Carbene stabilization by aryl substituents. Is bigger better?

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Organic Chemistry

Background:

• Carbenes are reactive intermediates with diverse electronic states.
• Aryl-substituted carbenes exhibit unique stability and reactivity patterns.
• Understanding carbene electronic structure is crucial for synthetic applications.

Purpose of the Study:

• To investigate the geometries and relative stabilities of singlet and triplet states in various aryl-substituted carbenes.
• To analyze the impact of substituent size and type on carbene electronic properties.
• To compare computational predictions with experimental data for singlet-triplet energy gaps.

Main Methods:

• Density functional theory (DFT) calculations using the B3LYP/6-311+G(d,p) + ZPVE level.
• Investigation of phenyl, diphenyl, naphthyl, di(naphthyl), and anthryl-substituted carbenes.
• Inclusion of an empirical correction for singlet-triplet energy gaps.

Main Results:

• Calculated singlet-triplet energy separations (DeltaEST) show good agreement with experimental values after empirical correction.
• Triplet di(9-anthryl)carbene exhibits an allene structure with significant biradical character.
• Resonance stabilization increases with larger aromatic substituents (e.g., naphthyl > phenyl).
• Steric effects and orbital interactions influence carbene geometries and bond lengths.

Conclusions:

• DFT calculations accurately predict singlet-triplet energy gaps for aryl carbenes.
• Larger aromatic substituents provide greater resonance stabilization to both singlet and triplet carbenes.
• Carbene geometry is modulated by steric hindrance and electronic interactions with aromatic rings.