科研之友
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摘要
Spin-coupled (SC) theory, which uses a compact and easy-to-interpret wavefunction that is comparable in quality to a complete active space self-consistent field construction, is used to obtain modern ab initio valence bond descriptions of the pi-electron systems of three benzodicyclobutadiene isomers. The geometries at the two energy minima for benzo[1,2:4,5]dicyclobutadiene (I and II) differ mostly in the lengths of the annulated CC bonds (ca. 1.41 and 1.55 angstrom, respectively). Analysis of the pi-space SC wavefunctions indicate that the electronic structure of isomer I resembles a fairly standard benzene ring augmented with two exocyclic double bonds; its resonance pattern is dominated by two Kekule-like and three less-important Dewar-like structures. Contrary to assumptions made in previous work, the resonance pattern for isomer II features two para-bonded structures which are at least as important as any of the Kekule-like structures. In the case of benzo[1,2:3,4]dicyclobutadiene, analysis of the pi-space SC wavefunction shows that the expected Kekule-like structure is indeed the most important, but this system provides further evidence that reliance on chemical intuition can sometimes lead to erroneous predictions about the relative importance of resonance structures; certain para-bonded structures turn out to be more important than any of the other four Kekule-like structures. The SC calculations suggest that benzo[1,2:4,5]dicyclobutadiene isomer I is the most aromatic, followed by benzo[1,2:4,5]dicyclobutadiene isomer II, whereas benzo[1,2:3,4]dicyclobutadiene is close to being a non-aromatic conjugated system.
- 出版日期2014-11-2
