 566KB)
The smallest stable fullerene, M@C28 (M = Ti, Zr, U): stabilisation and growth from carbon vapor
Sir Harry Kroto, "In this paper I neglected to refer to our paper in which the structure of C70 was suggested. I just did not think about it over a year later. It was the last thing on my mind as for me it was the exhilaration of discovering the IPR and realising it essentially proved C60 was a Buckyball as well as the special nature and beautiful tetravalency of C28 that was important.
"This unconscious omission was assumed to be a deliberate attempt by me to gain credit for the C70 structure. It was used as a pretext to cause me a LOT of trouble. I only discovered this many years later by an inadvertent comment by a co-worker who had accepted this contention as true.
"It is interesting to realise that you can be so affected through no fault of your own."