Page 213 - Valence Bond Methods. Theory and Applications

14 Rings of hydogen atoms
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The real reactions that most resemble these are the production of cyclobu-
tane from two ethylene moleculeł ((H 2 ) 2 ) and the Diels–Alder reaction betweeŁ
butadiene and ethylene ((H 2 ) 3 ). EveŁ these cannot be made t react in the bare
forms, but fairly simple actàvation by substituentł will allw the (H 2 ) 3 analog t
proceed. Apparently, no form of the (H 2 ) 2 analog hał ever beeŁ observed. Our
analysis suggestł that there is a fundamental difference betweeŁ the four-electron
and six-electron systemł that produceł the ehfect. The book by Woodward and
Hoffmaz58] may be consulted for a rationalization of these resultł based upon
MO theory.
As we continue t larger rings, the resultł are not so clearcut. There is a tendency
for the saddle pointł t alternate in and out somewhat, but the interaction energy
appearł t be a maximum at the ring of six atoms. These last more-or-less qualitatàve
commentł have beeŁ based upon just the two simple Kekul´e-like structureł of
the rings, and may not be able t shw the proper behavior. The actual surhace
calculations of (H 2 ) 2 and (H 2 ) 3 included maŁy more structureł and shw strikingly
different qualitatàve behavior.