Bonding in benzodicyclobutadiene isomers: Insights from modern valence bond theory

TY - JOUR

T1 - Bonding in benzodicyclobutadiene isomers

T2 - Insights from modern valence bond theory

AU - Cooper, David L.

AU - Karadakov, Peter B.

PY - 2014

Y1 - 2014

N2 - 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 πelectron 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 Å, respectively). Analysis of the π-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 Kekulé-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 Kekulé-like structures. In the case of benzo[1,2:3,4]dicyclobutadiene, analysis of the π-space SC wavefunction shows that the expected Kekulé-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 Kekulé-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.

AB - 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 πelectron 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 Å, respectively). Analysis of the π-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 Kekulé-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 Kekulé-like structures. In the case of benzo[1,2:3,4]dicyclobutadiene, analysis of the π-space SC wavefunction shows that the expected Kekulé-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 Kekulé-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.

KW - benzodicyclobutadiene

KW - bond-stretch isomers

KW - para-bonded structures

KW - spin-coupled theory

UR - http://www.scopus.com/inward/record.url?scp=84909947290&partnerID=8YFLogxK

U2 - 10.1080/00268976.2014.914598

DO - 10.1080/00268976.2014.914598

M3 - Article

AN - SCOPUS:84909947290

SN - 0026-8976

VL - 112

SP - 2840

EP - 2852

JO - Molecular Physics: An International Journal at the Interface Between Chemistry and Physics

JF - Molecular Physics: An International Journal at the Interface Between Chemistry and Physics

IS - 21

ER -