Dehydro[12]annulenes: structures, energetics, and dynamic processes.

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Theoretical Chemistry

Background:

• Monodehydroannulenes are cyclic hydrocarbons with potential for unique electronic properties.
• Understanding the stability and reactivity of these annulenes is crucial for synthetic chemistry.

Purpose of the Study:

• To identify the most stable isomer of neutral monodehydro[12]annulenes.
• To assess the kinetic stability and potential reaction pathways of the identified isomers.

Main Methods:

• Density functional theory (DFT) and coupled cluster (CC) calculations were employed.
• High-level computational methods like CCSD(T)/cc-pVDZ//BHLYP/6-31G* were utilized.
• Nuclear Magnetic Resonance (NMR) parameters were computed using GIAO-B3LYP/6-311+G**//RHF/6-31G*.

Main Results:

• The unsymmetrical CTCTC conformer (1a) was identified as the global minimum, lying at least 3 kcal/mol below other isomers.
• Isomer 1a exhibits rapid conformational automerization (E(a) = 3.9 kcal/mol) and presents distinct computed (1)H NMR parameters.
• Cumulenic isomers were found to be reactive intermediates, facilitating interconversion of alkyne forms.
• A significant overall barrier (28 kcal/mol) for the pathway from isomer 1a to biphenyl indicates kinetic stability.

Conclusions:

• Neutral monodehydro[12]annulenes have a kinetically stable global minimum (isomer 1a).
• Isomer 1a's stability suggests potential for isolation and further study.
• The computational approach provides valuable insights into the structure, stability, and reactivity of these annulene systems.