Quadripartite bond length rule applied to two prototypical aromatic and antiaromatic molecules

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Modeling

Background:

• A 2000 rule relates geometries of isomolecular states (ground singlet, excited triplet, radical anion, radical cation).
• This rule enables predicting one geometry if three others are known.
• Benzene and cyclobutadiene serve as test cases for aromatic and antiaromatic systems.

Purpose of the Study:

• To verify the applicability of the geometric rule for benzene and cyclobutadiene.
• To assess the rule's performance across different electronic states and molecular geometries.

Main Methods:

• Utilized density functional theory (DFT), coupled cluster (CC2), and multi-reference configuration interaction (CASSCF/CASPT2) methods.
• Employed Dunning's correlation-consistent basis sets (aug-cc-pVXZ).
• Used computational software Gaussian 16, Turbomole 7.1, and Molcas 8.0.

Main Results:

• The geometric rule shows good applicability for benzene.
• The rule functions independently for quinoid and anti-quinoid geometries in benzene.
• Challenges were noted in the theoretical description of radical anion species.

Conclusions:

• The geometric rule is validated for benzene and cyclobutadiene.
• The findings support the utility of the rule in predicting molecular geometries in computational studies.
• Further investigation into radical anion descriptions may be warranted.