Resonance and Aromaticity: An Ab Initio Valence Bond Approach

Abstract

Resonance energy is one of the criteria to measure aromaticity. The effect of the use of different orbital models is investigated in the calculated resonance energies of cyclic conjugated hydrocarbons within the framework of the ab initio Valence Bond Self-Consistent Field (VBSCF) method. The VB wave function for each system was constructed using a linear combination of the VB structures (spin functions), which closely resemble the Kekulé valence structures, and two types of orbitals, that is, strictly atomic (local) and delocalized atomic (delocal) p-orbitals, were used to describe the π-system. It is found that the Pauling-Wheland's resonance energy with nonorthogonal structures decreases, while the same with orthogonalized structures and the total mean resonance energy (the sum of the weighted off-diagonal contributions in the Hamiltonian matrix of orthogonalized structures) increase when delocal orbitals are used as compared to local p-orbitals. Analysis of the interactions between the different structures of a system shows that the resonance in the 6π electrons conjugated circuits have the largest contributions to the resonance energy. The VBSCF calculations also show that the extra stability of phenanthrene, a kinked benzenoid, as compared to its linear counterpart, anthracene, is a consequence of the resonance in the π-system rather than the H-H interaction in the bay region as suggested previously. Finally, the empirical parameters for the resonance interactions between different 4n+2 or 4n π electrons conjugated circuits, used in Randić's conjugated circuits theory or Herdon's semi-emprical VB approach, are quantified. These parameters have to be scaled by the structure coefficients (weights) of the contributing structures.

Predicting an Antiaromatic Benzene Ring in the Ground State Caused by Hyperconjugation

“…evaluated the aromatic character in the central six‐membered rings (6MRs) of 4 , 5 and 6 (Figure b), and found that the “benzene‐like” 6MR in 4 is aromatic whereas the “[6]radialene‐like” ones in 5 and 6 are non‐aromatic . In addition, the research for benzocyclobutadiene and the exploration of the origin of the aromaticity of benzene ring in benzocyclobutadiene have been studied by both experimental and theoretical chemists . The prevailing view supported that the benzene ring in benzocyclobutadiene is essentially nonaromatic.…”

“…[20] In addition, the research for benzocyclobutadiene and the exploration of the origin of the aromaticity of benzene ring in benzocyclobutadieneh ave been studied by both experimentala nd theoretical chemists. [21][22][23][24][25] The prevailing view supported that the benzene ring in benzocyclobutadiene is essentially nonaromatic. Our ongoing interest in aromaticity [26][27][28][29] has led us to probe ap ossibility to achievea na ntiaromatic benzene ring.…”

Aromaticity in catalysis: metal ligand cooperation via ligand dearomatization and rearomatization

“…99,100 Additionally, the Valence Bond Self-Consistent Field (VBSCF) method also can be useful in such calculations. 101…”

The Nature of the Chemical Bond from a Quantum Mechanical Interference Perspective

“…Benzene is among the most studied systems in chemistry. This emblematic molecule is an annulene composed of 6 carbon atoms displaced on a D 6h symmetric planar ring, and its geometric, electronic and magnetic properties are still reasons for scientific inquiry . From the energetic point of view, the molecule is more stable than their cyclic polyenes analogues.…”