Temporary anion states of pyrimidine and halopyrimidines

The empty-level electronic structures of pyrimidine and its 2-chloro, 2-bromo and 5-bromo derivatives have been studied with electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS) in the 0-5 eV energy range. The spectral features are assigned to the corresponding anion states with the support of theoretical calculations at the ab initio and density functional theory levels. The empty orbital energies supplied by simple Koopmans’ theorem calculations, scaled with empirical equations, reproduce quantitatively the energies of vertical electron attachment to pi* and sigma* empty orbitals measured in the ET spectra and predict a vertical electron affinity close to zero for the three halo derivatives. The total anion currents of the halo derivatives, measured at the walls of the collision chamber as a function of the impact electron energy, present intense maxima below 0.5 eV. The mass-selected spectra show that in this energy range the total anion current is essentially due to halide fragment anions. The DEA cross-section of the bromo derivatives is about six times larger than that of the chloro derivative. The absolute cross-sections at incident electron energy close to zero are evaluated to be 10 -16-10 -15 cm2.