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ABSTRACT The pursuit of sustainable organic synthesis has renewed interest in photochemistry, as sunlight‐driven reactions provide eco‐friendly alternative methods. Although the relationships among structure, properties, and reactivity are well established for ground‐state molecules, the understanding of excited states and reactive intermediates, such as triplet and singlet arylnitrenes, remains limited. Herein, we investigated the properties of triplet and singlet 4‐nitrenopyridine‐1‐pyridine oxide (1N), 3‐nitrenopyridine‐1‐pyridine oxide (2N), and phenylnitrene (PhN) using density functional theory (DFT), complete active space self‐consistent field (CASSCF(10,9)), and complete active space second‐order perturbation theory (CASPT2(10,9)) calculations. Bond length analysis demonstrated that31Nand11N, as well as12Nand1PhN, exhibit significant imine biradical character, whereas the structures of32Nand3PhNare better described as benzene‐like. Nucleus‐independent chemical shift (NICS(0), NICS(1.7)ZZ) and anisotropy of induced current density (ACID) calculations were performed to compare the induced magnetic currents in these molecules. These analyses demonstrated that31Nis weakly aromatic, whereas32Nand3PhNare best described as having Baird aromaticity. In contrast, singlet nitrenes11N,12N, and1PhNare nonaromatic. In addition, irradiation of1in argon matrices verified that31Nreacts photochemically to form corresponding ketenimine1K. Finally, the absorption difference spectrum of31Nin a frozen 2‐methyltetrahydrofuran (mTHF) matrix exhibited resolved vibrational structure, suggesting the vibrational coupling to another electronic state. These insights into the structure and aromaticity of heterocyclic nitrenes could provide new avenues for modulating the reactivity of triplet ground state and triplet excited molecules.
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Aromaticity of the triplet states of corannulene and coronene
Abstract Sunlight‐driven photochemical reactions are an important tool for sustainable organic synthesis. However, compared with ground states, for which the effects of structure on properties and reactivity are well established, the understanding of excited states is limited. In particular, an improved understanding of aromaticity and antiaromaticity in excited states is necessary to develop strategic photochemical methods for synthesizing polycyclic aromatic compounds. Herein, using density functional theory (DFT)‐optimized structures, the ground singlet (S0) and lowest triplet (T1) states of coronene and corannulene were compared. Bond length analysis demonstrated that both triplet corannulene and triplet coronene bear a partial resemblance to benzene. Nucleus‐independent chemical shift (NICS(0), NICS(1.7)ZZ, NICS scans) and anisotropy of the induced current density (ACID) calculations were carried out to compare the induced magnetic currents in these molecules. This analysis demonstrated rather weak π‐conjugation and partial antiaromaticity in the S0state of each molecule. In contrast, a combination of circular induced currents and pronounced antiaromaticity was found in the T1state of each molecule. However, the T1of corannulene exhibited higher stability, which should facilitate functionalization. Consequently, corannulene is considered more suitable for photochemical applications.
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- Award ID(s):
- 2102248
- PAR ID:
- 10379852
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Journal of Physical Organic Chemistry
- Volume:
- 36
- Issue:
- 1
- ISSN:
- 0894-3230
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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