Transient anion spectra of the potential radiosensitizers 5-cyanateuracil and 5-thiocyanateuracil

We report on the low energy anion spectra of 5-cyanateuracil (5-OCNU) and 5-thiocyanateouracil (5-SCNU), which have been the suggested potential radiosensitizers for use in cancer therapy [L. Chomicz et al., J. Phys. Chem. Lett. 4, 2853-2857 (2013)]. Employing bound state and scattering calculations, we obtained, for both molecules, a dipole bound state, a π^* valence bound state, and four π^* resonances, besides a σ_SCN^* resonance for 5-SCNU. The cyanate and thiocyanate substituents give rise to additional long-lived π^* resonances, compared to 5-halouracil radiosensitizers. From the reaction thresholds and the expected vibronic couplings among the anion states, efficient production of SCN and CN anions and radical fragments should be observed in dissociative electron attachment measurements for 5-SCNU. The corresponding dissociation processes in 5-OCNU are expected to be less effective in view of the lack of a long-lived σ_OCN^* shape resonance and the little σ^* admixture into the π^* resonances located on the cyanate group. The present results thus indicate 5-SCNU as a more promising radiosensitizer at sub-excitation energies.