Aromaticity and Homoaromaticity in Methano[10]annulenes

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Quantum Chemistry

Background:

• Methano[10]annulenes are intriguing polycyclic systems.
• Understanding their aromaticity and strain is crucial for predicting chemical behavior.

Purpose of the Study:

• To evaluate aromaticity and neutral homoaromaticity in 1,4-, 1,5-, and 1,6-methano[10]annulenes.
• To compare the stability and electronic properties of these isomers.

Main Methods:

• Density Functional Theory (DFT) calculations (B3LYP/6-311+G(d,p)).
• Analysis of C-C bond lengths, relative energies, and strain energies using homodesmotic reactions.
• Evaluation of aromaticity using magnetic susceptibilities (chiM), HOMA, NICS, and resonance energies (RE).
• Natural Bond Orbital (NBO) and Geometry-Dependent ביותר (GPA) analyses for homoaromaticity.

Main Results:

• 1,4-methano[10]annulene exhibits higher bond alternation than isomers 2 and 3.
• Isomer 3 is the most stable, with strain energies correlating to relative energies.
• Isomers 2 and 3 are strongly aromatic, while isomer 1 is fairly aromatic, based on HOMA, RE, and chiM.
• NICS values were unreliable; NBO and GPA analyses suggest neutral homoaromaticity, particularly in isomer 3.

Conclusions:

• The study provides a comprehensive analysis of aromaticity and homoaromaticity in methano[10]annulenes.
• Strain is primarily localized in the rings rather than the bridges.
• Neutral homoaromaticity is significant in isomer 3, offering new insights into these systems.