Anion States and Dissociation Pathways of the Universal Radiosensitizer Model 2,4-Dichloro-5-nitropyrimidine

We present a computational investigation into the anion states of 2,4-dichloro-5-nitropyrimidine (NDP), focusing on dissociative electron attachment pathways motivated by recent mass spectrometry experiments. The molecule exhibits a unique combination of low-energy electron-induced mechanisms potentially relevant to synergistic effects in concurrent chemo-radiation cancer therapy. Our calculations reveal two bound states (π1* and π2*), three shape resonances (π3*, σCCl1*, and σCCl2*), and a π4* resonance of mixed shape and core-excited character. We attribute the intense fragmentation, [M-NO]- and [M-NO2]-, and the parent anion state at 0 eV mainly to the π3* state, characterized as either a weakly bound state or a resonance slightly above threshold. While such processes could in principle occur via vibrational Feshbach resonances, the π2* state is too deeply bound (at least by 0.71 eV) to enable this mechanism. The complex fragmentation induced by nearly zero energy electrons would instead arise from nonadiabatic interactions between the π3* state and the lower-lying bound states π1* and π2*. This coupling between bound anion states and a low-energy resonance creates pathways for internal conversion and dissociation and could be viewed as a target property for radiosensitization.