Aromaticity and Photoreactivity of 4‐ and 3‐Nitrenopyridine 1‐Oxides and Phenylnitrene

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 that 3 1N and 1 1N , as well as 1 2N and 1 PhN , exhibit significant imine biradical character, whereas the structures of 3 2N and 3 PhN are 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 that 3 1N is weakly aromatic, whereas 3 2N and 3 PhN are best described as having Baird aromaticity. In contrast, singlet nitrenes 1 1N , 1 2N , and 1 PhN are nonaromatic. In addition, irradiation of 1 in argon matrices verified that 3 1N reacts photochemically to form corresponding ketenimine 1K . Finally, the absorption difference spectrum of 3 1N in 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.