Photoelectron spectroscopic study of the negative ions of 4-thiouracil and 2,4-dithiouracil.

We report the photoelectron spectra of the negative ions of 4-thiouracil (4-TU)(-) and 2,4-dithiouracil (2,4-DTU)(-). Both of these spectra are indicative of valence anions, and they are each dominated by a single broad band with vertical detachment energies of 1.05 and 1.4 eV, respectively. Complementary calculations by Dolgounitcheva, Zakrzewski, and Ortiz (see companion paper) are in accord with our experimental results and conclude that the (4-TU)(-) and (2,4-DTU)(-) anions, reported herein, are valence anions of canonical 4-thiouracil and canonical dithiouracil. Comparisons among the anions and corresponding neutrals of 4-thiouracil, 2,4-dithiouracil, 5-chlorouracil, 5-fluorouracil, and uracil itself show that both sulfur and halogen modifications of uracil give rise to significant changes in the electronic structure. The electron affinities of the first four are all substantially larger than that of the canonical uracil.