Structure of [18]Annulene Revisited: Challenges for Computing Benzenoid Systems

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Hartree-Fock (HF) and density functional theory (DFT) methods often produce erratic computational results for cyclic conjugated systems, particularly those with high HF-exchange contributions.
• This unreliability stems from issues related to molecular orbital (MO) stability, specifically near-zero eigenvalues.

Purpose of the Study:

• To explore the reasons behind the unreliability of computational methods for cyclic conjugated structures.
• To provide guidance on avoiding pitfalls and ensuring due diligence in computational investigations of such systems.
• To determine the equilibrium structure of [18]annulene.

Main Methods:

• Extensive computations on [18]annulene and related compounds.
• Careful examination of MO singlet-stability eigenvalues.
• Density-fitted frozen natural orbital coupled-cluster singles and doubles with perturbative triples (DF-FNO CCSD(T)) method using an aug-pVQZ/aug-pVTZ basis set.

Main Results:

• Near-zero (positive or negative) MO singlet-stability eigenvalues, rather than negative ones, can lead to dramatic errors in vibrational frequencies and related properties.
• DFT methods with lower HF admixture are generally more robust for benzenoid structures but may overestimate planarity and C-C bond equalization.
• [18]annulene exhibits a nonplanar equilibrium structure, with the C2 global minimum found to be 1.1 kcal mol-1 lower than the D6 stationary point.

Conclusions:

• A thorough examination of MO singlet-stability eigenvalues is crucial for reliable computational studies of cyclic conjugated systems.
• While DFT with lower HF exchange is often more stable, it can introduce biases.
• [18]annulene's equilibrium structure is nonplanar, as supported by high-level correlated calculations.