Pathways of the Dissociative Electron Attachment Observed in 5- and 6‑Azidomethyluracil Nucleosides: Nitrogen (N₂) Elimination vs Azide Anion (N₃^–) Elimination

5-Azidomethyl-2′-deoxyuridine (5-AmdU, 1) has been successfully employed for the metabolic labeling of DNA and fluorescent imaging of live cells. 5-AmdU also demonstrated significant radiosensitization in breast cancer cells via site-specific nitrogen-centered radical (π-aminyl (U-5-CH₂–NH^•), 2, and σ-iminyl (U-5-CHN^•), 3) formation. This work shows that these nitrogen-centered radicals are not formed via the reduction of the azido group in 6-azidomethyluridine (6-AmU, 4). Radical assignments were performed using electron spin resonance (ESR) in supercooled solutions, pulse radiolysis in aqueous solutions, and theoretical (DFT) calculations. Radiation-produced electron addition to 4 leads to the facile N₃^– loss, forming a stable neutral C-centered allylic radical (U-6-CH₂^•, 5) through dissociative electron attachment (DEA) via the transient negative ion, TNI (U-6-CH₂-N₃^•–), in agreement with DFT calculations. In contrast, TNI (U-5-CH₂–N₃^•–) of 1, via facile N₂ loss (DEA) and protonation from the surrounding water, forms radical 2. Subsequently, 2 undergoes rapid H-atom abstraction from 1 and produces the metastable intermediate α-azidoalkyl radical (U-5-CH^•-N₃). U-5-CH^•-N₃ converts facilely to radical 3. N₃^– loss from U-6-CH₂–N₃^•– is thermodynamically controlled, whereas N₂ loss from U-5-CH₂–N₃^•– is dictated by protonation from the surrounding waters and resonance conjugation of the azidomethyl side chain at C5 with the pyrimidine ring.