Tumor hypoxia hampers radiotherapy efficacy, necessitating radiosensitizers. Substituted nucleobases offer advantages as radiosensitizers. They enable DNA incorporation with minimal gene expression alteration, selectively targeting tumor cells and lower toxicity to normal tissues. They possess higher electron affinity than native DNA, facilitating rapid electron attachment for cancer cell damage. Despite advancements, exploration beyond uracil nucleobases remains limited. Herein, we investigated the electron attachment to potential radiosensitizers, specifically 5halo-2′-deoxycytidine-3′-monophosphate (5X-3′-dCMPH). Our findings indicate that 5X-3´-dCMPH nucleotides possess higher electron affinity than unsubstituted 3′-dCMPH, suggesting halogenated nucleotides are better electron acceptors. Moreover, the high vertical detachment energy (VDE) implies minimal auto-detachment, and the dissociative electron attachment (DEA) pathways suggest that dehalogenation is the favored process for halogenated systems, supported by low dissociation barriers. Notably, 5Br-3′-dCMPH and 5I-3′-dCMPH exhibit nearly barrier-free dissociation after electron attachment, and thus, they may preferentially act as superior radiosensitizers.