Tuning effects for some cyclic aromatic carbenes bearing remote amino groups

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Quantum Chemistry

Background:

• Cyclic aromatic carbenes are important molecules in organic chemistry.
• Yamamoto and co-workers synthesized novel cyclic aromatic carbenes with remote amino groups.
• Understanding singlet-triplet splitting is crucial for predicting carbene reactivity and stability.

Purpose of the Study:

• To theoretically investigate the effects of functional group variations on the singlet-triplet splitting (ΔE(S-T)) in cyclic aromatic carbenes.
• To explore methods for tuning the electronic ground state of these carbenes.
• To identify molecular designs that favor triplet electronic ground states.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to study a series of related cyclic aromatic carbene compounds.
• Systematic variations of functional groups, including substitutions on the X fragment and amino groups, were analyzed.
• Singlet-triplet energy differences (ΔE(S-T)) were calculated for each modified structure.

Main Results:

• The parent Yamamoto compound exhibits a singlet ground state (ΔE(S-T) = 15.7 kcal/mol).
• Replacing dimethylamino groups with hydrogen atoms reduced the singlet-triplet separation by approximately 7 kcal/mol.
• Substitutions on the X fragment (e.g., S, Se, Te, SO, SeO, TeO) modulated ΔE(S-T), with some yielding triplet ground states (e.g., X = BH, AlH, GaH, InH, TlH).
• Replacing N(CH3)2 with BH2 and using X = BH resulted in the strongest preference for a triplet state (ΔE(S-T) = -13.7 kcal/mol).
• Relocating amino groups to different ring positions also yielded significant changes in electronic properties.

Conclusions:

• Functional group modifications offer a powerful strategy for tuning the singlet-triplet splitting in cyclic aromatic carbenes.
• Specific substitutions can effectively shift the electronic ground state from singlet to triplet.
• The findings provide valuable insights for the rational design of carbenes with desired electronic and reactive properties.