Formation of negative ions from gas phase halo-uracils by low-energy (0-18 eV) electron impact

We report on low-energy electron-induced processes on gas phase halo-uracils (5-XU, X = F, Cl, Br, and I). The dominant dissociative electron attachment (DEA) channel is formation of X- + Uyl (uracil-yl radical) via a pronounced feature near 0 eV at estimated absolute cross-sections of 3 x 10(-14) cm(-2) (Cl-), 4 x 10-14 cm(-2) (Br-), and 9 x 10(-14) cm(-2) (I-). At that energy the complementary channel, with respect to the negative charge, namely formation of the closed shell uracil-yl anion (Uyl(-)) plus neutral X is also operative at weaker intensity. An exception is fluoro-uracil (5-FU) where both the formation of F- and Uyl(-) are absent. In addition 5-ClU, generates (Uyl - H)(-) + HCl rather than Uyl(-) + Cl as reported very recently [J. Chem. Phys. 118 (2003) 4170]. In competition to dissociation, all halo-uracils additionally form a long-lived parent anion (XU-) near 0 eV From the energy balance of the DEA reaction, we estimate the electron affinity of the uracil-yl radical (Uyl(.)) to be about 2.9-3.2 eV. At higher electron energies we observe various anion ring fragments with 68 and 57 amu, as well as OCN- and CN-. The two first anions are tentatively ascribed to H2C3NO- and CN2OH-, respectively. The present experiments are directly related to reveal the fundamental molecular mechanisms how the radiosensitizers 5-XU operate in cancer therapy. In radiotherapy, secondary electrons which are created in exceeding amounts along the radiation track are believed to play a crucial role. (C) 2003 Elsevier Science B.V. All rights reserved.